UC-NRLF 


B    M    DTI 


MANUAL  FOR 
MATE  ANATOMY 


DENTAL  DEPARTMENT 


Gift  of 
Dr.  Ellis  Jump, 


30 


Or  "^)      3 


A  LABORATORY  MANUAL  FOR  COMPARATIVE 
VERTEBRATE  ANATOMY 


THE  UNIVERSITY  OF  CHICAGO  PRESS 
CHICAGO,  ILLINOIS 


THE  BAKER   &  TAYLOR  COMPANY 
NEW  YORK 

THE  CAMBRIDGE  UNIVERSITY  PRESS 
LONDON 

THE   MARUZEN-KABUSHIKI-KAISHA 
TOKYO,  OSAKA,  KYOTO,  FUKUOKA,  SENDAI 

THE  COMMERCIAL  PRESS,  LIMITED 

SHANGHAI 


A  LABORATORY  MANUAL 

FOR 

COMPARATIVE  VERTEBRATE 
ANATOMY 


By 

LIBBIE  HENRIETTA  jHYMAN,  PH.D. 


THE  UNIVERSITY  OF  CHICAGO  PRESS 
CHICAGO,  ILLINOIS 


COPYRIGHT    1922   BY  THE    UNIVERSITY   OF   CHICAGO 

ALL  RIGHTS  RESERVED.    PUBLISHED  FEBRUARY  Ig22 

NINETEENTH  IMPRESSION   FEBRUARY   1938 


COMPOSED  AND  PRINTED  BY  THE  UNIVERSITY  OF  CHICAGO  PRESS 
CHICAGO,  ILLINOIS,   U.S.A. 


PREFACE 

Several  years  ago  the  method  of  procedure  in  the  laboratory  work  in  verte- 
brate zoology  in  this  University  was  changed  from  the  type  plan,  then  in  common 
use,  to  the  comparative  plan.  No  doubt  a  similar  change  has  been  made  in 
many  other  institutions.  A  suitable  manual  for  the  comparative  method  has, 
however,  hitherto  been  lacking;  the  present  publication  attempts  to  supply  this 
need.  There  can  scarcely  be  any  question  of  the  superiority  of  the  comparative 
method  of  study  of  vertebrate  anatomy,  for  by  this  method  the  student  not 
only  learns  all  of  the  anatomical  facts  brought  out  by  the  type  method  but  he 
also  acquires  an  understanding  of  vertebrate  and  human  structure  which  he  is 
quite  unable  to  attain  by  the  older  method  of  study.  In  view  of  the  fact  that 
the  majority  of  the  students  taking  courses  in  vertebrate  anatomy  at  the  present 
time  are  preparing  for  medicine,  it  seems  obligatory  that  they  be  taught  the 
" history  of  the  human  body"  as  revealed  by  the  study  of  the  anatomy  of  verte- 
brates. On  the  other  hand,  the  comparative  method  is  perhaps  faulty  in  that 
it  may  not  give  the  student  a  clear-cut  picture  of  the  characteristics  of  the 
various  classes  of  vertebrates.  Thus,  while  the  student  readily  learns  the  history 
of  the  aortic  arches,  for  example,  he  does  not  readily  associate  any  particular 
group  of  vertebrates  with  a  particular  condition  of  the  arches,  when  the  com- 
parative method  is  followed.  This  defect  should  probably  be  remedied  in  the 
lecture  part  of  the  course.  I  have  attempted  to  remedy  it  to  a  slight  extent  by 
introducing  the  section  on  the  general  features  of  typical  chordates. 

In  this  manual  I  have  attempted  not  only  to  give  the  laboratory  directions 
for  the  dissection  of  the  various  systems,  but  I  have  also  presented  in  connection 
with  each  system  a  very  brief,  generalized,  and  simplified  account  of  the  develop- 
ment and  evolution  of  that  system.  It  has  seemed  to  me  essential  that  such  an 
account  precede  or  accompany  the  laboratory  directions  in  order  to  give  a  signifi- 
cance to  the  facts  revealed  by  the  dissection  at  the  time  when  the  student 
becomes  aware  of  those  facts.  As  the  consultation  of  other  texts  during  a  dissec- 
tion is  inconvenient  and  time-consuming,  I  have  thought  it  the  most  practical 
plan  to  include  such  explanatory  matter  in  the  laboratory  manual.  Simple 
illustrations  have  been  added  to  clarify  further  the  explanations.  I  have  not, 
however,  in  the  least  intended  that  these  explanations  should  take  the  place  of 
reading  in  the  standard  texts  of  comparative  anatomy.  The  students  should 
understand  that  additional  outside  reading  is  expected  of  them. 

In  making  such  brief  and  generalized  explanations  as  are  given  here  it  is 
impossible  to  take  into  account  numerous  exceptions  and  variations.  I  must 
therefore  ask  the  indulgence  of  the  expert  in  vertebrate  anatomy  for  the  omission 
of  qualifying  clauses  in  the  explanatory  accounts  of  the  various  systems;  in 


vi  PREFACE 

some  cases,  no  doubt,  I  have  been  unaware  of  the  exceptions;  in  others  I  have 
knowingly  omitted  them  on  the  grounds  that  statements  of  exceptions  are  more 
confusing  than  informative  to  the  beginner. 

To  avoid  confusion  the  explanatory  matter  is  printed  in  slightly  smaller 
type  than  the  directions  for  the  dissections. 

I  have  included  in  the  manual  such  materials  as  seemed  to  me  to  bear  most 
directly  on  the  story  of  the  evolution  of  the  various  systems.  I  have  treated 
the  skeleton  and  the  coelom  at  greater  length  than  is  commonly  the  case.  The 
prevailing  neglect  of  the  study  of  the  skeleton  in  courses  in  vertebrate  anatomy 
seems  to  me  unjustifiable  in  view  of  the  importance  of  this  system  in  the  study  of 
phylogenetic  and  evolutionary  questions.  It  is  true  that  skeletal  material  is 
somewhat  expensive  to  purchase  and  maintain,  but  a  good  many  of  the  more 
important  materials  can  be  prepared  by  the  instructor  or  students.  In  presenting 
the  vertebral  column  I  have  adopted  Gadow's  conception  of  the  development 
of  the  vertebrae  from  separate  arcualia,  an  idea  also  adopted  by  Schauinsland  in 
his  account  of  the  development  of  the  vertebrae  in  Hertwig's  Handbuch.  The 
conception  would  appear  to  be  correct  in  the  main,  and  at  least  furnishes  a  simple 
explanation  for  the  variety  of  vertebral  columns  encountered  among  vertebrates. 
The  difficulties  attending  the  study  of  the  coelom  and  mesenteries  do  not  seem 
to  me  to  justify  one  in  disregarding  them.  I  hope  the  simplified  account  I  have 
presented  after  long  study  and  thought  on  the  matter  will  aid  in  the  understand- 
ing of  this  complex  subject.  As  to  the  animals  to  be  dissected,  the  elasmobranch 
must  naturally  form  the  point  of  departure  in  the  study  of  comparative  anatomy. 
I  have  described  the  skate  in  addition  to  the  dogfish,  since  some  teachers  prefer 
it;  it  is  certainly  more  favorable  than  the  dogfish  for  the  study  of  the  nervous 
and  urogenital  systems,  but  less  favorable,  in  my  opinion,  for  the  study  of  the 
circulation.  The  not  infrequent  scarcity  of  dogfish  in  recent  years  makes  it 
desirable  that  an  alternative  form  be  described.  The  bony  fish  is  omitted  because 
its  specialized  structure  would  confuse  rather  than  aid  the  student  in  following 
out  the  evolution  of  the  systems.  The  frog  is  so  often  used  in  general  and  begin- 
ning courses  that  it  seems  superfluous  to  consider  it  here.  Further,  the  urodeles 
serve  our  purpose  much  better.  I  should  like  to  have  included  Cryptobranchus 
as  alternative  to  Necturus,  but  the  limits  of  space  forbade.  The  reptile  is  impor- 
tant for  the  purposes  of  a  comparative  course,  and  the  turtle  is  perhaps  the  most 
readily  obtainable  form  of  sufficient  size.  The  bird  has  been  included  since  it 
seemed  inadvisable  to  omit  altogether  an  entire  vertebrate  class.  I  have  de- 
scribed both  the  cat  and  rabbit,  as  the  former  animal,  though  perhaps  preferable 
in  some  respects,  is  not  always  readily  obtainable. 

At  the  University  of  Chicago  the  course  consists  of  a  brief  glance  at  the 
external  features  of  the  lower  chordates  and  of  representative  vertebrates;  of  all  of 
the  work  given  in  the  manual  on  the  exoskeleton,  endoskeleton,  and  muscles, 
with  the  exception  of  the  muscles  of  Necturus;  and  of  the  complete  dissection 


PREFACE  vii 

of  the  dogfish,  turtle,  and  mammal,  except  the  peripheral  distribution  of  the 
cranial  nerves  in  the  mammal.  The  elasmobranch,  urodele,  and  mammal  would 
make  a  combination  nearly  as  desirable. 

The  general  statements  and  explanations  given  in  the  manual  have  been 
taken  from  standard  works  and  original  papers  on  comparative  anatomy,  verte- 
brate zoology  and  embryology,  and  human  anatomy.  I  have  adopted,  for  the 
most  part,  the  usual  accounts  of  the  evolution  of  the  various  systems  and  parts, 
not  regarding  myself  as  competent  to  criticize  them.  In  a  few  cases  I  have  pre- 
sented some  recent  views  appearing  in  the  literature.  The  laboratory  directions 
and  descriptions  of  the  anatomy  of  the  several  animals  have,  however,  been 
written  entirely  from  the  specimens.  Practically  the  whole  of  the  dissection 
has  been  performed  twice,  some  of  it  more  than  twice.  The  dissections  have  been 
carried  on  simultaneously  with  the  writing  of  the  directions.  In  locating  and 
naming  the  structures  I  have  been  assisted  by  various  texts,  manuals,  and  original 
papers.  In  a  number  of  cases  I  have  found  it  desirable  to  devise  additional 
names  or  to  change  old  names.  I  have  employed  the  terms  dorsal,  ventral, 
anterior,  and  posterior  as  they  are  used  in  vertebrate  anatomy,  abandoning  the 
usage  common  in  human  anatomy.  This  has  involved  a  change  in  some  familiar 
names,  such  as  that  of  the  "anterior"  abdominal  vein. 

I  have  made  a  particular  effort  to  use  all  technical  words  in  a  very  precise 
and  exact  manner  and  to  define  each  such  word  where  it  first  occurs.  One  is 
continually  surprised  and  annoyed  in  a  study  of  vertebrate  structure  to  note 
the  loose  and  inexact  way  in  which  many  terms  are  employed.  It  is,  in  fact, 
practically  impossible  to  find  any  precise  usage  for  many  terms,  such  as  urethra, 
bulbus  arteriosus,  peritoneum,  olfactory  lobe,  perineum,  and  others.  In  such 
cases  I  have  been  compelled  to  adopt  such  a  definition  as  seems  consistent  with 
the  majority  of  the  anatomical  facts. 

I  have  attempted  nothing  in  this  manual  but  a  plain  account  of  the  anatomy 
of  the  several  animals,  which  account  the  student  follows.  This  "  verification " 
type  of  laboratory  manual  has  been  recently  subjected  to  much  criticism,  much 
of  it,  in  my  opinion,  undeserved.  The  critics  seem  to  forget  that  the  student  is 
not  in  reality  engaged  in  "  verifying"  the  statements  in  the  manual;  he  is  engaged 
in  learning  the  anatomy  of  an  animal  by  the  shortest  and  easiest  route,  a  route 
which  the  critics  themselves  would  follow  if  confronted  with  an  animal  with 
which  they  were  not  familiar.  It  is  my  opinion  that  human  beings  in  general 
see  chiefly  that  which  is  pointed  out  to  them;  this  has  been  proved  over  and  over 
again  in  the  history  of  biology.  The  large  number  and  complexity  of  the 
anatomical  facts  to  be  acquired,  the  limited  time  allowed  for  their  acquisition, 
the  large  size  of  the  classes,  and  the  limited  number  of  laboratory  assistants 
available  seem  to  me  to  necessitate  that  detailed  and  specific  laboratory  direc- 
tions be  provided.  If  the  directions  are  not  given  in  the  manual,  then  the  labora- 
tory instructors  are  compelled  to  provide  them  verbally.  Personally  I  am  unable 


viii  PREFACE 

to  see  any  pedagogical  difference  between  directions  and  explanations  written 
in  the  manual  and  those  given  out  verbally  by  the  instructor,  but  I  do  see  a  grea  t 
deal  of  difference  as  concerns  the  time,  patience,  and  energy  of  instructors  and 
students.  Our  experience  with  laboratory  manuals  of  the  type  in  which  the 
burden  of  discovery  is  left  to  the  student  is  that  the  student  becomes  highly 
dissatisfied  and  that  the  instructors  are  brought  into  a  state  of  irritation  and 
fatigue  by  the  continuous  demands  for  assistance  with  which  they  are  bombarded. 
Frankly,  I  believe  in  the  conservation  of  instructors,  and  have  written  this  manual 
with  that  end  in  view.  In  place  of  inserting  questions  in  the  laboratory  manual 
our  method  is  to  hold  thorough  oral  quizzes  on  the  laboratory  and  textbook  work 
at  frequent  intervals. 

Although  a  number  of  drawings  are  called  for  in  the  manual,  it  is  probable 
that  each  instructor  will  prefer  to  decide  for  himself  what  drawings  are  to  be 
made.  Drawings  might  profitably  be  omitted  altogether,  their  place  being 
taken  by  rigorous  practical  individual  quizzes  on  the  dissected  specimen. 

I  am  indebted  to  a  number  of  authors  and  publishers  for  permission  to 
reproduce  figures  from  their  publications.  Due  acknowledgment  is  made  in 
the  legends  to  these  figures.  I  have  not  listed  the  numerous  original  papers  to 
which  I  have  referred,  since  most  of  them  are  given  in  the  bibliographies  appended 
to  Kingsley's  Comparative  Anatomy  of  Vertebrates  and  Goodrich's  account  of  the 
fishes  in  Part  IX,  first  fascicle,  of  Lankester's  A  Treatise  on  Zoology.  I  am  in- 
debted to  Dr.  C.  R.  Moore,  Dr.  B.  H.  Willier,  and  Dr.  J.  W.  Buchanan  for  caUing 
my  attention  to  errors  and  omissions  in  the  first  draft  of  the  manual  which  has 
been  used  in  the  laboratory  under  their  direction  during  the  past  two  years. 
My  thanks  are  also  due  to  Mr.  Kenji  Toda  for  his  patience  and  skill  in  drawing 
the  illustrations.  Finally,  I  wish  to  acknowledge  the  fact  that  the  previous 
organization  of  the  laboratory  work  along  comparative  lines,  chiefly  through 
the  efforts  of  Dr.  J.  W.  MacArthur  and  Mr.  J.  G.  Sinclair,  has  facilitated  the 
task  of  preparing  this  manual. 

So  laborious  has  been  this  task  and  so  great  is  the  number  of  facts  to  be  con- 
sidered that  I  can  scarcely  hope  to  have  avoided  errors,  omissions,  and  state- 
ments lacking  in  clarity.  I  shall  be  more  than  grateful  to  have  my  attention 
called  to  them. 

L.  H.  HYMAN 

UNIVERSITY  or  CHICAGO 
November,  1921 


GENERAL  DIRECTIONS 

Supplies 

1.  Dissecting  instruments  necessary  for  the  course  are: 
Medium-sized  scalpel. 

Fine  scissors. 

Coarse  scissors. 

Stout  probe  for  dissecting. 

Long  slender  probe  for  probing. 

Medium-sized  forceps  with  straight  points. 

A  towel  and  a  laboratory  coat  or  gown  to  protect  the  clothes  are  desirable. 

Bone  scissors  or  forceps  will  be  provided  in  the  laboratory. 

2.  Drawing  materials  necessary  for  the  course  are: 

Drawing  paper,  No.  6.    This  paper  must  be  stiff  and  hard  and  have  a  smooth  surface. 

Hard  drawing  pencil,  6  or  8H. 

Eraser. 

Ruler. 

Red,  yellow,  and  blue  pencils. 

Pad  of  emery  paper  to  sharpen  the  hard  pencil. 

3.  Obtain  the  supplies  named  above  and  present  yourself  with  a  complete  outfit  at  the 
first  laboratory  period.    Do  not  handicap  yourself  at  the  start  by  neglecting  to  provide 
yourself  with  the  necessary  materials. 

Drawings 

1.  All  drawings  must  be  made  with  a  hard  pencil  on  good  quality  drawing  paper,  unless 
otherwise  specified.     Colors  are  to  be  used  only  when  specified  in  the  directions.     Shading, 
crosshatching,  etc.,  are  undesirable  and  are  to  be  avoided.    Drawings  made  otherwise  than 
as  here  specified  will  not  be  accepted. 

2.  Drawings  are  to  be  line  drawings  only,  that  is,  only  the  outlines  of  the  structures  are 
to  be  drawn.    Every  line  must  represent  a  structure  actually  present  on  the  specimen.    Lines 
must  be  smooth  and  clean.     Correct  proportions  are  of  the  utmost  importance  and  are  to  be 
obtained  by  use  of  a  ruler.    In  making  a  drawing  it  is  best  to  outline  the  drawing  first  with 
very  light  lines,  correcting  these  until  accurate  appearance  and  proportions  are  obtained. 
Then  erase  the  light  lines  until  they  are  barely  visible  and  go  over  them  with  a  well-sharpened 
pencil,  making  the  final  lines  firm  and  clear. 

3.  Drawings  are  not  to  be  diagrammatized  unless  so  directed  in  the  manual.     Many 
students  do  not  seem  to  understand  the  difference  between  a  diagram  and  a  drawing.     As 
an  illustration,  Figure  9,  page  15,  in  Kingsley's  Comparative  Anatomy  of  Vertebrates,  is  a  diagram, 
and  Figure  45,  page  51,  is  a  drawing  of  the  upper  half  of  the  same  structures  represented  in  the 
diagram.    The  latter  shows  what  the  object  actually  looks  like;  the  former  is  for  purposes  of 
explanation  only. 

4.  All  drawings  must  be  made  directly  from  the  object  with  the  object  before  the  student 
and  completed  in  the  laboratory.    The  making  of  rough  sketches  in  the  laboratory  to  be 
"improved"  elsewhere  is  unscientific,  inaccurate,  and  absolutely  not  permitted. 

5.  Remember  that  the  prime  requisite  of  a  drawing  is  accuracy.     A  drawing  is  for  the 
instructor  a  record  of  what  you  have  actually  seen  upon  your  specimen.   If  you  have  not 

ix 


x  GENERAL  DIRECTIONS 

dissected  the  structures  called  for,  then  it  is  obvious  that  you  cannot  draw  them  accurately. 
Poor  laboratory  work  invariably  reflects  itself  in  the  quality  of  the  drawings. 

6.  Drawings  must  contain  all  of  the  details  mentioned  in  the  manual.    If,  after  honest 
effort  and  with  the  aid  of  the  laboratory  assistants,  you  are  unable  to  identify  certain  structures, 
omit  them  from  the  drawing  and  make  a  note  to  the  effect  that  you  were  unable  to  find  them. 
An  unreasonable  amount  of  time  should  not  be  spent  in  locating  small  or  unimportant  details. 

7.  All  drawings  must  be  thoroughly  labeled.    Every  drawing  must  be  completely  labeled 
regardless  of  whether  the  same  structures  have  already  been  labeled  in  some  preceding  draw- 
ing.   Labels  are  to  be  written  or  printed  in  hard  pencil  parallel  to  the  top  and  bottom  edges  of 
the  page  and  lines  drawn  with  a  ruler  from  the  labels  to  the  parts  indicated. 

8.  Draw  on  the  right-hand  surface  of  the  page  only. 

9.  Remember  that  the  laboratory  instructors  are  familiar  with  all  of  the  figures  in  the 
various  textbooks  and  that  undue  resemblance  between  your  drawings  and  such  figures  will 
reflect  upon  your  honesty  and  raise  a  suspicion  that  you  have  not  been  exerting  yourself  in  the 
laboratory. 

10.  The  drawings  will  be  called  in  at  intervals.    The  dates  on  which  they  are  due  will  be 
announced  in  advance  by  the  laboratory  instructors. 

Notes  and  Quizzes 

1.  No  notes  are  required  in  this  course.    The  notebooks  will  consist  of  drawings  only. 

2.  Oral  and  .written  quizzes  upon  the  subject-matter  of  the  laboratory  work  are  to  be 
expected  at  any  time.     These  quizzes  will  deal  with  the  anatomy  of  the  animals  you  are  dissect- 
ing and  with  comparative  anatomy.    You  will  be  expected  to  know  thoroughly  the  animals 
and  materials  which  you  dissect  and  study  in  the  laboratory,  and  to  be  able  to  compare  them 
with  one  another,  stating  resemblances  and  differences.     You  will  be  required  to  exhibit  your 
dissections  and  to  be  able  to  identify  the  structures  present  on  the  dissections. 

3.  An  important  quiz  will  follow  the  completion  of  each  section  of  the  laboratory  work 
and  will  deal  with  that  section. 

4.  Reading  in  the  textbooks  of  comparative  anatomy  is  expected  as  a  part  of  the  labora- 
tory work.    Quizzes  will  include  material  in  such  textbooks. 

Dissection 

1.  Dissection  does  not  consist  in  cutting  an  animal  to  pieces.     Dissection  consists  in 
separating  the  parts  of  an  animal  so  that  they  are  more  clearly  visible,  leaving  the  parts  as 
intact  as  practicable. 

2.  In  dissecting  an  animal  very  little  cutting  is  required.     Cleaning  away  the  connective 
tissue  which  binds  together  and  conceals  structures  is  the  chief  process  in  dissection.     In 
doing  this,  use  blunt  instruments,  as  the  probe,  forceps,  or  fingers.    Avoid  the  use  of  scalpel 
and  scissors.    You  will  probably -cut  something  you  will  need  later  on.    In  short,  do  not  cut; 
separate  the  parts. 

3.  Have  the  animal  firmly  fastened.     Smaller  animals  are  generally  pinned  to  wax- 
bottomed  dissecting  pans.    Larger  animals,  such  as  are  used  in  the  greater  part  of  this  course, 
are  tied  to  screw  eyes  in  the  corners  of  the  dissecting  pan.     Put  the  particular  part  you  are 
dissecting  on  a  stretch. 

4.  Do  your  own  dissecting.    Do  not  watch  somebody  else  do  it.     Begin  at  the  most 
easily  accessible  point  of  the  system  you  are  studying  and  follow  out  your  structure,  cleaning 
away  the  tissues  that  conceal  it. 

5.  Exercise  patience  and  care.     Clean  the  structures  by  small  portions. 


GENERAL  DIRECTIONS  xi 

6.  Follow  the  directions  precisely.     Do  not  cut  anything  or  remove  anything  unless 
specifically  directed  to  do  so. 

7.  Your  laboratory  grade  is  partly  determined  by  the  kind  of  dissections  you  make. 

Materials 

1.  But  one  specimen  of  each  animal  is  allowed  to  each  student.     Each  student  will  be 
given  the  necessary  specimens  and  will  retain  them  to  the  end  of  the  course.     Do  not  discard 
any  animal  until  the  manual  so  directs. 

2.  The  smaller  materials  which  are  provided  for  the  class  as  a  whole  should  be  returned 
to  the  bottles  or  jars  from  which  they  came  as  soon  as  you  have  finished  studying  them. 

3.  The  larger  specimens  will  be  kept  in  large  cans.    Each  table  will  be  allotted  the  necessary 
number  of  cans.     Students  will  attach   tags  bearing   their  names  to  their  specimens  and 
keep  them  in  the  cans  when  they  are  not  in  use. 

4.  Specimens  must  always  be  kept  moist  and  must  never  be  allowed  to  dry  up,  as  this 
ruins  them  for  dissection.    Do  not  go  away  and  leave  your  specimens  out  on  the  table.    When 
ready  to  leave  the  laboratory,  wrap  the  animals  in  moistened  cheesecloth  provided  in  the  labora- 
tory and  put  them  into  the  cans.     See  that  the  cans  are  always  covered. 

5.  Students  who,  through  their  own  carelessness,  render  their  specimens  unfit  for  further 
dissection  will  have  to  pay  for  new  specimens. 

6.  The  skeletal  material  provided  in  this  course  is  expensive.    Handle  it  with  care.    Be 
particularly  careful  with  skeletons  preserved  in  fluid. 

References 

Throughout  this  manual  reference  is  made  to  a  number  of  texts  and  manuals  which  may 
profitably  be  consulted  by  the  student.    They  are  obtainable  in  the  library.     These  references 
are  indicated  as  follows: 
B — Bensley,  Practical  Anatomy  of  the  Rabbit. 
CNH—The  Cambridge  Natural  History:  Volume  VII,  "  Fishes,  Ascidians, "  etc. ;   Volume  VIII, 

"Amphibia  and  Reptiles." 

D — Davison,  Mammalian  Anatomy  with  Special  Reference  to  the  Cat. 
H — Hertwig,  Manual  of  Zoology,  translated  by  Kingsley,  sd  ed.,  1912. 
K — Kingsley,  Comparative  Anatomy  of  Vertebrates,  3d  ed.,  1926. 

L — Lankester,  A  Treatise  on  Zoology.    Part  IX,  first  fascicle,  "  Cyclostomes  and  Fishes." 
N — Newman,  Vertebrate  Zoology. 

P  and  H— Parker  and  Haswell,  A  Textbook  of  Zoology,  Volume  II,  3d  ed.,  1921. 
R— Reynolds,  The  Vertebrate  Skeleton. 
R  and  J — Reighard  and  Jennings,  Anatomy  of  the  Cat. 
W— Wilder,  The  History  of  the  Human  Body. 
Wd  -Wiedersheim,  Comparative  Anatomy  of  Vertebrates. 


TABLE  OF  CONTENTS 

MM 

I.  GENERAL  CONSIDERATIONS  ON  ANIMAL  FORM i 

A.  Descriptive  Terms i 

B.  Planes  and  Axes i 

C.  Symmetry i 

D.  Metamerism  or  Segmentation a 

E.  Cephalization 3 

F.  Homology  and  Analogy 3 

II.  THE  PHYLUM  CHORDATA 5 

A.  The  Characteristics  of  the  Chordates .  •      .       .  5 

B.  The  Characteristics  of  the  Vertebrates 5 

C.  The  Classification  of  the  Chordates 5 

III.  GENERAL  STUDY  OF  TYPICAL  CHORDATES 9 

A.  A  mphioxus 9 

B.  A  Tunicate n 

C.  Balanoglossus 14 

D.  Anatomy  of  a  Lamprey 14 

E.  External  Anatomy  of  the  Dogfish 16 

F.  External  Anatomy  of  the  Skate 18 

G.  External  Anatomy  of  a  Teleost 19 

H.  Some  Ganoid  Fishes 21 

I.  External  Anatomy  of  Necturus 22 

J.  External  Anatomy  of  a  Lizard 23 

K.  External  Anatomy  of  the  Turtle 24 

L.  External  Anatomy  of  the  Pigeon 25 

M.  External  Anatomy  of  a  Mammal *   .  27 

N.  Summary 29 

IV.  GENERAL  FEATURES  OF  CHORDATE  DEVELOPMENT 31 

A.  The  Chordate  Egg 31 

B.  The  Cleavage  of  the  Egg  and  the  Formation  of  the  Blastula  ....  32 

C.  Formation  of  the  Gastrula 34 

D.  Formation  of  the  Third  Germ  Layer,  the  Neural  Tube,  and  the  Notochord  36 

E.  Further  History  of  the  Mesoderm 39 

F.  The  Fate  of  the  Ectoderm 41 

G.  The  Fate  of  the  Entoderm      . 41 

H.  The  Fate  of  the  Mesoderm  and  the  Formation  of  Mesenchymc      ...  42 

V.  THE  COMPARATIVE  ANATOMY  OF  THE  INTEGUMENT  AND  THE  EXOSKELETON   .  45 

A.  General  Considerations  on  the  Skeleton 45 

B.  The  Structure  of  the  Skin 45 

C.  The  Exoskeleton  in  General 47 

D.  Exoskeleton  of  Fishes 47 

E.  Exoskeleton  of  Amphibia S° 

F    Exoskeleton  of  Reptiles 5° 

dii 


xiv  TABLE  OF  CONTENTS 

PAGE 

G.  Exoskeleton  of  Birds 52 

H.  Exoskeleton  of  Mammals 54 

I.  Summary     .     ' ...:..  55 

VI.  THE  ENDOSKELETON:  THE  COMPARATIVE  ANATOMY  OF  THE  VERTEBRAL  COLUMN 

AND  RIBS 57 

A.  General  Considerations  on  the  Endoskeleton 57 

B.  The  Embryonic  Origin  of  the  Vertebrae  and  Ribs 58 

C.  Some  Primitive  Vertebral  Columns 63 

D.  The  Vertebral  Column  of  the  Dogfish 64 

E.  Vertebral  Column  of  Teleosts 67 

F.  Vertebral  Column  of  Amphibia 68 

G.  Vertebral  Column  of  Reptiles 70 

H.  Vertebral  Column  of  Birds 72 

I.  Vertebral  Column  of  Mammals      . 73 

J.  Summary  of  the  Vertebral  Column  and  Ribs 76 

VII.  THE   ENDOSKELETON:    THE   COMPARATIVE  ANATOMY  OF  THE   GIRDLES,   THE 

STERNUM,  AND  THE  PAIRED  APPENDAGES 78 

A.  General  Considerations 78 

B.  The  Pelvic  Girdle  and  the  Posterior  Paired  Appendages 79 

C.  The  Pectoral  Girdle,  the  Sternum,  and  the  Anterior  Paired  Appendages       .  85 

D.  General  Summary  of  the  Girdles,  the  Sternum,  and  the  Paired  Appendages  94 

VIII.  THE  ENDOSKELETON:    THE  COMPARATIVE  ANATOMY  OF  THE  SKULL  AND  THE 

VISCERAL  SKELETON 96 

A.  The  Cartilage  Stage  of  the  Skull 96 

B.  The  Visceral  Skeleton 99 

C.  The  Formation  of  the  Membrane  Bones  of  the  Skull 100 

D.  The  Formation  of  the  Cartilage  Bones  of  the  Skull  and  the  Composition  of 

the  Complete  Skull 103 

E.  The  Skull  of  Neciurus,  a  Partially  Ossified  Skull 107 

F.  The  Skull  of  the  Alligator,  a  Typical  Modern  Skull 112 

G.  The  Bones  of  the  Mammalian  Skull 116 

H.  General  Summary  of  the  Skull  and  Visceral  Skeleton        .       .       .       ...  126 

IX.  THE  COMPARATIVE  ANATOMY  OF  THE  MUSCULAR  SYSTEM 128 

A.  General  Considerations    .       . .       .128 

B.  The  Muscles  of  the  Dogfish 128 

C.  The  Muscles  of  Neciurus         . 130 

D.  The  Muscles  of  the  Cat  and  Rabbit 133 

E.  Summary 157 

X.  THE  COMPARATIVE  ANATOMY  OF  THE  COELOM,  DIGESTIVE,  AND  RESPIRATORY 

SYSTEMS 158 

A.  The  Origin  and  Parts  of  the  Coelom  and  the  Mesenteries        .       .       .       .  1 58 

B.  The  Digestive  Tract  and  Its  Derivatives 159 

C.  The  Coelom,  Digestive,  and  Respiratory  Systems  of  Elasmobranchs     .       .  163 

D.  The  Coelom,  Digestive,  and  Respiratory  Systems  of  Necturus         .       .       .  168 

E.  The  Coelom,  Digestive,  and  Respiratory  Systems  of  the  Turtle      .       .       .  172 

F.  The  Coelom,  Digestive,  and  Respiratory  Systems  of  the  Pigeon      .        .       .  116 


TABLE  OF  CONTENTS  xv 

PAGE 

G.  The  Coelom,  Digestive,  and  Respiratory  Systems  of  a  Mammal     .       .       .  183 

H.  The  Comparative  Anatomy  of  the  Coelom  and  the  Mesenteries      .       .       .  194 

I.  Summary 198 

XI.  THE  COMPARATIVE  ANATOMY  OF  THE  CIRCULATORY  SYSTEM       ....  200 

A.  General  Considerations 200 

B.  The  Circulatory  System  of  Elasmobranchs 207 

C.  The  Circulatory  System  of  Neclurus 222 

D.  The  Circulatory  System  of  the  Turtle 230 

E.  The  Circulatory  System  of  the  Pigeon 241 

F.  The  Circulatory  System  of  the  Mammal 248 

G.  Summary  of  the  Circulatory  System 270 

XII.  THE  COMPARATIVE  ANATOMY  OF  THE  UROGENITAL  SYSTEM         ....  273 

A.  Embryonic  Origin  and  Evolution  of  the  Urogenital  System     .        .       .       .  273 

B.  The  Urogenital  System  of  Elasmobranchs 280 

C.  The  Urogenital  System  of  Necturus 283 

D.  The  Urogenital  System  of  the  Turtle 284 

E.  The  Urogenital  System  of  the  Pigeon 286 

F.  The  Urogenital  System  of  the  Mammal 288 

G.  The  Embryonic  Membranes 292 

H.  Summary  of  the  Urogenital  System 294 

XIII.  THE  COMPARATIVE  ANATOMY  OF  THE  NERVOUS  SYSTEM  AND  THE  SENSE  ORGANS  296 

A.  General  Considerations 296 

B.  The  Nervous  System  and  Sense  Organs  of  Elasmobranchs       .       .       .       .  301 

C.  The  Nervous  System  and  Sense  Organs  of  Necturus 318 

D.  The  Nervous  System  and  Sense  Organs  of  the  Turtle 321 

E.  The  Nervous  System  and  Sense  Organs  of  the  Pigeon 328 

F.  The  Nervous  System  and  Sense  Organs  of  the  Mammal 333 

G.  Summary  of  the  Nervous  System  and  the  Sense  Organs 359 

APPENDIX  A.  PRONUNCIATION  AND  DERIVATION  OF  TECHNICAL  WORDS   ....  362 

APPENDIX  B.  PREPARATION  OF  MATERIALS    .       .       .       . 37° 

INDEX                                        «  373 


I.     GENERAL  CONSIDERATIONS  ON  ANIMAL  FORM 

A.  DESCRIPTIVE   TERMS 

The  body  of  vertebrates  is  carried  in  the  horizontal  position  and  the  various  surfaces 
are  designated  as  follows  with  reference  to  this  position: 

Dorsal — the  back  or  upper  side  (posterior  in  human  anatomy). 

Ventral — 'the  under  side  (anterior  in  human  anatomy). 

Lateral— the  sides,  right  and  left. 

Anterior,  cephalic,  or  cranial — the  head  end  of  the  animal  (superior  in  human  anatomy). 

Posterior  or  caudal — the  tail  end  of  the  animal  (inferior  in  human  anatomy). 

Median — the  middle. 

Adverbs  made  by  substituting  d  for  the  terminal  letter  of  these  words  mean  "in  the  direc- 
tion of, "  as  craniad,  toward  the  head,  caudad,  toward  the  tail,  etc. 

Other  descriptive  terms  are: 

Central — the  part  of  a  system  nearest  the  middle  of  the  animal. 

Peripheral — the  part  nearest  the  surface. 

Proximal — near  the  main  mass  of  the  body,  as  the  thigh. 

Distal — away  from  the  main  mass  of  the  body,  as  the  toes. 

Superficial — on  or  near  the  surface. 

Deep — some  distance  below  the  surface. 

Superior — above. 

Inferior — below. 

B.  PLANES  AND  AXES 

The  structures  of  most  animals  are  arranged  symmetrically  with  reference  to  certain 
imaginary  planes  and  axes. 

1.  The  median  plane  is  a  vertical  longitudinal  plane  passing  from  head  to  tail  through  the 
center  of  the  body  from  dorsal  to  ventral  surfaces.    It  divides  the  body  into  two  nearly  identi- 
cal halves,  right  and  left. 

2.  The  sagittal  plane  or  section  is  any  vertical  longitudinal  plane  through  the  body — 
that  is,  the  median  plane  or  any  plane  parallel  to  it.     Sagittal  planes  other  than  the  median 
plane  are  sometimes  designated  as  parasagittal  to  avoid  misunderstanding. 

3.  The  horizontal  or  frontal  plane  or  section  is  any  horizontal  longitudinal  section  through 
the  body — that  is,  all  planes  at  right  angles  to  the  median  plane  and  parallel  to  the  dorsal  and 
ventral  surfaces. 

4.  The  transverse  or  cross  plane  or  section  cuts  vertically  across  the  body  at  right  angles 
to  the  sagittal  and  horizontal  planes. 

5.  The  longitudinal  or  antero posterior  axis  is  a  line  in  the  median  sagittal  plane  extending 
from  head  to  tail;  a  sagittal  or  dorsoventral  axis  is  any  line  in  the  median  sagittal  plane  extend- 
ing from  dorsal  to  ventral  surfaces;  a  transverse  or  mediolateral  axis  is  any  line  in  the  transverse 
plane  running  from  side  to  side. 

C.      SYMMETRY 

The  forms  of  symmetrical  animals  are  dependent  upon  the  arrangement  of  their  parts 
with  regard  to  the  foregoing  axes  and  planes.  There  are  four  fundamental  types  of  animal 
symmetry — spherical,  radial,  biradial,  and  bilateral.  Since  all  vertebrates  are  bilaterally 


2         LABORATORY  MANUAL  FOR  VERTEBRATE  ANATOMY 

symmetrical,  the  other  types  of  symmetry  will  not  be  considered  here.     For  them  consult  H, 
pages  123-25. 

Bilateral  symmetry. — The  parts  of  a  bilaterally  symmetrical  animal  are  arranged 
symmetrically  with  reference  to  three  axes,  the  longitudinal,  transverse,  and  sagittal  axes; 
the  two  ends  of  the  sagittal  axis  in  any  given  cross-section  are  unlike.  There  is  but  one  plane 
of  symmetry  in  such  an  animal — that  plane  which  passes  through  the  longitudinal  and  sagittal 
axes — namely,  the  median  sagittal  plane.  It  divides  the  animal  into  approximately  identical 
right  and  left  halves,  which  are  mirror  images  of  each  other.  The  structures  of  vertebrates 
are  either  cut  in  half  by  the  median  sagittal  plane,  in  which  case  they  are  spoken  of  as  unpaired 
structures,  or  they  are  placed  symmetrically  on  each  side  of  this  plane,  equidistant  from  it, 
in  which  case  they  are  paired  structures.  The  digestive  tract  is  the  only  system  which  does  not 
exhibit  a  symmetrical  relation  to  the  median  plane  in  the  adult,  although  it,  too,  is  bilaterally 
symmetrical  in  early  embryonic  stages. 

D.      METAMERISM   OR   SEGMENTATION 

Segmentation  or  metamerism  is  that  structural  condition  occurring  in  certain  groups  of 
animals  in  which  all  or  most  of  the  paired  parts  or  structures  are  repeated  at  regular  intervals 
along  the  anteroposterior  axis.  The  body  of  such  animals  consequently  is  composed  of  a 
longitudinal  series  of  divisions  or  elements,  in  each  of  which  all  or  most  of  the  systems  of  the 
body  are  represented,  either  by  entire  paired  organs  or  structures  or  by  a  portion  of  the  median 
unpaired  structures.  Each  such  division  or  element  of  the  body  is  called  a  metamere,  somite, 
or  segment.  The  anterior  and  posterior  boundaries  of  each  segment  may  or  may  not  be 
marked  externally  by  a  constriction  of  the  body  wall.  In  the  former  case  the  animal  is  said 
to  exhibit  both  external  and  internal  metamerism;  in  the  latter  case  internal  metamerism  alone 
is  present. 

In  an  ideal  segmented  animal  all  of  the  segments  are  identical  with  each  other  in  all  of 
the  details  of  structure.  No  such  animal  exists  because  both  the  head  and  the  terminal 
segments  must  of  necessity  differ,  if  only  slightly,  from  the  other  segments,  but  the  ringed 
worms,  such  as  Nereis  and  the  earthworm,  closely  approach  the  ideal.  Such  segmented  animals 
in  which  the  various  segments  are  nearly  alike  are  said  to  possess  homonomous  segmentation. 
The  majority  of  segmented  animals,  however,  display  heteronomous  segmentation,  in  which  the 
various  segments  have  become  unlike  each  other  in  many  respects. 

The  segmented  groups  of  animals  are  the  annelids,  the  arthropods,  and  the  vertebrates 
and  their  relatives.  In  the  evolution  of  segmented  animals  there  has  been  a  continuous  pro- 
gression from  the  homonomous  to  the  extreme  heteronomous  condition.  Homonomous 
segmentation  represents  a  primitive  and  generalized  type  of  structure  in  which  the  various 
segments  are  more  or  less  independent  of  each  other  and  each  is  capable  of  performing  all 
of  the  necessary  functions.  But  with  the  evolution  of  heteronomy,  the  segments  become 
unlike  and  there  is  a  division  of  labor  among  them,  some  portions  of  the  body  elaborating 
certain  functions  and  others  other  functions.  Each  segment  is  then  no  longer  capable  of 
performing  all  of  the  functions,  but  is  dependent  upon  its  fellow-segments  with  a  resulting 
unification  and  organization  which  is  lacking  in  the  homonomously  segmented  forms. 

The  heteronomous  condition  is  derived  from  the  homonomous  through  a  number  of 
different  processes,  such  as  loss  of  segments,  fusion  of  adjacent  segments,  enlargement  or 
reduction  of  segments,  loss  of  organs  or  parts  from  some  segments  with  their  retention  in  other 
segments,  structural  changes  among  the  repeated  organs  or  parts  so  that  those  of  different  seg- 
ments become  unlike,  etc. 

The  vertebrates  are  heteronomously  segmented  animals  with  internal  segmentation  only. 
The  embryos  of  vertebrates  much  more  nearly  approach  the  homonomous  condition,  an 


GENERAL  CONSIDERATIONS  ON  ANIMAL  FORM  3 

indication  that  the  vertebrates  arose  from  homonomously  segmented  forms.  In  the  embryonic 
development  of  vertebrates  the  change  from  a  somewhat  homonomous  condition  to  an  extreme 
heteronomy  can  be  directly  followed. 

E.      CEPHALIZATION 

In  the  evolution  of  animals  there  is  a  pronounced  tendency  for  the  anterior  end  of  the 
body  to  become  more  and  more  distinctly  separated  and  differentiated  from  the  rest  of  the 
body  as  a  head.  This  differentiation  of  the  head  consists  chiefly  of  the  localization  within 
the  head  of  the  main  part  of  the  nervous  system — i.e.,  the  brain — and  of  the  most  important 
sense  organs.  Since  the  brain  and  the  sense  organs  control,  to  a  very  large  degree,  the  activi- 
ties and  responses  of  the  rest  of  the  body,  the  head  thus  becomes  the  dominant  part  of  the 
organism.  This  centralization  or  localization  of  nervous  structures  and  functions  in  the  head 
with  accompanying  dominance  of  the  head  is  called  cephalization.  Cephalization  is  more 
and  more  marked  the  higher  one  ascends  in  the  animal  kingdom,  and  is  particularly  prominent 
as  a  structural  and  functional  feature  of  the  vertebrates. 

In  segmented  animals  the  advance  in  cephalization  is  correlated  with  the  progression 
of  the  heteronomous  condition.  Heteronomy,  in  fact,  appears  first  hi  the  head  region  and 
gradually  progresses  posteriorly.  The  anterior  end  thus  retains  the  least  and  the  posterior 
end  the  most  resemblance  to  the  original  homonomous  condition.  This  results  in  an  illu- 
sion of  a  retreat  of  certain  systems  toward  the  posterior  regions  of  the  body,  whereas  the 
situation  in  reality  arises  from  the  fact  that  these  systems  have  disappeared  from  the  anterior 
segments  and  are  retained  in  the  posterior  segments.  In  the  case  of  certain  vertebrate  organs, 
as  the  heart,  a  real  posterior  descent  occurs  during  the  evolution  of  the  vertebrates.  In 
the  vertebrates,  as  in  other  heteronomously  segmented  animals,  the  head  is  produced  through 
the  fusion  of  a  certain  number  of  the  most  anterior  segments  with  a  loss  of  some  segments 
or  of  parts  of  segments  and  the  disappearance  from  these  head  segments  of  nearly  all  systems 
except  the  nervous  system.  As  cephalization  progresses  the  head  appropriates  more  and 
more  of  the  adjacent  segments,  incorporating  them  into  its  structure,  so  that  in  general  it 
may  be  said  that  the  higher  the  degree  of  cephalization,  the  greater  is  the  number  of  segments 
composing  the  head.  In  advanced  cephalization,  such  as  is  possessed  by  vertebrates,  it  is 
very  difficult,  indeed,  almost  impossible,  to  decipher  the  number  and  boundaries  of  the  segments 
which  originally  went  into  the  composition  of  the  head;  in  fact,  the  problem  of  the  segmenta- 
tion of  the  vertebrate  head  has  not  been  completely  solved,  although  it  has  received  the  atten- 
tion of  the  foremost  vertebrate  anatomists. 

The  vertebrates  are,  then,  animals  characterized  by  the  possession  of  bilateral  symmetry, 
internal  and  markedly  heteronomous  segmentation,  and  a  high  degree  of  cephalization.  The 
details  of  their  structure  are  understandable  only  in  the  light  of  these  three  broad  anatomical 
conditions. 

F.      HOMOLOGY  AND  ANALOGY 

Homologous  structures  are  those  which,  however  unlike  in  function  or  superficial  appear- 
ance, have  the  same  origin,  as  demonstrated  by  the  study  of  their  embryonic  origin  and  develop- 
ment and  their  paleontological  history.  Thus  the  wing  of  birds,  the  flipper  of  the  seals,  and  the 
fore  leg  of  the  cat  are  homologous  structures  because  they  are  all  modifications  of  the  original 
fore  limb  and  develop  in  the  same  way  up  to  a  certain  point.  The  whole  aim  of  comparative 
anatomy  is  to  discover  what  structures  are  homologous  and  to  trace  the  modifications  of  such 
homologous  structures  in  the  course  of  evolution. 

Analogous  structures  are  those  which  resemble  each  other  either  as  to  superficial  appear- 
ance or  in  function  but  which  have  had  different  origins.  Thus,  both  fish  and  snakes  are 


4        LABORATORY  MANUAL  FOR  VERTEBRATE  ANATOMY 

covered  with  scales  for  protective  purposes,  but  these  scales  are  not  the  same  morphologically 
because  they  originate  from  the  skin  in  different  ways.  Such  functional  correspondences  are 
usually  the  result  of  environmental  conditions,  i.e.,  they  are  "  adaptations. "  Not  only  parts  of 
animals  but  whole  animals  may  come  to  resemble  each  other  through  the  action  of  the  same 
environment,  as  whales  and  fishes.  Such  resemblance,  not  based  on  actual  relationship,  is 
called  convergence.  On  the  other  hand,  animals  closely  related  to  each  other  may  differ  greatly 
in  appearance,  owing  to  the  different  environments  in  which  they  live,  as  seals  and  cats.  This 
is  known  as  divergence.  (See  further,  N,  pp.  15-20;  W,  pp.  16  ff.) 


II.  THE  PHYLUM  CHORDATA 

A.   THE  CHARACTERISTICS  OF  THE  CHORDATES 

While  the  vertebrates  comprise  the  greater  part  of  the  phylum  Chordata,  three  small 
groups  of  animals  are  united  with  them  in  this  phylum  because  they  possess  certain  char- 
acteristics in  common  with  the  vertebrates.  These  characteristics  are: 

1.  The  wall  of  the  pharynx  of  the  embryo  or  adult  is  pierced  by  openings,  the  gill  slits, 
originally  for  respiratory  purposes. 

2.  A  notochord  is  present  in  embryo  or  adult.    The  notochord  is  a  rod  lying  dorsal  to  the 
intestine,  extending  from  anterior  to  posterior  end,  and  serving  as  a  skeletal  support.     In 
vertebrates  the  notochord  is  partially  or  wholly  replaced  by  the  skull  and  vertebral  column. 

3.  The  central  nervous  system  is  nearly  always  hollow  (in  the  tunica tes  in  the  embryo 
only),  containing  usually  a  single  continuous  cavity  but  in  some  cases  a  number  of  isolated 
spaces,  and  is  situated  entirely  on  the  dorsal  side  of  the  body  (except  in  the  Hemichordata 
where  there  are  both  dorsal  and  ventral  portions).    In  the  invertebrates,  the  central  nervous 
system  is  always  solid  and  lies  mainly  ventral  in  the  body. 

For  further  discussion  of  these  characters  see  P  and  H,  pages  1-2;  N,  pages  31-32; 
K,  pages  1-2. 

B.   THE  CHARACTERISTICS  OF  THE  VERTEBRATES 

The  morphological  characters  of  the  vertebrates  are  the  following:  animals  with  bilateral 
symmetry,  internal  heteronomous  segmentation,  and  cephalization;  with  generally  two  pairs  of 
paired  jointed  locomotor  appendages,  in  the  form  of  fins  or  limbs,  and  sometimes  with  unpaired 
appendages  in  addition;  skin  separable  from  the  rest  of  the  body  wall  and  commonly  producing 
protective  structures,  such  as  scales,  feathers,  hair,  etc.,  cellular  in  nature;  muscle  layer  of  the 
body  wall  decidedly  metameric  in  arrangement;  with  an  internal  skeleton,  of  cartilage  or 
bone,  consisting  of  a  skull  and  gill  supports  in  the  head,  vertebral  column,  ribs,  and  breast- 
bone in  the  body,  and  supports  for  the  appendages;  vertebral  column  highly  metameric, 
composed  of  successive  rings  around  the  notochord;  central  nervous  system  consisting  of  a 
brain,  much  enlarged,  within  the  skull,  and  a  spinal  cord  within  the  vertebral  column;  nerves 
highly  metameric  in  arrangement;  head  with  three  pairs  of  sense  organs,  eyes,  ears,  and  nose; 
digestive  tract  giving  rise  by  outgrowth  to  two  digestive  glands,  the  liver  and  the  pancreas; 
pharynx  intimately  connected  with  the  respiratory  system,  either  opening  to  the  exterior 
by  openings,  the  gill  slits,  in  the  walls  of  which  the  gills  are  borne,  or  giving  rise  by  outgrowth 
to  the  lungs;  heart  always  ventral  in  the  body;  circulatory  system  closed,  always  with  a  median 
dorsal  artery,  the  aorta,  and  with  one  or  two  portal  systems;  genital  and  excretory  systems 
closely  related,  the  excretory  ducts  generally  serving  as  genital  ducts;  excretory  and  genital 
chicts  opening  in  common  with  the  intestine  into  a  cloaca,  or  opening  separately  near  the  anus; 
with  a  well-developed  coelom,  never  segmented,  and  divided  in  the  adult  into  two  or  four 
compartments;  viscera  supported  by  mesenteries.  (See  further,  K,  pp.  2-4;  P  and  H, 
p.  119;  H,  pp.  575-76;  Wd,  pp.  11-13;  and  study  the  diagrams  in  P  and  H,  p.  69.) 

C.      THE   CLASSIFICATION   OF   THE   CHORDATES 

Since  in  this  manual  reference  by  their  scientific  names  to  groups  of  chordates  is  unavoid- 
able, it  is  essential  that  the  student  learn  at  once  the  following  scheme  of  classification.  Only 


6        LABORATORY  MANUAL  FOR  VERTEBRATE  ANATOMY 

those  groups  are  included  which  are  actually  met  with  or  referred  to  in  the  manual.  For 
complete  classification  see  the  various  textbooks,  as  N,  P  and  H,  CNH,  etc.  Speci- 
mens of  these  different  groups  will  be  seen  in  the  laboratory  in  the  next  section  of  the 
manual. 

Phylum  Chordata 

Subphylum  I.    Cephalochordata  (Acrania),  Amphioxus  and  its  aDies. 
Subphylum  II.     Urochordata  or  Tunicata,  the  tunicates  or  sea  squirts. 
Subphylum  III.    Hemichordata  (Enter  o  pneusta) ,  Balanoglossus  and  its  allies. 
Subphylum  IV.     Vertebrata,  all  animals  with  a  vertebral  column. 

Class  i.    Cyclostomata,  the  cyclostomes  or  round-mouthed  fishes,  fishlike  animals  without 
lower  jaws  or  paired  fins. 

Class  2.    Pisces,  the  true  fishes,  with  jaws  and  paired  fins. 

Subclass  i.    Elasmobranchii,  fishes  with  a  cartilaginous  skeleton  and  exposed  gill 

slits,  including  the  dogfish,  skates,  sharks,  etc. 

Subclass  2.     Teleostomi,  fishes  with  a  more  or  less  bony  skeleton  and  with  gill  slits 
concealed  under  an  operculum. 

Order  i.    Crossopterygii,  paired  fins  with  a  basal  stalk;  Polypterus.     (See  Fig.  i.) 


stalk  of  the  fin 


B 


FIG.  i. — A,  anterior  end  of  a  crossopterygian  fish  (Polypterus)  to  show  the  lobe  or  stalk  on  which 
the  fin  is  borne.  B,  anterior  end  of  a  common  teleost  fish  (trout)  to  show  the  absence  of  such  a  stalk. 
This  difference  is  not,  however,  an  important  distinguishing  character  between  groups  of  fishes.  (From 
Bridge  and  Boulenger  in  the  Cambridge  Natural  History,  courtesy  of  the  Macmillan  Company.) 


Order  2.  Chondrostei,  paired  fins  without  a  stalk  (Fig.  i),  skeleton  largely  carti- 
laginous, with  a  spiral  valve  in  the  intestine,  heart  with  a  conus  arteriosus; 
the  sturgeon  (Acipenser)  and  the  spoonbill  (Polyodon). 

Order  3.    Holostei,  like  the  preceding,  but  skeleton  well  ossified;   the  gar  pike 
(Lepidosteus)  and  the  river  dogfish  or  bowfin  (Amia). 
These  three  orders  are  commonly  referred  to  as  the  ganoid  fishes  owing  to 

the  shiny  scales  (ganoid  scales)  with  which  most  of  them  are  covered. 

Order  4.  Teleostei,  paired  fins  without  a  stalk,  skeleton  well  ossified,  without 
spiral  valve  or  conus  arteriosus;  all  of  the  common  fishes.  This  order  is 
commonly  referred  to  as  the  teleosts  or  bony  fishes.  The  three  orders  Chon- 
drostei, Holostei,  and  Teleostei  are  often  grouped  together  as  the  Actinopterygii 
or  ray-finned  fishes  in  which  the  fin  rays  spring  directly  from  the  body  in 
contrast  to  the  Crossopterygii  or  fringe-finned  fishes,  in  which  the  fin  rays 
spring  from  a  stalk  and  form  a  sort  of  fringe  on  the  end  of  the  stalk. 


THE  PHYLUM  CHORDATA  7 

Class  3.     Amphibia,  amphibians,  lowest  four-legged  vertebrates,  skins  naked  and  slimy, 
or  with  bony  plates  (extinct),  living  in  or  near  water. 
Subclass  i.     Stegocephala,  extinct  amphibia,  with  tails,  and  covered  with  an  armor 

of  bony  plates. 

Subclass  2.    Lissamphibia,  present-day  amphibia,  with  naked  slimy  skins  (a  few  with 
minute,  concealed  scales). 

Order  i.     Urodela,  with  tails;  the  salamanders  and  newts,  Necturus,  Amblystoma, 

Cryptobranchus,  etc. 
Order  2.    Anura,  tailless;  frogs,  toads. 
Class  4.    Reptilia,  .reptiles,  air-breathing  vertebrates  covered  with  horny  scales. 

Order  i.    Cotylosauria.    Most  primitive  group  of  extinct  reptiles,  resembling 

Stegocephala  in  skeletal  characters.    Seymouria. 
Order  2.    Chelonia,  the  turtles,  body  inclosed  in  a  hard  case. 
Order  3.    Rhyncocephalia,  including  but  one  animal,  the  Sphenodon  (Hatteria) 
or  tuatara  of  New  Zealand,  a  lizard-like  animal  with  primitive  skeletal  char- 
acters. 

Order  4.    Squamata,  usually  of  small  or  moderate  size  and  covered  with  horny 
scales. 
Suborder  i.    Lacertilia,   the   lizards,   nearly   always   with   limbs,   eyelids 

movable. 

Suborder  2.    Ophidia,  the  snakes,  devoid  of  limbs,  eyelids  immovable. 
Order  5.    Crocodilia,   the   crocodiles,   alligators,   gavials,   and   caimans,   large 
reptiles  with  both  horny  scales  and  bony  plates  in  the  skin. 

Class  5.    Aves,  birds,  vertebrates  with  feathers. 

Class  6.    Mammalia,  mammals,  vertebrates  with  hair  and  milk  glands. 
Subclass  i.    Prototheria,  mammals  laying  eggs. 

Order  i.     Monotremata,  the  monotremes,  or  egg-laying  mammals,  including  only 
the  duckbill  (Ornithorhynchus)  and  the  spiny  anteaters  (Echidna  and  Pro- 
echidna)  of  the  Australian  region. 
Subclass  2.     Eutheria,  mammals  bearing  the  young  alive. 

Division  i.     Didelphia  or  Metatheria. 

Order  i.     Marsupialia,  the  marsupials,  mammals  bearing  the  young  in  a  very 

immature  state,  and  carrying  them  in  a  pouch  formed  by  a  fold  of  skin  on  the 

abdomen,  placenta  absent  or  primitive;  kangaroos,  opossums,  etc. 

Division  2.    Monodelphia  or  Placentalia,  the  placental  mammals,  without  a 

pouch,  young  nourished  in  the  uterus  by  a  placenta,  which  is  produced 

by  a  fusion  between  certain  parts  of  the  embryo  and  certain  parts  of  the 

maternal  uterus.    This  division  includes  sixteen  orders  which  are  described 

in  detail  in  N,  pages  346-404.    In  this  manual,  we  meet  with  three  orders: 

Order  Carnivora,  the  carnivorous  mammals,  with  claws  and  sharp,  cutting  teeth ; 

the  bears,  raccoons,  minks,  martens,  weasels,  otters,  dogs,  foxes,  wolves, 

cats,  lions,  tigers,  hyaenas,  seals,  walruses. 

Order  Rodentia,  the  rodents,  with  chisel-like  front  teeth,  and  back  teeth  with 
flat,  grinding  surfaces;  the  hares,  rabbits,  squirrels,  rats,  mice,  porcupines, 
guinea  pigs. 
Order  Edentata,  teeth  lacking  or  degenerate;  the  ant  bears,  sloths,  and  armadillos. 


8         LABORATORY  MANUAL  FOR  VERTEBRATE  ANATOMY 

Additional  scientific  names  used  for  convenience  in  grouping  the  classes  of  vertebrates 
are: 

Ichthyopsida.  This  term,  meaning  fishlike  animals,  includes  the  two  classes,  Pisces  and 
Amphibia. 

Sauropsida.  This  term,  meaning  reptile-like  animals,  includes  the  two  classes,  ReptUia 
and  Aves. 

Anamniota  or  Anamnia.  This  term  includes  the  classes  Pisces  and  Amphibia  and  refers 
to  the  fact  that  their  embryos  are  naked. 

Amniota.  This  embraces  the  three  classes,  ReptUia,  Aves,  and  Mammalia,  and  refers  to 
the  fact  that  the  embryos  of  these  groups  are  covered  by  a  membrane,  the  amnion. 

Acraniata  or  Acrania.  This  name  is  synonymous  with  Cephalochordata  and  is  derived  from 
the  absence  of  the  skull  in  Amphioxus. 

Craniata.  This  term  is  synonymous  with  Vertebrata  and  refers  to  the  fact  that  all  verte- 
brates possess  a  skull. 

Agnathostomata.    This  term,  meaning  without  jaws,  is  synonymous  with  Cyclostomata. 

Gnathostomata.  This  name,  meaning  jawed,  includes  all  of  the  classes  of  vertebrates, 
except  Cyclostomata. 

Tetrapoda.  This  term,  meaning  four  footed,  includes  all  of  the  land  vertebrates,  i.e., 
the  classes  Amphibia,  ReptUia ,  Aves,  and  Mammalia. 


III.     GENERAL  STUDY  OF  TYPICAL  CHORDATES 

A.      AMPHIOXUS 

i.  External  anatomy  of  Amphioxus. — Obtain  a  specimen  and  place  in  a 
dish  of  water.  The  body  is  slender,  fishlike,  pointed  at  each  end,  and  compressed 
laterally.  The  more  blunt  end  is  anterior,  the  more  pointed  end,  posterior; 
the  dorsal  surface  is  sharp,  the  ventral  surface,  for  the  greater  part  of  its  length, 
flattened.  The  anterior  end  represents  a  poorly  developed,  somewhat  degenerate 
head.  The  ventral  and  greater  part  of  the  head  consists  of  an  expanded  mem- 
brane, the  oral  hood,  which  incloses  a  cavity,,  the  stomodaeum  or  vestibule,  at  the 
bottom  of  which  the  mouth  is  located.  The  borders  of  the  oral  hood  are  extended 
into  a  series  of  stiff  tentacles  or  cirri. 

Turn  the  animal  ventral  side  up  and  observe  that  the  flattened  portion  of 
the  ventral  surface  is  bounded  laterally  by  two  membranous  folds,  the  meta- 
pleural  folds,  or  lateral  fins,  extending  posteriorly  from  the  oral  hood.  These 
folds  meet  at  a  point  nearly  three-fourths  of  the  distance  from  anterior  to  pos- 
terior end,  behind  a  median  opening,  the  atriopore.  From  this  point  a  median 
membranous  fold,  the  fin,  passes  to  the  posterior  end  of  the  body,  around  to 
the  dorsal  side,  and  forward  along  the  dorsal  side  to  the  anterior  end.  The 
slightly  wider  portion  of  this  fin  which  surrounds  the  pointed  posterior  end  is 
the  caudal  fin,  that  along  the  dorsal  side,  the  dorsal  fin.  The  anal  opening  will 
be  found  on  the  ventral  side  very  near  the  posterior  end,  just  behind  the  point 
where  the  fin  widens.  The  anus  is  on  the  left  side. 

The  body  is  covered  by  a  thin  epidermis  under  which  is  a  muscle  layer.  The 
greater  part  of  the  muscle  layer  consists  of  the  lateral  muscles,  forming  the 
side  walls  of  the  body,  and  divided  into  a  large  number  of  V-shaped  muscle 
segments,  or  myotomes,  clearly  visible  through  the  transparent  epidermis.  Each 
myotome  is  separated  from  its  neighbor  by  a  connective  tissue  partition,  the 
myocomma.  Note  that  the  myotomes  extend  nearly  to  the  tip  of  the  anterior 
end,  diminishing  in  size  above  the  oral  hood.  The  number  of  myotomes  in 
Amphioxus  is  definite,  about  sixty.  The  myotomes  are  one  expression  of  the 
metamerism  of  the  Amphioxus  body.  The  ventral  portion  of  the  body  is 
provided  with  a  thin  layer  of  transverse  or  ventral  muscles,  whose  fibers  run 
circularly;  these  muscles  are  not  visible  externally. 

Immediately  below  the  ventral  terminations  of  the  myotomes  will  be  seen, 
in  some  individuals  at  least,  a  row  of  square  white  masses,  the  gonads  or  reproduc- 
tive organs.  Their  arrangement  is  metameric.  The  ventral  part  of  the  body, 
extending  posteriorly  from  the  oral  hood,  appears  somewhat  clear  and  is  occupied 
by  a  large  cavity,  the  atrium,  which  surrounds  the  digestive  tract  and  opens  to 


io        LABORATORY  MANUAL  FOR  VERTEBRATE  ANATOMY 

the  exterior  through  the  atriopore.  Amphioxus  maintains  a  continuous  circula- 
tion of  water  through  its  digestive  tract;  water  enters  the  mouth,  passes  through 
the  walls  of  the  pharynx  into  the  atrium,  and  out  of  the  atriopore.  Make  a 
drawing  of  the  animal  from  the  side. 

2.  Internal  anatomy. — The  internal  anatomy  is  most  easily  studied  on  small 
mounted  specimens.     Examine  with  the  low  power  of  the  microscope.     Identify : 
the  various  parts  of  the  fin,  containing  rectangular  bodies,  the  fin  rays,  serving 
as  the  skeletal  support  of  the  fins;  the  muscle  segments;  and  the  digestive  tract, 
occupying  the  ventral  half  of  the  body.     The  parts  of  the  digestive  tract  may 
be  studied  in  some  detail.     Note  again  the  oral  hood  with  its  cirri.     The  vesti- 
bule is  narrowed  posteriorly  by  a  membrane,  the  velum,  pierced  by  a  small  open- 
ing, the  mouth.     The  mouth  opens  into  a  wide  cavity,  the  pharynx,  extending 
half  the  length  of  the  body.     Its  walls  are  perforated   by  numerous  oblique 
slits,  the  gill  slits  or  pharyngeal  clefts.     The  solid  portions  of  the  pharyngeal  wall 
between  the  gill  clefts  are  called  the  branchial  bars  and  each  is  supported  by  an 
internal  skeletal  branchial  rod.     Surrounding  the  pharynx  is  a  large  cavity,  the 
atrium,  the  ventral  boundary  of  which  is  visible  as  a  line  below  the  pharynx. 
This  line  may  be  traced  to  the  atriopore.    The  posterior  end  of  the  pharynx 
opens  into  a  tubular  intestine  which  extends  straight  to  the  anus.     The  first  part 
of  this  intestine  for  a  short  distance  posterior  to  the  pharynx  is  narrow  and  dor- 
sally  located;   the  intestine  then  widens  suddenly  and  from  this  widened  part 
a  blind  sac,  the  liver,  extends  forward  beneath  the  narrowed  portion. 

Immediately  dorsal  to  the  digestive  tract  and  nearly  as  wide  as  the  pharynx 
is  a  rod,  the  notochord,  extending  the  length  of  the  body.  It  will  be  seen  by 
focusing  down  through  the  myotomes  and  is  most  distinct  in  the  head,  where  it 
runs  forward  nearly  to  the  extreme  tip.  Just  above  the  notochord  is  situated 
the  much  smaller  neural  tube,  best  recognized  by  the  row  of  black  pigment  spots 
which  it  bears.  These  pigment  spots  have  been  shown  to  be  sensitive  to  light. 
Draw  the  whole  mount,  showing  its  structure. 

3.  Cross-section    through    the    pharyngeal    region. — Examine    the    cross- 
section  with  the  low  power  and  identify  the  following:    (a)  The  epidermis,  the 
outer  covering  of  the  body  composed  of  a  single  layer  of  columnar  epithelial  cells. 
(b)  The  dorsal  median  projection,  the  dorsal  fin,  containing  an  oval  mass,  the 
fin  ray,  which  supports  it.     (c)  The  two   ventrolateral  projections,  the  meta- 
pleural  folds.     There  are  a  number  of  smaller  folds  in  the  ventral  wall  between 
the  two  metapleural  folds,     (d)  The  myotomes,  a  series  of  circular  masses  filling 
the  dorsal  and  lateral  portions  of  the  body  wall,  and  separated  from  each  other 
by  connective  tissue  partitions.     The  myotomes  are  thick  dorsally  and  thin  out 
ventrally.     Transverse  muscles  are  present  in  the  ventral  body  wall,  just  above 
the  small  folds  of  the  epidermis,     (e)  The  neural  tube,  a  median  dorsal  mass, 
oval  or  trapezoidal  in  section,  lying  between  the  dorsal  portions  of  the  myotomes, 
below   the  fin  ray.     Observe  that  it  contains  a  central   canal,  the  neurocoel. 


GENERAL  STUDY  OF  TYPICAL  CHORD ATES  II 

Black  spots  in  the  neural  tube  are  the  pigment  cups  of  simple  eyes  or  light- 
perceiving  organs.  These  are  distributed  along  the  anterior  part  of  the  neural 
tube  of  Amphioxus  and  are  similar  in  structure  to  the  eyes  of  planarians,  consist- 
ing of  a  pigment  cup  filled  with  nerve  cells.  Experiment  has  shown  that  these 
optic  cups  are  sensitive  to  light  while  the  eyespot  on  the  anterior  end  of  the 
brain  is  insensitive  to  light.  (/)  The  notochord,  an  oval  mass,  larger  than  the 
neural  tube  and  directly  ventral  to  it.  (g)  The  atrium,  the  large  cavity  occupying 
the  ventral  half  of  the  section,  (ti)  The  pharynx,  the  structure  occupying  the 
center  of  the  atrium.  It  is  elongated  in  some  regions,  heart  shaped  in  others.  It 
consists  of  separate  pieces,  the  branchial  bars,  each  inclosing  in  its  outer  extremity 
a  stiff  support,  the  branchial  rod.  The  spaces  between  adjacent  gill  bars  are  the 
gill  slits  or  pharyngeal  clefts,  by  means  of  which  the  cavity  of  the  pharynx  com- 
municates with  the  atrium.  In  the  median  dorsal  line  of  the  pharynx  is  a  deep 
groove,  the  epibranchial  groove;  in  the  median  ventral  line,  a  broad  slightly 
depressed  structure,  the  hypobranchial  groove,  or  endostyle.  These  grooves 
secrete  mucus  in  which  the  minute  food  particles  are  caught,  (i)  The  liver,  an 
oval  hollow  structure  present  in  some  sections  to  the  right  of  the  pharynx.  It 
is  composed  of  tall  epithelial  cells,  which  probably  produce  a  digestive  secretion. 
(j)  The  gonads.  In  cross-sections  of  larger  specimens  these  are  found  as 
masses,  one  on  each  side  of  and  projecting  into  the  atrium.  Draw  the  cross- 
section. 

For  further  details  of  the  anatomy  of  Amphioxus  consult  P  and  H,  pages 
44-56;  CNH,  Vol.  VII,  pages  112-30;  N,  pages  30-42. 

B.      A   TUNICATE 

i.  External  anatomy. — Obtain  a  specimen  and  place  in  a  dish  of  water. 
It  is  an  oval  saclike  creature,  scarcely  recognizable  as  an  animal.  The  animal 
is  in  life  permanently  attached  to  rocks  or  other  objects.  The  end  which  was 
attached  is  recognizable  by  its  rough  and  irregular  form  and  by  the  fragments 
of  wood  or  other  materials  which  adhere  to  it.  The  opposite  end,  which  in  the 
living  state  extends  free  into  the  water,  bears  two  openings,  the  siphons.  When 
the  animal  is  gently  squeezed,  jets  of  water  are  observed  to  squirt  from  the 
siphons;  hence  the  name  sea  squirt  popularly  applied  to  these  animals.  The 
upper  of  the  two  siphons  is  the  oral  or  incurrent  siphon  and  is  the  degenerate 
anterior  end  of  the  animal;  the  lower  siphon  is  the  atrial  or  excurrent  siphon 
and  represents  the  posterior  end.  The  anteroposterior  axis  of  the  tunicates  is 
bent  into  a  U -shape.  The  region  of  the  body  between  the  two  siphons  is  there- 
fore dorsal,  and  the  remaining  and  much  larger  part  of  the  surface  is  ventral. 
When  the  animal  is  undisturbed,  the  siphons  are  widely  expanded,  and  a  current 
of  water  passes  into  the  oral  siphon  and  out  of  the  atrial  siphon.  When  disturbed 
and  also  generally  in  the  preserved  state,  the  siphons  are  closed  and  retracted. 
Closure  and  reaction  is  accomplished  by  circular  and  longitudinal  muscles  located 


12       LABORATORY  MANUAL  FOR  VERTEBRATE  ANATOMY 

in  the  body  wall.  In  closing,  the  rim  of  the  siphons  is  thrown  into  folds,  whose 
number  and  arrangement  varies  with  different  species. 

The  outer  layer  of  the  body  wall  consists  of  a  thick,  tough,  sometimes  some- 
what gelatinous,  often  warty,  covering,  the  tunic  or  test,  which  is  secreted  by 
the  ectoderm  beneath  it.  In  some  tunicates  (dona,  Corella)  the  test  is  so  trans- 
parent that  practically  all  of  the  internal  organs  are  visible  through  it.  The 
test  is  attached  to  the  underlying  body  only  at  the  siphons. 

Draw  the  animal. 

2.  Dissection  of  a  tunicate. — (A  demonstration  dissection  may  be  substituted 
for  individual  dissection  by  the  student.)  As  different  tunicates  vary  in  the 
details  of  their  internal  structure,  the  following  account  is  of  necessity  somewhat 
generalized.  It  is  based  chiefly  on  the  anatomy  of  Ciona.  Make  a  cut  through 
the  base  of  the  tunic,  avoiding  injury  to  the  underlying  wall,  and  peel  off  the 
tunic.  Note  its  attachments  at  the  siphons,  and  cut  through  these  attachments, 
discarding  the  tunic.  The  soft  body  wall  or  mantle  is  thus  revealed,  the  tunic 
being,  as  already  explained,  a  secretion  of  the  outer  layer  of  the  mantle.  The 
muscle  fibers  which  operate  the  siphons  may  be  visible  as  stripes  in  the  mantle; 
they  are  conspicuous  in  Ciona. 

Fasten  the  animal  in  a  wax-bottomed  dissecting  pan  by  pins  through  the 
rims  of  the  siphons  and  through  the  extreme  basal  end.  Fill  the  pan  with  water 
to  cover  the  animal. 

The  mantle  incloses  the  viscera  which  tend  to  adhere  to  it,  so  that  its  removal 
without  injury  to  the  viscera  is  difficult.  In  transparent  forms  the  viscera  may 
be  studied  through  the  mantle.  In  case  it  is  necessary  to  remove  the  mantle, 
make  a  longitudinal  slit  in  it  from  the  atrial  siphon  to  the  basal  end  and  pull  it 
off  in  small  strips,  separating  each  strip  gently  from  the  underlying  parts.  The 
principal  internal  parts  are  the  following.  The  oral  siphon  leads  into  a  large 
thin-walled  bag,  which  in  some  forms,  as  Ciona,  extends  the  length  of  the  body. 
This  bag  is  the  pharynx.  Its  wall  appears  to  the  naked  eye  like  mosquito  netting, 
because  it  is  pierced  by  numerous  minute  openings,  the  gill  slits  or  visceral  clefts. 
The  cavity  outside  of  the  pharynx  is  the  atrium;  it  communicates  with  the 
exterior  by  way  of  the  atrial  siphon.  The  lower  end  of  the  pharynx  narrows 
abruptly  into  a  short  esophagus  which  opens  into  a  widened  stomach,  situated 
below  or  to  one  side  of  the  basal  end  of  the  pharynx.  The  stomach  is  curved 
so  that  its  long  axis  lies  at  right  angles  to  the  long  axis  of  the  pharynx.  In  some 
cases  (Molgula)  brown  digestive  glands  can  be  seen  covering  the  stomach.  The 
stomach  leads  into  an  intestine  which  immediately  makes  a  loop,  doubling 
back  so  as  to  lie  parallel  to  the  stomach.  It  then  bends  and  extends  straight 
upward  toward  the  atrial  siphon,  terminating  by  an  anus  situated  within  the 
atrial  cavity. 

The  reproductive  organs  consist  of  a  single  or  paired  mass,  each  being  her- 
maphroditic, that  is,  composed  in  part  of  an  ovary  and  in  part  of  a  testis.  This 
hermaphroditic  gonad  is  located  either  in  the  space  between  the  intestinal  loop 


GENERAL  STUDY  OF  TYPICAL  CHORD ATES  13 

and  the  stomach  (Ciona,  Corella,  Ascidia)  in  which  case  it  is  commonly  single,  or 
may  be  attached  to  the  inside  of  the  mantle  (Cynthia,  Molgula),  in  which  case 
a  pair  of  such  glands  is  often  present.  From  the  ovarian  part  arises  an  oviduct 
and  from  the  testicular  part  a  vas  deferens.  The  two  ducts  run  in  close  contact 
with  each  other  (and  in  some  cases  fuse  to  one  duct)  and  open  into  the  atrial 
cavity  near  the  atrial  siphon.  In  Ciona  the  two  genital  ducts  are  found  closely 
attached  to  that  side  of  the  intestine  which  faces  the  pharynx,  and  in  this  posi- 
tion run  far  forward,  opening  into  the  atrium  some  distance  above  the  anal  open- 
ing. The  genital  opening  in  Ciona  is  colored  red,  due  to  the  presence  in  its  walls 
of  red  vesicles,  supposed  to  have  an  excretory  function.  In  Molgula  and  other 
members  of  the  family  Molgulidae  there  is  a  large  saclike  organ  on  the  right  side, 
believed  to  be  excretory  in  nature,  but  the  presence  of  excretory  organs  in  the 
tunicates  is  more  or  less  uncertain. 

In  the  dorsal  part  of  the  mantle,  between  the  two  siphons,  will  be  found  an 
elongated  mass,  the  ganglion,  which  constitutes  the  entire  central  nervous  system 
of  the  adult  tunicate.  Nerves  may  be  seen  extending  from  its  ends  to  the  siphons. 

Draw  the  dissection. 

3.  The  structure  of  the  pharynx. — Make  a  longitudinal  slit  in  the  side  of 
the  pharynx  forward  through  the  oral  siphon  and  spread  out  its  walls.  In  the 
median  ventral  line  of  the  pharynx  there  is  present  a  conspicuous  white  cord 
extending  the  length  of  the  pharynx.  This  is  the  endostyle  or  hypobranchial 
groove,  whose  walls  are  composed  of  glandular  and  ciliated  cells,  the  former 
producing  mucus.  Directly  opposite  the  endostyle  in  what  may  be  designated 
the  median  dorsal  line  of  the  pharynx  is  located  the  dorsal  lamina.  This  in 
some  tunicates  (Ascidia,  Cynthia,  Molgula)  consists  of  a  delicate  membranous 
fold  of  the  pharyngeal  wall,  while  in  others  (Ciona,  Corella)  it  consists  of  a  row 
of  processes  called  languets  and  resembles  a  fringe.  At  the  anterior  end  of  the 
pharynx  at  the  bottom  of  the  oral  siphon  there  generally  occurs  a  circlet  of 
tentacles  or  irregular  processes  of  the  wall,  and  posterior  to  this  is  a  grooved 
ridge,  the  peri  pharyngeal  band,  in  which  the  anterior  ends  of  endostyle  and 
dorsal  lamina  terminate.  The  lateral  walls  of  the  pharynx  in  some  tunicates 
(Molgula,  Cynthia)  exhibit  a  definite  number  of  longitudinal  folds. 

Cut  out  a  small  piece  of  the  pharyngeal  wall,  mount  in  water,  spread  out 
flat,  and  examine  under  the  low  power  of  the  microscope,  or  examine  prepared 
slides.  The  wall  is  found  divided  into  squares  by  means  of  longitudinal  and 
cross  bars.  Within  each  square  so  formed  are  seen  several  of  the  elongated 
gill  slits  or  visceral  clefts,  separated  from  each  other  by  smaller  bars.  The 
gill  slits  are  commonly  parallel  to  the  longitudinal  bars  but  may  be  curved 
(Molgula).  On  the  longitudinal  bars  there  are  often  present  finger- like  processes 
or  papillae  which  have  been  shown  to  aid  in  pushing  the  food-containing  mucous 
strands  toward  the  esophagus.  Draw  a  portion  of  the  pharyngeal  wall. 

For  further  details  of  the  anatomy  of  tunicates  consult  P  and  H,  pages  14-21 ; 
CNH,  Vol.  VII,  pages  35-62;  N,  pages  50-52,  H>  Pa8es  5°5~6- 


14  LABORATORY  MANUAL  FOR  VERTEBRATE  ANATOMY 

C.      BALANOGLOSSUS 

i.  External  anatomy. — Obtain  a  specimen,  place  in  a  dish  of  water,  and 
examine.  (As  different  species  differ  somewhat  in  appearance,  the  following 
description  applies  only  to  Balanoglossus  [Dolichoglossus]  kowalevskii.)  It  is  an 
elongated  wormlike  animal  about  five  or  six  inches  in  length.  The  body  is 
divided  into  three  parts — the  proboscis,  the  collar,  and  the  trunk.  The  proboscis 
is  the  slender,  conical,  pointed  structure  at  the  anterior  end,  used  by  the  animal 
in  burrowing  into  the  sand.  The  collar  is  the  band  encircling  the  body  just 
posterior  to  the  proboscis.  The  mouth  is  concealed  within  the  collar.  The 
trunk  comprises  the  greater  part  of  the  animal  and  presents  a  more  or  less 
ruffled  appearance.  The  median  ventral  line  is  marked  by  a  distinct  longitudi- 
nal band;  the  median  dorsal  line  in  a  similar  but  less  distinct  manner.  The 
sides  of  the  trunk  for  about  an  inch  and  a  half  posterior  to  the  collar  project 
somewhat  and  are  slightly  different  in  appearance  from  the  rest  of  the  trunk; 
these  projections  contain  the  reproductive  organs  and  hence  are  named  the 
genital  ridges.  Between  the  anterior  portions  of  the  genital  ridges  on  the  dorsal 
side  will  be  found  two  longitudinal  rows  of  small  parallel  slits,  the  gill  pores. 
(The  true  gill  slits,  leading  from  the  pharynx,  are  U-shaped  and  each  opens  into 
a  chamber,  the  gill  pouch,  which  in  turn  communicates  with  the  exterior  by  means 
of  the  gill  pore,  which  is  somewhat  smaller  than  the  gill  slit.  In  some  balano- 
glossids,  as  Ptychodera,  the  U-shaped  gill  slits  open  directly  to  the  exterior,  as 
is  the  case  in  the  larva  of  Amphioxus.  The  structure  and  development  of  the 
branchial  region  of  balanoglossids  is  strikingly  similar  to  that  of  Amphioxus.) 
The  remainder  of  the  trunk  is  occupied  by  the  digestive  tract  and  possesses  a 
terminal  anus.  The  species  under  consideration  lacks  the  hepatic  caeca,  a 
series  of  outgrowths  of  the  intestinal  wall,  figured  in  P  and  H.  Make  a  drawing 
of  the  animal  from  the  dorsal  side. 

For  further  details  consult  P  and  H,  pages  2-7;  CNH,  Vol.  VII,  pages  3-21; 
H.  pages  512-14;  N,  pages  60-65. 

D.      ANATOMY   OF   A   LAMPREY 

The  lampreys  belong  to  the  lowest  class  of  true  vertebrates,  the  class  Cyclo- 
stomata,  or  round-mouthed  fishes. 

i.  External  anatomy  of  a  lamprey. — Place  the  specimen  in  a  dissecting 
tray.  The  body  consists  of  a  stout  cylindrical  head  and  trunk  and  a  flattened 
tail.  From  the  median  dorsal  line  in  the  posterior  portion  of  the  body  arise 
two  dorsal  fins,  while  the  tail  is  provided  with  a  caudal  fin,  continuous  with 
the  second  dorsal  fin.  The  fins  are  supported  by  numerous  fin  rays,  slender 
parallel  cartilages  usually  visible  through  the  skin.  There  are  no  paired 
ventral  fins  such  as  are  found  in  the  true  fishes.  The  skin  is  soft,  slimy,  and 
naked. 


GENERAL  STUDY  OF  TYPICAL  CHORD ATES  15 

The  anterior  end  of  the  head  presents  a  peculiar  appearance,  owing  to  the 
apparent  absence  of  a  lower  jaw  (the  jaw  is  probably  present  in  a  degenerate 
condition)  and  the  presence  on  the  ventral  side  of  the  head  of  a  large  bowl- 
shaped  depression,  the  buccal  funnel.  The  edges  of  the  buccal  funnel  are  pro- 
vided with  soft  papillae,  and  its  interior  is  studded  with  brown,  horny  teeth, 
definitely  arranged.  At  the  bottom  of  the  funnel  is  a  projection,  called  the 
tongue,  also  covered  with  teeth.  The  mouth  is  just  dorsal  to  the  tongue.  Lamp- 
reys attach  themselves  to  the  bodies  of  fishes  by  means  of  the  buccal  funnel  and 
rasp  off  their  flesh  by  filing  movements  of  the  tongue. 

On  the  dorsal  surface  of  the  head  is  an  opening,  the  nasal  aperture  or  nostril, 
which  leads  into  the  olfactory  sac.  Cyclostomes  differ  from  other  vertebrates 
in  possessing  but  one  nostril,  although  this  is  probably  a  secondary  condition. 
On  each  side  of  the  head  on  a  level  with  the  olfactory  organ  is  an  eye,  not  provided 
with  eyelids.  Posterior  to  the  eye  on  each  side  of  the  head  is  a  row  of  seven 
oval  openings,  the  gill  slits. 

Along  the  sides  of  the  body  the  boundaries  of  the  myotomes  are  generally 
noticeable  through  the  skin.  In  the  median  ventral  line  at  the  junction  of  trunk 
and  tail  will  be  found  a  pit.  In  the  anterior  end  of  this  pit  is  the  anal  opening 
and  immediately  behind  the  anus  a  projecting  papilla,  the  urogenital  papilla, 
which  receives  the  products  of  the  kidneys  and  reproductive  organs  and  dis- 
charges them  to  the  exterior  through  an  opening  at  its  tip,  the  urogenital 
aperture. 

2.  Sagittal  section  of  the  anterior  end. — Make  a  median  sagittal  section  of 
your  specimen  to  a  point  about  an  inch  posterior  to  the  last  gill  slit  or  study  a 
section  so  prepared.  Examine  the  cut  surface  and  identify  the  following: 

a)  Digestive  tract:  Observe  again  the  buccal  funnel  with  its  teeth  and  tongue. 
Note  the  large  muscle  masses  extending  posteriorly  from  the  tongue,  by  means 
of  which  the  rasping  movements  of  the  tongue  are  brought  about.     Find  the 
mouth  opening  above  the  tongue  and  follow  it  into  a  passage,  the  buccal  cavity 
which  slopes  ventrally.     The  buccal  cavity  opens  at  its  posterior  end  into  two 
tubes,  an  upper  smaller  one,  the  esophagus,  and  a  larger  ventral  one,  the  pharynx, 
the  wall  of  which  is  pierced  by  seven  oval  openings.     A  fold,  the  velum,  is  present 
at  the  entrance  of  the  buccal  cavity  into  the  pharynx.     The  esophagus  leads 
into   the  remainder  of  the   digestive   tract,   but  the  pharynx,   which   in  the 
embryo  constituted  the  anterior  part  of  the  digestive  tract,  ends  blindly  in 
the  adult. 

b)  Respiratory  system:   The  seven  openings  in  the  wall  of  the  pharynx  are 
the  internal  gill  slits.     They  open  into  much  enlarged  gill  pouches  which  bear 
the  gills  on  their  walls  and  which  communicate  with  the  exterior  through  the 
external  gill  slits  already  noted.     Probe  into  one  of  the  gill  pouches  and  note  the 
leaves  or  gill  lamellae  borne  on  its  walls.     The  dorsal  portions  of  the  gill  pouches 
inclose  the  narrow  esophagus  between  them. 


16       LABORATORY  MANUAL  FOR  VERTEBRATE  ANATOMY 

c)  Notochord:  The  notochord  is  the  broad,  brown  rod  situated  just  dorsal  to 
the  esophagus.     It  is  the  chief  axial  skeleton  of  the  animal,  the  vertebral  column 
being  very  embryonic. 

d)  Nervous  system:  Above  the  notochord  is  a  narrow  canal,  the  neural  canal, 
in  which  the  spinal  cord  and  brain  are  located.     The  spinal  cord  is  a  slender 
cord  occupying  the  canal;   the  brain  is  an  enlarged  lobed  structure  situated  just 
dorsal  to  the  anterior  extremity  of  the  notochord. 

e)  Olfactory  apparatus:    The  section  should  bisect  the  olfactory  aperture. 
Note  that  a  canal  extends  from  this  aperture  and  opens  into  a  sac,  the  olfactory 
sac,  situated  just  anterior  to  the  tip  of  the  brain.     The  folds  in  the  wall  of  the 
sac  bear  the  olfactory  mucous  membrane.     From  the  anterior  margin  of  the 
olfactory  sac  a  tube  arises  which  bends  ventrally  and  posteriorly  and  widens 
into  an  elongated  sac,  the  pituitary  pouch,  lying  ventral  to  the  anterior  end  of 
the  notochord.     The  peculiar  dorsal  position  of  the  olfactory  sac  as  well  as  its 
relation  to  the  pituitary  sac  (which  in  other  vertebrates  has  no  connection  with 
the  olfactory  cavities)  are  due  to  a  shifting  of  these  organs  during  development 
from  their  original  ventral  position   (see  P  and  H,  Fig.   799,  n.   133).     The 
pituitary  sac  corresponds  to  the  anterior  lobe  of  the  pituitary  body  or  hypophysis 
of  other  vertebrates. 

/)  Pericardial  cavity:  Posterior  to  the  last  gill  pouch  is  a  somewhat  conical 
cavity,  the  pericardial  cavity,  within  which  the  heart  is  situated. 

Make  a  detailed  drawing  of  the  sagittal  section. 

For  a  complete  account  of  the  anatomy  of  cycles  tomes  consult  P  and  H, 
pages  119-39;  CNH,  Vol.  VII,  pages  150-52,  216-22,  247-48,  279-82,  371-73, 
385-95;  N,  pages  87-94. 


E.      EXTERNAL  ANATOMY  OF  THE  DOGFISH 

The  dogfish  or  dog  shark  is  a  true  fish,  belonging  to  the  lowest  group  of 
fishes,  the  subclass  Elasmobranchii.  It  is  a  very  generalized  vertebrate,  and 
hence  a  knowledge  of  its  structure  is  indispensable  for  an  understanding  of 
vertebrate  anatomy.  There  are  two  common  dogfishes  usually  obtainable  for 
laboratories,  the  smooth  dogfish,  Mustelus  canis,  and  the  spiny  dogfish,  Acanthias 
vulgaris.  These  two  species  are  slightly  different  in  several  respects;  attention 
will  be  called  to  these  differences  where  necessary. 

i.  Body  and  skin. — The  body  of  the  dogfish  has  the  shape  and  proportions 
which  we  recognize  as  most  advantageous  for  free-swimming  animals — fusiform 
(spindle  shaped)  and  pointed  at  each  end,  thus  offering  little  resistance  to  the 
water.  The  body  is  divided  into  head,  trunk,  and  tail,  which  are  not,  however, 
distinctly  bounded  from  each  other.  Trunk  and  tail  are  provided  with  fins  for 
purposes  of  locomotion.  The  body  is  clothed  with  minute  scales,  each  of  which 
bears  a  tiny  spine.  Pass  the  hand  over  the  skin  of  the  dogfish  and  note  the 


GENERAL  STUDY  OF  TYPICAL  CHORD ATES  17 

rough  feeling  due  to  the  spines.  A  whitish  line,  the  lateral  line,  extends  along 
each  side  of  the  body;  it  contains  sensory  cells  whose  function  is  the  detection 
of  vibration  in  the  water.  Through  the  skin  the  zigzag  muscle  segments  or 
myo tomes  similar  to  those  of  Amphioxus  can  often  be  perceived. 

2.  The  head. — The  head  is  triangular  and  somewhat  flattened;  its  pointed 
extremity  is  known  as  the  rostrum.  On  the  ventral  side  of  the  head  is  the  narrow 
crescentic  mouth,  bounded  as  in  all  vertebrates  except  cycles  tomes  by  the  upper 
and  lower  jaws,  both  of  which  bear  a  number  of  teeth  arranged  in  diagonal  rows. 
The  head  further  bears  the  three  pairs  of  sense  organs  characteristic  of  verte- 
brates— the  olfactory  organs,  the  eyes,  and  the  ears.  The  nostrils  are  a  pair  of 
openings  on  the  ventral  side  of  the  rostrum.  A  little  flap  of  skin  extends  over 
the  center  of  each  nostril,  dividing  the  opening  into  two  passages,  by  means  of 
which  a  current  of  water  circulates  through  the  olfactory  sac,  a  rounded  sac 
into  which  each  nostril  leads.  The  oval  eyes  are  situated  on  the  sides  of  the  head. 
They  have  immovable  upper  and  lower  eyelids;  in  the  smooth  dogfish  the  lower 
lid  is  extended  into  a  thin  membrane,  the  nictitating  membrane,  or  third  eyelid, 
which  can  be  drawn  up  over  the  eye.  Behind  each  eye  is  a  slight  prominence, 
which  is  best  perceived  by  feeling  with  the  finger;  within  this  the  ear  is  located. 
There  is  no  external  ear,  ear  opening,  or  drum  membrane,  but  the  ears  are  con- 
nected with  the  surface  of  the  head  by  two  canals,  the  endolymphatic  ducts, 
which  open  by  a  pair  of  small  pores  in  the  center  of  the  dorsal  surface  of  the  head 
just  back  of  the  level  of  the  eyes.  These  may  be  difficult  to  find  in  some  speci- 
mens. The  rostrum  and  adjacent  parts  of  the  head  are  punctured  by  many 
pores,  which  are  the  openings  of  long  mucous  canals  imbedded  under  the  skin 
and  of  uncertain  function. 

3.  Gill  slits. — Just  behind  each  eye  is  a  circular  opening,  the  first  gill  slit 
or  spiracle  and  a  short  distance  posterior  to  this  a  row  of  five  elongated  slits,  the 
second  to  sixth  gill  slits.     The  gill  slits  communicate  with  the  cavity  of  the  phar- 
ynx.   In  the  respiratory  movements  water  enters  through  the  mouth  and  exits 
through  the  gill  slits. 

4.  Fins. — These  are  of  two  kinds,  the  unpaired  or  median  fins,  arising  from 
the  median  lines  of  the  animal,  and  the  paired  or  lateral  fins,  located  on  the  ventral 
side  of  the  trunk  near  the  median  line.    The  unpaired  fins  consist  of  an  anterior 
and  a  posterior  dorsal  fin,  arising  from  the  median  dorsal  line,  each,  in  the  case 
of  the  spiny  dogfish,  being  provided  with  a  spine  in  front;  a  ventral  fin  on  the 
ventral  side  of  the  tail,  present  in  the  smooth  dogfish  only;  and  the  tail  fin  or 
caudal  fin  surrounding  the  posterior  end  of  the  body.     In  the  dogfish  and  other 
elasmobranchs  the  caudal  fin  is  asymmetrical  with  respect  to  the  tail,  which 
bends  upward  in  it,  dividing  the  caudal  fin  into  a  narrow  dorsal  part  and  a  broader 
ventral  portion ;  this  type  of  tail  is  known  as  heterocercal.    There  are  two  pairs  of 
paired  fins;  they  correspond  to  the  limbs  of  land  vertebrates.     The  anterior  pair, 
just  behind  the  gill  slits,  is  named  the  pectoral  fins,  the  posterior  pair  at  the  junction 


i8       LABORATORY  MANUAL  FOR  VERTEBRATE  ANATOMY 

of  trunk  and  tail,  the  pelvic  fins.  In  male  dogfishes,  the  medial  side  of  each  pelvic 
nn  is  modified  into  a  stout  process  directed  posteriorly.  These  processes  are 
called  claspers  and  are  used  by  the  males  in  mating  with  the  females.  All  of 
the  fins  are  supported  by  slender  flexible  rays,  the  dermal  fin  rays,  imbedded  in 
the  skin  of  the  fins. 

5.  Anus. — Between  the  two  pelvic  fins  is  a  large  opening,  the  anus  or  cloacal 
aperture.  This  is  the  opening  of  a  chamber,  the  cloaca,  which  receives  the  end 
of  the  intestine  and  the  terminations  of  the  ducts  of  the  kidneys  and  reproduc- 
tive organs.  The  anus  of  the  dogfish  and  of  the  majority  of  vertebrates  does  not 
quite  correspond  to  the  anus  of  man  and  other  mammals,  which  receives  the 
intestine  only,  but  the  term  is  commonly  used  to  designate  both  classes  of 
opening. 

F.   EXTERNAL  ANATOMY  OF  THE  SKATE 

The  skates  or  rays  are,  like  the  dogfish,  elasmobranch  fishes  but  highly 
modified  as  regards  external  form  and  proportions.  Obtain  a  specimen  and 
examine. 

1.  Body  and  skin. — The  body  is  divided  into  a  greatly  flattened  anterior 
portion  comprising  head  and  trunk  and  a  slender  posterior  portion,  the  tail. 
The  broad,  flat  form  is  characteristic  of  bottom-feeding  fishes  (see  N,  pp.  97-99) 
and  results  from  a  shortening  of  the  dorsoventral  axis  and  an  elongation  of  the 
transverse  axis.     The  tough  skin  contains  scattered  scales  of  which  the  spines 
project  conspicuously.     These  scales  are  of  the  same  type  already  noted  in  the 
dogfish,  consisting  of  a  basal  plate  imbedded  in  the  skin  and  a  projecting  spine, 
directed  posteriorly.     The  scales  of  the  skate  are  much  larger  and  fewer  in  number 
than  those  of  the  dogfish.     They  are  definitely  arranged  in  lines  and  groups,  the 
arrangement  differing  somewhat  in  the  two  sexes.     In  females  there  are  scales 
over  the  lateral  expansions  of  the  trunk  and  several  rows  of  scales  on  the  median 
dorsal  part  of  the  trunk  and  dorsal  surface  of  the  tail,  while  in  males  the  greater 
part  of  the  lateral  expansions  is  devoid  of  scales,  there  are  fewer  rows  along  the 
middle  of  the  back  and  tail,  and  the  scales  on  the  margins  of  the  head  are  enlarged. 
In  males,  furthermore,  there  are  two  rows  of  curious  erectile  spines  on  the  lateral 
expansions  of  the  trunk  about  an  inch  in  from  the  margin;   these  can  be  erected 
and  lowered  into  depressions  in  the  skin.     Are  there  any  scales  on  the  ventral 
surface  ?    Note  the  marked  differences  in  color  between  the  dorsal  and  ventral 
surfaces. 

2.  Fins. — Like  the  dogfish  the  skate  is  provided  with  median  and  paired 
fins,  but  the  former  are  much  reduced.     They  consist  of  two  small  dorsal  fins 
on  the  dorsal  side  of  the  end  of  the  tail.     The  pectoral  fins  are  enormously 
enlarged,  forming  the  lateral  expansions  of  the  trunk  already  mentioned  several 
times;   they  are  confluent  anteriorly  with  the  margins  of  the  head.     The  pelvic 
fins  are  smaller  and  immediately  posterior  to  the  pectoral  fins  with  which  they 


GENERAL  STUDY  OF  TYPICAL  CHORD ATES  19 

are  continuous  in  some  species.  They  consist  of  two  lobes,  and  in  the  males 
bear  large  stout  daspers,  deeply  grooved  along  their  posterior  lateral  margins. 
The  claspers  are  employed  in  mating  with  the  females. 

3.  Head.— The  head  like  the  trunk  is  greatly  flattened,  its  margins  con- 
tinuous with  the  pectoral  fins.     It  terminates  in  a  pointed  rostrum.    It  bears 
dorsally  a  pair  of  large  projecting  eyes,  without  lids.     Behind  each  eye  is  the 
large  spiracle  or  first  gill  slit,  bearing  a  valve  on  its  anterior  face,  marked  by 
parallel  ridges  which  represent  a  rudimentary  gill.     On  the  ventral  side  of  the 
head  is  the  mouth  bounded  by  tooth-bearing  jaws.     The  jaws  and  teeth  are 
commonly  larger  in  the  males  than  in  the  females.     In  front  of  the  mouth  are 
the  two  nostrils,  each  provided  with  a  fringed  ear-shaped  flap.     Extending 
posteriorly  from  each  nostril  to  the  angle  of  the  mouth  is  a  flap  with  a  fringed 
posterior  margin.     This  is  the  nasofrontal  process.     On  lifting  up  this  process  a 
deep  groove,  the  oronasal  groove,  will  be  found  extending  from  the  nostril  into 
the  mouth  cavity.     This  arrangement  foreshadows  the  appearance  of  a  closed 
passage  from  the  nostrils  into  the  mouth  such  as  is  present  in  higher  vertebrates. 
Posterior  to  the  mouth  are  five  pairs  of  gill  slits.     In  skates  the  pectoral  fins  have 
grown  forward  above  the  gill  slits  and  fused  with  the  sides  of  the  head,  thus 
shoving  the  gill  slits  to  the  ventral  surface. 

4.  Anus. — The  anus  or  cloacal  aperture  is  a  large  opening  between  the 
bases  of  the  pelvic  fins. 

G.      EXTERNAL  ANATOMY  OF  A  TELEOST 

For  this  purpose  any  common  bony  fish  can  be  used,  but  the  following 
description  is  based  upon  the  perch.  The  perch  like  other  common  fishes  is  a 
member  of  the  great  order  Teleostei.  Obtain  a  specimen  and  note  the  following 
points. 

1.  Body  and  skin. — The  body  has  the  shape  typical  of  aquatic  animals, 
thickest  in  the  middle  and  tapering  to  each  end.     It  is  somewhat  compressed 
laterally.     It  is  indistinctly  divided  into  head,  trunk,  and  tail.     Trunk  and  tail 
are  clothed  with  scales  arranged  in  diagonal  rows,  overlapping  each  other. 
These  scales  are  set  in  pockets  in  the  deeper  part  of  the  skin  (dermis) ,  as  may  be 
determined  by  removing  one  of  them,  and  the  superficial  layer  of  the  skin  (epi- 
dermis) forms  a  thin  film  over  their  surfaces.     The  head  is  covered  by  the  soft 
epidermis  and  in  some  regions  bears  small  scales  like  those  on  the  remainder  of 
the  body.     Beneath  the  scaleless  portions  of  the  epidermis  of  the  head  will  be 
noted  large,  thin,  flat  bones.     These  bones,  which  are  the  outer  bones  of  the 
skull,  are  in  reality  nothing  but  enlarged  scales  which  have  sunk  from  their 
original  superficial  position  to  a  deeper  location.     A  lateral  line  is  present  along 
each  side  of  the  body. 

2.  Head. — The  head  bears  a  terminal  mouth  bounded  by  well-developed 
jaws.     The  terminal  position  of  the  mouth  is  probably  more  primitive  than  the 


20        LABORATORY  MANUAL  FOR  VERTEBRATE  ANATOMY 

ventral  position  found  in  the  elasmobranchs.  On  the  dorsal  side  of  the  anterior 
end  of  the  head  are  two  pairs  of  nostrils,  a  pair  to  each  olfactory  sac.  This 
arrangement  permits  a  current  of  water  to  circulate  through  the  olfactory  sac. 
The  large  eyes  are  without  lids.  The  ears,  situated  behind  the  eyes,  are  invisible 
externally.  The  posterior  and  lateral  margins  of  the  head  are  formed  by  a  large 
flap,  the  gill  cover  or  operculum,  which  is  supported  by  several  opercular  bones, 
large,  flat,  scalelike  bones,  already  noted.  It  covers  a  wide  slit  in  the  body  wall 
known  as  the  gill  opening.  Attached  to  the  ventral  margin  of  the  operculum 
is  a  membrane,  the  branchiostegal  membrane,  supported  by  seven  bony  rays,  the 
branchioslegal  rays.  Grasp  the  membrane  with  a  forceps  and  spread  it  out  to 
see  the  rays.  Lift  up  the  operculum  and  look  within  the  cavity  which  it  covers. 
Four  curved  structures,  the  gill  arches,  which  should  be  separated  from  each 
other  with  a  forceps,  will  be  seen.  Each  bears  on  its  outer  surface  a  gill,  con- 
sisting of  a  double  row  of  soft  filaments,  and  on  its  inner  margin  a  series  of  short 
toothlike  processes,  the  gill  rakers.  Thrust  a  probe  inward  between  two  gill 
arches,  open  the  mouth  of  the  fish,  and  observe  that  the  end  of  the  probe  has 
entered  the  mouth  cavity.  The  cavity  of  the  pharynx  is  thus  in  communication 
with  the  exterior  through  the  spaces  between  the  gill  arches.  These  spaces  are 
gill  slits  corresponding  to  those  which  we  saw  in  the  dogfish  and  skate,  but  here 
the  portions  of  the  body  wall  between  successive  gill  slits  have  disappeared,  and 
all  open  into  a  common  cavity  covered  by  the  opercuhim.  This  condition  is 
characteristic  of  all  fishes  except  elasmobranchs.  When  the  fish  respires,  the 
mouth  opens,  the  opercula  move  outward,  the  branchiostegal  membrane  unfolds 
and  closes  the  gill  opening;  water  is  thus  drawn  into  the  mouth  and  bathes 
the  gills.  The  mouth  then  closes,  the  opercula  move  inward,  the  branchioste- 
gal membrane  folds  up,  and  the  water  passes  out  through  the  gill  slits  and 
gill  opening. 

3.  Fins. — The  body  is  provided  with  median  and  paired  fins.     Of  the  former 
there  are  an  anterior  and  a  posterior  dorsal  fin,  a  caudal  fin,  and  a  ventral  or 
anal  fin.     (The  number  and  position  of  the  median  fins  are  very  variable  in  differ- 
ent fishes.)     The  caudal  fin  is  apparently  symmetrical  with  the  end  of  the  tail, 
forming  a  homocercal  tail.    The  paired  fins  are  the  same  as  in  the  dogfish.     The 
pectoral  fins  are  located  just  behind  the  operculum,  but  the  pelvic  fins  have 
moved  forward  from  their  normal  position  at  the  level  of  the  anus  to  a  position 
nearly  level  with  the  pectoral  fins.     Such  a  forward  migration  of  the  pelvic  fins 
is  very  common  in  the  teleost  fishes  and  is  often  associated  with  diminution  and 
degeneration  of  these  fins.     The  anterior  dorsal  fin  is  supported  by  sharp  hard 
spines,  and  these  are  also  present  in  one  border  of  some  of  the  other  fins,  but  the 
latter  are  supported  chiefly  by  flexible  bony  fin  rays. 

4.  Openings. — In  the  median  ventral  line  just  in  front  of  the  ventral  fin 
is  a  large  opening,  the  anus.     Behind  this  is  a  depression  into  which  projects  a 
small  elevation,  the  urogenital  papilla.     In  the  perch  there  is  no  cloaca,  but  the 


GENERAL  STUDY  OF  TYPICAL  CHORDATES  21 

intestine  and  urogenital  systems  have  separate  openings;  this  is  one  of  the  marked 
differences  between  elasmobranch  and  teleost  fishes. 

H.      SOME   GANOID  FISHES 

As  certain  ganoid  fishes  are  frequently  referred  to  in  the  study  of  the  skeleton, 
it  is  advisable  that  the  student  become  familiar  with  their  appearance.  The 
ganoid  fishes  belong  to  the  lower  and  more  primitive  orders  of  the  Teleostomi 
and  hence  are  placed  in  the  scheme  of  classification  below  the  teleosts,  but  the 
latter  have  been  considered  first  to  introduce  the  terminology.  We  shall  examine 
briefly  four  common  ganoids. 

1.  The  gar  pike  (Lepidosteus) . — The  body  is  clothed  in  a  complete  armor 
of  rhomboid  scales  set  in  diagonal  rows.     Such  scales  are  called  ganoid  scales 
and  consist  of  bone  with  an  outer  coat  of  a  shiny  substance  named  ganoin. 
These  scales  pass  onto  the  head  as  enlarged  plates,  which  constitute  the  bones  of 
the  roof  of  the  skull.     The  jaws  are  very  elongated,  producing  a  snout  on  the 
tip  of  which  are  the  small  nostrils.     The  jaws  bear  numerous  sharp  teeth.     There 
is  an  operculum  covering  the  gills.    A  spiracle  is  absent.     Note  median  and 
paired  fins,  with  their  stout,  jointed  fin  rays.     The  tail  is  heterocercal  but 
approaches  the  homocercal  type. 

2.  The  sturgeon  (Acipenser). — There  is  a  covering  of  ganoid  scales  arranged 
in  five  rows  on  the  trunk  with  areas  of  apparently  naked  skin  (which  really  bear 
small  scales)  between  the  rows.     The  scales  lack  the  outer  coat  of  ganoin.     Most 
of  them  bear  sharp  posteriorly  directed  spines.     They  pass  onto  the  head, 
transforming  into  skull  bones.     The  head  is  broad  and  terminates  in  a  large 
rostrum,  bearing  many  small  scales  on  its  dorsal  surface.     The  nostril  is  divided 
by  a  partition  into  two  openings.    The  operculum  contains  a  single  bone.    There 
is  a  slitlike  spiracle  above  each  eye.     On  the  ventral  side  of  the  rostrum  are  four 
branched  processes,  the  barbels,  used  as  sense  organs  for  the  detection  of  food. 
The  mouth  is  curious  in  form,  distensible,  and  with  sensory  papillae  on  its 
borders.     The  jaws  are  degenerate,  and  there  are  no  teeth.     Note  paired  and 
median  fins;  the  tail  is  heterocercal. 

3.  The  spoonbill  (Polyodori). — In  this  curious  fish  the  rostrum  is  expanded 
into  a  broad,  thin,  spatulate  structure,  provided  with  sense  organs  for  the  detec- 
tion of  food.     At  the  base  of  the  rostrum  just  in  front  of  the  eyes  are  the  nostrils, 
each  with  two  openings.     Behind  the  eyes  is  the  small  spiracle.     The  operculum 
contains  no  opercular  bones  and  is  extended  into  a  pointed  process;  the  branchio- 
stegal  membrane,  continuous  with  the  opercula,  has  no  branchiostegal  rays. 
Lift  up  the  operculum  and  note  the  numerous  long  and  fine  gill  rakers  on  the  gill 
arches,  used  by  the  fish  in  separating  food  particles  from  mud.     The  jaws  open 
widely  and  are  provided  with  minute  teeth.     The  body  is  naked.     Note  the  fins. 
The  tail  is  typically  heterocercal.     The  fish  is  singularly  lacking  in  external 
hard  parts. 


22       LABORATORY  MANUAL  FOR  VERTEBRATE  ANATOMY 

4.  The  bowfin  or  river  dogfish  (Amia). — The  body  is  clothed  in  flexible 
scales,  similar  to  those  of  the  perch.  There  are  large  scales  on  the  head.  The 
operculum  and  branchiostegal  membranes  contain  bones.  The  nostrils  have  two 
openings,  the  anterior  one  borne  on  the  tip  of  the  process  located  on  the  anterior 
end  of  the  head,  the  posterior  opening  situated  in  front  of  the  eye.  There  is  no 
spiracle.  There  is  a  very  long  dorsal  fin  and  a  nearly  homocercal  tail.  The 
fish  strongly  resembles  the  ordinary  teleost  type. 

I.      EXTERNAL  ANATOMY  OF  NECTURUS 

Necturus  is  a  salamander;  it  belongs  to  the  class  Amphibia,  order  Urodela. 
It  is  an  example  of  the  lowest  land  vertebrates — those  which  left  the  water, 
acquired  lungs  and  the  air-breathing  habit,  and  developed  legs  in  place  of  the 
paired  fins.  Obtain  a  specimen  and  note  the  following  points. 

1.  Body  and  skin. — The  body  is  typically  vertebrate  in  form.     It  is  divisible 
into  head,  trunk,  and  tail,  as  in  fishes.     A  neck  is  not  present.     The  skin  is 
naked  and  very  slimy,  without  scales  or  other  hardened  parts  such  as  are  com- 
monly found  in  vertebrates. 

2.  Head. — The  head  is  broad  and  flat  and  has  a  terminal  mouth  provided 
with  lips.     It  bears  the  usual  three  pairs  of  sense  organs.     The  nostrils  or  external 
nares  are  a  pair  of  widely  separated  openings  just  back  of  the  margin  of  the  upper 
lip.     By  probing  into  the  nostrils  determine  that  they  communicate  with  the 
mouth  cavity  by  means  of  openings  known  as  the  internal  nares.     This  arrange- 
ment permits  air  to  enter  the  mouth  cavity  through  the  nostrils,  and  differs 
greatly  from  the  condition  found  in  the  majority  of  fishes  where  the  olfactory 
sacs  end  blindly  and  have  no  connection  with  the  mouth  cavity.     In  some  fishes, 
however,  as  the  skate,  there  is  an  external  groove,  the  oronasal  groove,  extending 
from  each  olfactory  sac  to  the  mouth  cavity;  and  by  the  fusion  of  the  borders 
of  this  groove  a  closed  passage  from  the  nostrils  to  the  mouth  cavity  is  produced 
(see  K,  pp.  206-8).     The  small  eyes  without  eyelids  are  situated  on  the  sides  of 
the  head.     The  ears,  as  in  fishes,  are  internal  only. 

From  each  side  of  the  posterior  margin  of  the  head  spring  three  gills,  each  con- 
sisting of  a  fringe  of  filaments  dependent  from  a  dorsal  process.  They  are 
external  gills  and  do  not  correspond  to  the  gills  of  fishes  which  are  internal. 
Between  the  first  and  second,  and  the  second  and  third  gills  are  the  gill  slits 
which  open,  as  in  fishes, into  the  cavity  of  the  pharynx.  The  animal,  however,  does 
not  pass  water  through  the  gill  slits  but  respires  by  means  of  the  external  gills 
which  are  kept  in  constant  motion,  through  the  general  surface  of  the  body,  and 
to  some  extent  by  means  of  its  lungs. 

3.  Appendages. — The  trunk  bears  the  two  pairs  of  appendages.     These 
correspond  to  the  paired  fins  of  fishes  but  have  evolved  into  typical  walking 
limbs.     Each  consists  of  the  following  parts:  upper  arm,  forearm,  wrist,  and  hand, 


GENERAL  STUDY  OF  TYPICAL  CHORDATES  23 

in  the  case  of  the  fore  limb;  thigh,  shank,  ankle,  and  foot,  in  the  hind  limb.  Both 
hand  and  foot  bear  four  digits  (fingers,  toes),  although  five  is  the  typical  verte- 
brate number;  the  first  digit  is  the  one  which  is  missing.  The  position  of  the 
limbs  with  reference  to  the  body  is  very  primitive,  especially  in  the  case  of  the 
hind  limb,  and  should  be  carefully  studied.  Note  that  the  hind  limb  projects 
out  at  right  angles  to  the  body,  all  of  its  parts  on  a  plane  parallel  to  the  ground. 
In  this  primitive  position  the  limb  has  an  anterior  or  preaxial  border,  a  posterior 
or  postaxial  border,  and  dorsal  and  ventral  surfaces.  In  the  fore  limb,  however, 
the  forearm  is  bent  downward,  and  the  hand  is  directed  slightly  forward.  This 
alteration  of  position  is  brought  about  chiefly  by  a  torsion  of  the  upper  arm, 
whose  former  preaxial  surface  now  looks  dorsally.  The  preaxial  border  of  the 
forearm  is  turned  medially,  its  postaxial  border  laterally.  As  a  result  of  these 
changes  the  animal  is  able  to  lift  itself  to  a  slight  extent  above  the  ground. 

The  flattened  tail  is  bordered  by  a  tail  fin  which  differs  from  the  fins  of  fishes 
in  that  it  contains  no  fin  rays.  The  tail  is  diphycercal,  that  is,  truly  symmetrical 
with  respect  to  the  vertebral  column. 

4.  Anus. — At  the  junction  of  the  trunk  and  tail  in  the  median  ventral  line 
is  the  large  anus  or  cloacal  aperture,  with  fimbriated  borders.  Amphibia,  like 
the  elasmobranchs,  have  a  cloaca. 

J.      EXTERNAL  ANATOMY  OF  A  LIZARD 

The  lizards  are  typical  reptiles,  class  Reptilia,  order  Squamata. 

1.  Body  and   skin. — The  body  is  characteristically  vertebrate  in  form, 
thickest  in  the  middle,  tapering  to  each  end.     It  consists  of  head,  neck,  trunk, 
and  a  long  tail.     It  should  be  recalled  that  a  neck  is  absent  in  fishes  and 
Amphibia;  its  appearance  is  correlated  with  the  assumption  of  the  land  habitat. 
The  body  is  completely  clothed  in  horny  scales,  which  are  thickenings  of  the  outer 
layer  of  the  skin  (epidermis)  and  are  not  homologous  with  the  scales  of  fishes. 
The  scales  are  larger  on  the  head  where  they  are  known  as  head  shields. 

2.  Head. — The  head  bears  the  usual  three  pairs  of  sense  organs.     The 
external  nares  or  nostrils  are  located  at  the  tip  of  the  head  and  lead,  as  in  the 
Amphibia,  into  the  mouth  cavity  where  they  open  by  the  internal  nares.     Pos- 
terior to  the  nares  are  the  eyes,  each  provided  with  an  upper  and  lower  movable 
eyelid  and  with  a  third  eyelid,  the  nictitating  membrane,  a  thin  transparent 
membrane  concealed  from  view  when  not  in  use  in  the  anterior  corner  of  the 
eye  where  it  should  be  sought  with  a  forceps.     Halfway  between  the  eye  and 
the  base  of  the  fore  limb  is  a  slight  depression  in  the  skin  across  the  bottom  of 
which  stretches  a  thin  membrane.     This  membrane  is  the  tympanic  membrane 
or  drum  membrane,  and  it  covers  a  cavity,  the  tympanic  cavity  or  middle  ear. 
The  depression  may  be  regarded  as  the  beginning  of  an  external  ear.     The  lizard 
therefore  possesses  a  middle  and  an  external  ear  in  addition  to  the  internal  ear 
present  in  fishes  and  Necturus. 


24       LABORATORY  MANUAL  FOR  VERTEBRATE  ANATOMY 

3.  Appendages. — The  trunk  bears  two  pairs  of  limbs,  which  have  the  same 
parts  as  those  of  Necturus.     Five  digits,  the  typical  vertebrate  number,  are 
present  on  each  limb;    they  terminate  in  horny  claws.     The  student  should 
note  that  the  limbs  depart  still  further  from  the  primitive  position  described  in 
connection  with  Necturus,  thus  lifting  the  animal  above  the  ground  to  a  greater 
extent.     In  the  hind  limb  the  thigh  still  extends  out  at  right  angles  to  the  body, 
but  is  slightly  twisted  on  its  longitudinal  axis  so  that  the  original  dorsal  surface 
is  becoming  anterior.     The  shank  is  directed  ventrally  and  posteriorly,  and  has 
undergone  the  same  sort  of  torsion  as  the  thigh.     The  foot  retains  the  primitive 
position.     In  the  fore  limb  the  upper  arm  is  rotated  in  the  opposite  direction 
from  that  observed  in  the  thigh,  so  that  the  original  preaxial  surface  is  now 
dorsal;   the  upper  arm  is  directed  posteriorly.     The  forearm  and  hand  are 
directed  downward  and  forward,  and  are  so  rotated  that  their  original  preaxial 
borders  look  inward  instead  of  forward. 

4.  Anus. — On  the  ventral  side  of  the  trunk  behind  the  bases  of  the  hind 
limbs  is  the  anus  or  cloacal  aperture.     Its  shape,  a  transverse  slit,  is  character- 
istic of  lizards  and  snakes. 

K.   EXTERNAL  ANATOMY  OF  THE  TURTLE 

The  turtle  is  a  reptile  belonging  to  the  order  Chelonia.    Obtain  a  specimen 
and  examine. 

1.  Body  and  skin. — The  form  of  the  body  is  considerably  modified  from 
the  typical  vertebrate  shape.     It  is  divided  into  head,  neck,  trunk,  and  tail. 
The  head  is  similar  in  form  to  that  of  other  reptiles,  but  the  neck  is  unusually 
long  and  flexible;  the  trunk  is  remarkably  flat  and  broad,  and  the  tail  diminished 
in  diameter  and  length.     The  turtle  is  thus  one  of  those  forms  in  which,  as  in 
the  skate,  the  transverse  axis  is  elongated  while  the  anteroposterior  and  dorso- 
ventral  axes  are  shortened.     The  skin  of  the  legs,  tail,  and  other  exposed  parts 
of  the  body  is  provided  with  small  horny  scales  or  horny  thickenings  as  is  charac- 
teristic of  reptiles.    The  trunk  is  inclosed  in  a  hard  shell,  the  outer  surface  of 
which  consists  of  greatly  enlarged  horny  scales,  definitely  arranged.     The  skin 
of  the  head  in  most  of  our  common  turtles  is  bare,  but  in  some  turtles  is  marked 
off  into  large  head  shields. 

2.  Head. — The  anterior  end  of  the  head  is  pointed  and  elevated  and  bears 
at  its  tip  the  two  external  nares,  close  together;    this  position  of  the  nostrils 
enables  the  animal  to  breathe  air  with  only  a  slight  exposure  of  the  head 
above  water.     The  jaws  are  clothed  with  hard,  horny  beaks,  teeth  being  absent. 
The  large  eyes  are  provided  as  in  the  lizard  with  upper  and  lower  eyelids  and  a 
nictitating  membrane,  located  in  the  anterior  corner  of  the  eye.    Just  posterior 
to  the  angles  of  the  mouth  can  be  observed  a  circular  area,  covered  by  skin;  this 
is  the  tympanic  membrane  beneath  which  is  located  the  cavity  of  the  middle 
ear.     This  membrane  is  not,  however,  sunk  below  the  surface,  as  in  the  lizard. 


GENERAL  STUDY  OF  TYPICAL  CHORDATES  25 

3.  Trunk. — The  trunk  is  incased  in  a  strong,  thick  shell  consisting  of  the 
following  parts:  the  dorsal  arched  portion,  the  carapace;  the  ventral,  flat  portion, 
the  plastron;  and  the  lateral  bridges  connecting  the  carapace  and  plastron.     The 
shell  is  composed  of  bony  and  horny  plates  which  will  be  studied  in  detail  later. 
The  trunk  bears  two  pairs  of  limbs,  stout  and  strong,  and  composed  of  the  same 
parts  as  in  the  animals  already  studied.     There  are  five  digits  to  each  limb, 
each  bearing  a  claw,  except  the  fifth  digits  of  the  hind  limbs.     The  digits  are 
more  or  less  united  by  webs.     The  parts  of  the  limbs  have  undergone  a  torsion 
and  alteration  from  the  primitive  position  similar  to  that  observed  in  the  case 
of  the  lizard.     The  student  may  determine  for  himself  the  details  of  these  changes. 

4.  Anus. — At  the  base  of  the  ventral  side  of  the  tail  is  the  rounded  anus  or 
cloacal  aperture. 

L.      EXTERNAL  ANATOMY  OF  THE   PIGEON 

The  pigeon  is  a  typical  representative  of  the  class  Aves,  or  birds.  Examine 
specimens  with  and  without  feathers. 

1.  Body  form  and  skin. — The  proportions  of  the  body,  as  seen  in  the  plucked 
specimen,  bear  a  general  resemblance  to  those  of  the  turtle.     The  head  is  well 
developed,  the  neck  long  and  flexible;    the  trunk  is  shorter  and  considerably 
plumper  than  normal,  and  the  tail  reduced  to  a  stump,  the  uropygium.    The 
ancestors  of  birds  had  long  tails  like  lizards,  bearing  feathers  along  their  entire 
length. 

The  body  is  clothed  with  a  covering  of  feathers,  which  conceal  its  shape. 
These  feathers  are  called  contour  feathers.  The  contour  feathers  are  of  two 
kinds,  the  large,  stout  feathers  of  the  wings  and  tail,  called  quills,  and  the  smaller 
feathers,  the  coverts,  which  cover  the  bases  of  the  wings  and  tail,  and  the  general 
surface  of  the  body.  On  the  plucked  bird  from  which  the  contour  feathers  have 
been  removed  note  the  presence  of  hairlike  processes,  the  hair  feathers  or 
filoplumes.  In  the  plucked  bird,  also,  note  the  delicacy  of  the  skin  and  the 
presence  in  it  of  numerous  deep  pits,  the  feather  follicles,  into  which  the  contour 
feathers  were  set. 

2.  Head. — The  head  terminates  in  the  elongated  beak,  which  consists  of 
the  upper  and  lower  jaws  incased  in  horny  sheaths.     Teeth  are  absent  on  all 
modern  birds,  although  extinct  birds  possessed  them.    The  base  of  the  upper  beak 
bears  a  cushion-like  protuberance,  the  cere,  a  structure  occurring  only  in  certain 
families  of  birds.     Under  the  anterior  margins  of  the  ceres  are  the  slitlike  external 
nares.     The  remarkably  large  eyes  are  provided  with  upper  and  lower  lids  and 
with  a  nictitating  membrane  which  may  be  drawn  across  the  eye  from  the  anterior 
corner.     The  ear  is  behind  and  below  the  eye  and  is  observable  only  on  the 
plucked  specimen.     There  is  visible  only  a  deep  and  narrow  passage,  the  external 
auditory  meatus.     The  middle  ear,  which  is  flush  with  the  skin  in  the  frog  and 
most  reptiles  or  only  slightly  depressed,  in  birds  has  sunk  below  the  surface  to 


26       LABORATORY  MANUAL  FOR  VERTEBRATE  ANATOMY 

such  an  extent  that  the  tympanic  membrane  is  no  longer  visible.  The  skin 
around  the  entrance  to  the  meatus  tends  to  elevate  as  a  fold;  this  fold  together 
with  the  meatus  constitutes  the  external  ear. 

3.  Trunk. — The  trunk  is  very  firm  and  inflexible,  owing  to  a  fusion  of  the 
bones  of  the  back.     Pass  your  fingers  along  the  back  of  the  plucked  bird  and 
feel  the  skeleton.     Feel  also  in  the  median  ventral  line  of  the  trunk  the  pro- 
jecting keel  of  the  breastbone,  to  which  are  attached  the  great  wing  muscles. 
It  is  the  presence  of  these  muscles,  the  " breast"  of  the  bird,  which  produces  the 
plump  contour  of  the  trunk.     The  trunk  bears  the  two  pairs  of  limbs,  of  which 
the  anterior  pair  is  remarkably  modified  into  wings  or  organs  of  flight.     The 
hind  limbs  have  also  undergone  considerable  modification  as  a  result  of  the 
biped  mode  of  walking  used  by  birds. 

The  parts  of  the  wing,  which  are  homologous  to  those  of  the  fore  limb  of 
other  vertebrates,  should  be  studied  on  the  plucked  bird.  In  the  folded  condi- 
tion the  sections  of  the  wing  make  angles  with  each  other  like  the  letter  Z. 
The  upper  arm  is  short,  directed  posteriorly,  and  slightly  twisted  on  its  axis  so 
as  to  bring  the  preaxial  margin  on  the  dorsal  side.  The  forearm  is  longer  and 
directed  forward.  The  wrist  and  hand  are  fused  together,  and  the  whole  is  con- 
siderably elongated  and  directed  caudad.  There  are  but  three  digits,  which 
are  regarded  as  the  second,  third,  and  fourth.  The  second  digit  is  the  projection 
found  just  below  the  joint  between  forearm  and  wrist;  the  third  digit  forms  the 
terminal  point  of  the  wing;  the  fourth  cannot  be  seen  externally.  When  the 
wing  is  extended,  its  parts  have  nearly  the  primitive  position  described  under 
Necturus.  The  great  quills  of  the  wings  are  known  as  remiges:  those  of  the 
hand  are  called  primaries;  of  the  forearm,  secondaries;  and  of  the  upper  arm, 
humerals.  The  primaries  differ  from  the  others  in  that  the  soft  part  of  the  feather 
is  wider  on  the  posterior  side  of  the  central  axis  than  on  the  anterior  side.  The 
remiges  are  borne  on  the  postaxial  margin  of  the  wing,  and  the  deep  large  feather 
follicles  exposed  by  their  removal  should  be  noted  on  the  plucked  bird. 

The  hind  limb  is  partially  clothed  with  feathers  and  partially  with  horny 
scales,  identical  with  those  found  in  reptiles.  The  digits,  of  which  there  are  but 
four — the  fifth  being  absent — terminate  in  claws.  The  position  of  the  hind 
limb  with  reference  to  the  body  is  greatly  altered.  The  whole  limb,  instead  of 
extending  straight  out  laterally  from  the  body,  is  directed  ventrally,  thus  raising 
the  animal  high  above  the  ground.  In  order  to  achieve  this  result  it  is  obvious 
that  the  limb  must  have  been  rotated  90°  from  the  primitive  position  so  that  the 
original  dorsal  surface  now  faces  anteriorly — that  is,  has  become  preaxial.  The 
toes  are  consequently  directed  forward,  with  the  exception  of  the  first  which, 
through  a  secondary  modification  connected  with  the  perching  habit,  points 
posteriorly. 

4.  Tail. — The  tail  stump  bears  a  half-circle  of  large  quills,  known  as  rectrices. 
Under  the  base  of  the  tail  is  the  anus  or  cloacal  aperture,  a  transverse  opening 


GENERAL  STUDY  OF  TYPICAL  CHORD ATES  27 

with  protruding  lips.  On  the  dorsal  side  just  in  front  of  the  base  of  the  tail 
will  be  seen  by  lifting  up  the  tail  coverts  or,  on  the  plucked  bird,  a  papilla,  the 
opening  of  the  uropygial  gland,  from  which  the  bird  obtains  oil  for  preening 
its  feathers. 

M.      EXTERNAL   ANATOMY   OF   A   MAMMAL 

For  this  purpose  either  the  cat  or  the  rabbit  may  be  used.  Both  are 
mammals  belonging  to  the  class  Mammalia.  The  rabbit  is  a  rodent,  the  cat  a 
carnivore. 

1.  Body  form  and  skin. — The  body  is  divisible  into  head,  neck,  trunk,  and 
tail.     Its  proportions  depart  somewhat  from  the  typical  vertebrate  form  in  the 
larger  size  of  the  head  and  the  reduction  of  the  tail,  the  latter  feature  particularly 
noticeable  in  the  rabbit.     The  body  is  clothed  with  closely  set  hairs,  forming  a 
covering  of  fur,  characteristic  of  mammals.     Upon  the  head  are  a  number  of 
especially  long  and  stout  hairs,  the  whiskers  or  vibrissae,  which  serve  as  important 
organs  of  touch. 

2.  Head. — The  large  size  of  the  head  is  due  to  the  great  development  of 
the  brain  within  it.     The  head  may  consequently  be  divided  into  an  anterior 
facial  region,  in  front  of  the  eyes,  and  an  enlarged  posterior  cranial  region. 
The  mouth  is  provided  with  well-developed  lips.    The  upper  lip  is  cleft  in  the 
center,  deeply  so  in  the  rabbit,  exposing  the  incisor  teeth.     The  external  nares 
are  large  and  elongated,  overhung  by  the  mobile  nose.     The  eyes  have  upper 
and  lower  lids  and  a  nictitating  membrane.     The  latter  should  be  sought  with 
a  forceps  in  the  anterior  corner  of  the  eye  and  drawn  over  the  eyeball.     The 
ears  are  provided  with  a  long  and  flexible  external  fold,  the  pinna,  which  springs 
from  the  rim  of  an  opening,  the  external  auditory  meatus,  which  descends  into 
the  interior  of  the  skull.     Pinna  and  meatus  constitute  the  external  ear. 

3.  Trunk. — The  trunk  is  divisible  into  an  anterior  chest  or  thorax,  supported 
by  the  ribs,  and  a  posterior  abdomen.     On  the  ventral  surface  of  the  trunk  are 
four  or  five  pairs  of  teats  or  nipples  in  female  specimens;   they  are  the  openings 
of  the  milk  glands  or  mammary  glands.    The  trunk  bears  two  pairs  of  limbs, 
which  consist  of  the  same  parts  as  other  vertebrate  limbs.     The  upper  section 
of  each  limb — that  is,  thigh  or  upper  arm — is  more  or  less  included  in  the  trunk. 
The  limbs  terminate  in  clawed  digits,  five  in  front,  four  behind,  the  first  hind 
toe  being  absent.     The  claws  of  the  cat  and  its  relatives  are  retractile. 

The  limbs  have  undergone  a  marked  change  from  the  primitive  position. 
Instead  of  extending  laterally  from  the  body  they  project  ventrally  and  are 
elongated,  so  that  the  body  of  the  animal  is  carried  high  above  the  ground.  This 
change  has  involved  a  rotation  of  90°  in  each  limb.  The  hind  limb  is  rotated 
forward  so  that  the  original  dorsal  surface  is  anterior  and  the  toes  point  forward. 
The  knee  or  joint  between  the  thigh  and  shank  is  likewise  directed  anteriorly. 
The  fore  limb,  on  the  contrary,  has  rotated  posteriorly,  so  that  the  original 


28       LABORATORY  MANUAL  FOR  VERTEBRATE  ANATOMY 

dorsai  surface  faces  posteriorly  and  the  preaxial  surface  faces  laterally.  Conse- 
quently, the  elbow  or  joint  between  upper  arm  and  forearm  is  directed  caudad. 
As  a  result  of  the  rotation  of  the  upper  arm  the  toes  would  point  posteriorly,  but 
by  an  additional  torsion  they  have  been  brought  around  to  the  front  again. 
This  torsion  involves  a  crossing  of  the  two  bones  of  the  forearm,  the  distal  end 
of  the  preaxial  bone  being  brought  internal  to  the  distal  end  of  the  postaxial 
bone  (see  Fig.  2  and  read  R,  p.  26).  This  position  of  the  forearm  is  known  as 


postaxial      preaxial 


postaxial 


preaxial 


B 


FIG.  2. — Diagrams  to  illustrate  the  torsion  of  the  limb;  preaxial  surface  white,  postaxial  surface 
black.  Ay  primitive  position  of  the  limbs,  seen  from  above,  the  limbs  extending  at  right  angles  to  the 
body,  the  preaxial  surface  facing  anteriorly,  the  postaxial  posteriorly.  B,  position  of  the  limbs  in 
mammals,  seen  from  the  side;  the  limbs  extend  vertically  below  the  body;  the  upper  arm  has  rotated 
outward  and  backward  so  that  the  preaxial  surface  now  faces  laterally;  the  forearm  has  rotated  forward 
again,  resulting  in  a  crossing  of  its  bones;  the  hind  limb  has  rotated  outward  and  forward  so  that  the 
postaxial  surface  faces  laterally.  (From  Flower's  Osteology  of  the  Mammalia.) 


the  prone  position,  and  is  imitated  in  the  human  arm  when  the  arm  hangs  by 
the  side  with  the  back  of  the  hand  directed  forward.  In  this  position  the  crossing 
of  the  two  long  bones  of  the  forearm  can  be  felt.  If  now  the  arm  is  raised  side- 
wise,  shoulder  high  with  the  palm  facing  forward,  the  two  bones  return  to  the 
primitive  parallel  position,  known  as  the  supine  position.  Thus  man  can  change 
his  forearm  from  the-prone  to  the  supine  position,  but  in  most  mammals  the  fore- 
arm is  fixed  in  the  prone  position. 

The  position  of  the  parts  of  the  foot  in  walking  is  different  in  different 
mammals.  The  rabbit  and  the  cat  walk  on  the  digits,  with  the  remainder  of 
the  hand  and  foot  elevated.  This  type  of  gait  is  known  as  digUigrade.  Man 
walks  on  the  whole  sole  of  the  foot,  the  primitive  method,  known  as  plantigrade. 
Horses  and  cattle  and  other  ungulates  walk  on  their  nails,  which  are  broadened 
into  hoofs,  a  mode  of  walking  called  unguligrade. 


GENERAL  STUDY  OF  TYPICAL  CHORD Arj  J2S  29 

4.  Perineal  region.— In  mammals  the  region  which  includes  both  the  anal 
and  urogenital  openings  is  designated  the  perineum.  The  anus  is  situated  in  the 
ventral  median  line  just  in  front  of  the  base  of  the  tail.  On  each  side  of  the  anus 
in  the  rabbit  is  a  deep,  hairless  depression,  the  inguinal  or  perineal  space,  onto 
which  open  the  inguinal  glands  (not  visible  externally)  whose  secretion  produces 
the  odor  characteristic  of  the  animal.  In  the  female,  the  urogenital  opening  is 
situated  immediately  anterior  and  ventral  to  the  anus;  it  is  similar  in  appearance 
to  the  anus.  It  is  inclosed  by  a  fold  of  skin  extending  around  the  rim;  this  fold 
is  named  the  greater  lips  or  labia  majora.  The  labia  together  with  the  urogenital 
aperture  constitute  the  vulva.  In  the  male  rabbit  there  is  a  hillock  in  front  of 
the  anus.  In  the  center  of  this  is  usually  visible  a  pointed  projection,  the  end 
of  the  penis  or  organ  of  copulation;  the  penis  bears  at  its  tip  an  opening,  the 
urogenital  aperture.  The  hillock  of  skin  which  folds  up  about  the  penis  is  named 
the  prepuce  or  foreskin.  To  each  side  of  the  prepuce  and  extending  forward  is 
an  oval  swelling,  caused  by  the  male  gonad,  or  testis,  which  is  inclosed  within  it. 
The  double  pouch  of  the  body  wall  which  incloses  the  two  testes  is  named  the 
scrotum  or  scrotal  sac.  In  the  male  cat  there  is  a  pair  of  rounded  eminences 
anterior  to  the  anus;  each  eminence  contains  a  testis  or  male  gonad,  and  the 
double  pouch  of  the  body  wall  inclosing  the  testes  is  named  the  scrotum  or  scrotal 
sac.  Anterior  to  the  scrotum  is  a  hillock  of  skin,  the  prepuce  or  foreskin.  In 
the  center  of  the  prepuce  is  an  opening,  which  is  not,  however,  as  might  be  sup- 
posed, the  urogenital  opening.  It  is  merely  the  depression  left  by  the  withdrawal 
into  the  prepuce  of  the  penis;  the  penis  is  generally  so  far  withdrawn  in  male 
cats  as  to  be  invisible  externally. 


N.      SUMMARY 

Our  study  of  the  external  features  of  representative  vertebrates  may  be  utilized  to  direct 
the  student's  attention  to  the  following  points. 

1.  The  vertebrate  body  is  typically  fusiform,  that  is,  moderately  thick  through  the  trunk, 
tapering  toward  each  end.    In  every  group  of  vertebrates  forms  may  be  found  which  deviate 
from  this  typical  shape,  but  such  deviations  bear  no  relation  to  the  position  of  the  animal  in 
the  vertebrate  scale,  being  rather  adaptations  to  particular  modes  of  life. 

2.  The  body  is  divided  into  head,  trunk,  and  tail  in  the  lowest  vertebrates.    A  neck  is 
added  in  land  vertebrates. 

3.  The  head  tends  to  increase  relatively  in  size  and  the  tail  to  decrease  as  one  ascends  the 
vertebrate  series.    The  former  change  is  associated  with  increase  in  the  size  of  the  brain;  the 
latter  with  greater  speed  and  agility  of  movement. 

4.  The  skin  of  vertebrates  is  commonly  clothed  with  protective  structures,  such  as  scales, 
feathers,  or  hairs.    These  are  more  complex  in  structure  in  the  higher  vertebrates  and  better 
fitted  for  keeping  the  body  warm. 

5.  The  head  throughout  bears  three  pairs  of  sense  organs.     Of  these  the  eyes  undergo 
little  change  throughout  the  series.    The  olfactory  sacs  are  blind  in  fishes  and  open  to 
the  exterior  only,  by  means  of  the  external  nares.    As  soon,  however,  as  vertebrates  left  the 
aquatic  environment,  internal  nares  were  developed,  connecting  the  olfactory  sacs  with  the 


30  LABOR^'tyORY  MANUAL  FOR  VERTEBRATE  ANATOMY 

mouth  cavity.  This  arrangement  permits  the  animal  to  breathe  without  opening  the  mouth, 
a  decided  advantage  in  air-breathing  animals,  since  thereby  the  drying  of  the  mouth  cavity  is 
avoided.  In  the  lower  vertebrates  an  internal  ear  only  is  present.  To  this  is  added,  begin- 
ning with  the  anuran  Amphibia,  a  middle  ear,  closed  externally  by  the  tympanic  membrane. 
In  Anura  and  many  reptiles  the  tympanic  membrane  is  level  with  the  surface  of  the  head,  but 
in  some  reptiles  it  begins  to  sink  below  the  surface.  In  birds  and  mammals  it  has  descended 
deeply  into  the  head,  forming  a  narrow  passage,  the  external  auditory  meatus.  Around  the 
external  rim  of  the  meatus  in  mammals  the  skin  elevates  to  form  a  sound-catching  device,  the 
pinna.  Pinna  and  meatus  constitute  the  external  ear. 

6.  The  gill  slits  and  gills  present  in  the  fishes  and  lower  Amphibia  disappear  in  the  adults 
of  the  higher  Amphibia  and  all  forms  above  them.    This  is  due  to  the  assumption  of  the  air- 
breathing  habit. 

7.  The  trunk  bears  two  pairs  of  appendages.     These  are  fins  in  fishes,  but  become  limbs 
in  all  vertebrates  above  fishes.     Stages  in  this  transformation  are  very  imperfectly  known. 
The  parts  of  the  limbs  are  the  same  through  all  of  the  vertebrates,  although  they  are  subject 
to  considerable  modification.    The  most  primitive  limbs,  in  structure,  form,  and  position  with 
reference  to  the  body  occur  in  the  urodele  Amphibia.    In  higher  vertebrates  the  position  of  the 
limbs  is  altered  by  bending  and  torsion,  resulting  in  an  elevation  of  the  body  above  the  ground 
with  a  correspondingly  more  rapid  progression.    As  a  still  further  aid  to  rapid  movement  the 
digitigrade  or  unguligrade  mode  of  walking  has  been  adopted  in  many  cases.    Loss  of  digits 
is  quite  common  among  vertebrates;  the  missing  digits  are  nearly  always  the  first  or  last  ones, 
rarely  the  middle  ones. 

8.  In  nearly  all  vertebrates  except  mammals  the  intestine  and  the  urogenital  ducts  open 
into  a  common  chamber,  the  cloaca,  which  communicates  with  the  exterior  by  a  single  opening, 
the  anus  or  cloacal  aperture.    In  all  placental  mammals  the  intestine  and  the  urogenital  system 
open  by  separate  apertures,  the  urogenital  opening  being  situated  always  anterior  to  the  anus. 
The  term  anus,  therefore,  does  not  have  the  same  significance  in  mammals  as  in  other 
vertebrates. 


IV.     GENERAL  FEATURES  OF  CHORDATE  DEVELOPMENT 

Since  it  is  impossible  to  understand  the  comparative  anatomy  of  vertebrates, 
which  forms  the  main  object  of  study  of  this  course,  without  some  knowledge 
of  the  way  in  which  a  vertebrate  develops,  it  is  necessary  that  the  student  learn 
some  of  the  elementary  facts  about  vertebrate  development.  Every  student 
must  read  either  K,  pages  7-16,  or  W,  pages  60-71.  He  will  be  expected  to 
know  the  contents  of  these  pages  thoroughly  and  to  be  able  to  answer  at  all 
times  questions  relating  to  them. 


A.      THE   CHORDATE  EGG 


The  manner  of  development  of  the  chordate  egg  is  dependent  upon  the 
amount  of  food  material,  or  yolk,  which  it  contains.  On  this  basis  chordate 
eggs  are  classified  as  follows:  (i)  Isolecithal  eggs,  with  little  yolk;  Amphioxus 
and  mammals.  (2)  Telolecithal  eggs  with  total  cleavage.  There  is  a  moderate 


animal  pole  \  germinal  disk , 

,  vegetal  pole 

,yolk 


FIG.  3. — Three  types  of  chordate  eggs.  The  black  coloring  represents  the  yolk.  A,  isolecithal 
egg  with  small  and  evenly  distributed  yolk  particles.  B,  telolecithal  egg  with  total  unequal  cleavage, 
with  the  yolk  more  abundant  in  the  vegetal  than  in  the  animal  half.  C,  telolecithal  egg  with  mero- 
blastic  cleavage,  consisting  completely  of  yolk  except  for  the  small  germinal  disk  of  protoplasm  at  one 
pole.  The  size  of  the  germinal  disk  in  the  figure  is  greatly  exaggerated  with  respect  to  the  size  of  the 
yolk. 

amount  of  yolk  which  accumulates  in  one-half  of  the  egg  and  retards  its  develop- 
ment; Amphibia,  cyclostomes,  and  ganoid  fishes.  (3)  Telolecithal  eggs  with 
meroblastic  cleavage.  There  is  an  enormous  amount  of  yolk,  and  the  protoplasm 
is  reduced  to  a  small  disk  which  floats  on  the  surface  of  the  yolk.  This  spot  is 
called  the  germinal  disk.  Such  eggs  are  characteristic  of  birds,  reptiles,  and 
teleosts.  The  true  egg  in  the  case  of  birds  is  the  yolk,  the  white  being  merely 
a  nutritive  envelope. 

These  three  types  of  eggs  are  illustrated  diagrammatically  in  Figure  3. 
(See  further,  H,  pp.  141-48.) 

31 


32  LABORATORY  MANUAL  FOR  VERTEBRATE  ANATOMY 

B.      THE  CLEAVAGE   OF  THE   EGG  AND   THE   FORMATION   OF  THE   BLASTULA 

Development  begins  by  the  division  of  the  egg  into  two,  four,  etc.,  cells  until 
a  large  number  of  cells  has  been  produced.  This  process  of  division  of  the  egg 
is  called  cleavage,  and  the  way  in  which  it  occurs  depends  on  the  amount  of  yolk 
which  the  egg  contains.  It  should  be  understood  that  the  yolk  is  inert  material 
and  that  the  process  of  development  is  carried  out  only  by  the  living  proto- 
plasmic portions  of  the  egg. 

1.  Holobastic  equal  cleavage. — In  the  case  of  isolecithal  eggs  the  entire 
egg  divides  and  produces  a  number  of  approximately  equal  cells.     Such  cleavage 
is  said  to  be  holoblastic  and  equal.     The  cells,  as  they  increase  in  number,  gradually 
withdraw  from  the  center  and  arrange  themselves  in  a  single  layer  on  the  surface, 
thus  producing  a  hollow  ball  of  cells.     This  ball  is  called  the  blastula;  its  cavity 
is  known  as  the  segmentation  cavity  or  blastocoel.     Such  a  blastula  is  produced 
in  the  development  of  Amphioxus.     Cleavage  and  formation  of  the  blastula 
in  Amphioxus  are  illustrated  in  Figure  $A.    Similar  figures  will  also  be  found  in 
the  standard  textbooks,  as  P  and  H,  K,  W,  and  H.     Study  also  the  models  of 
the  cleavage  of  Amphioxus  provided  in  the  laboratory. 

2.  Holobastic  unequal  cleavage. — This  type  occurs  in  those  telolecithal 
eggs  which  contain  a  moderate  amount  of  yolk.    The  half  of  the  egg  which  con- 
tains  the  majority  of   the  yolk  is   called  the  vegetal  hemisphere;   that  which 
contains  the  majority  of  the  protoplasm  is  the  animal  hemisphere.     The  early 
cleavage  planes  are  shifted  toward  the  animal  hemisphere,  and  further,  the 
cleavage  processes  are  delayed  in  the  vegetal  hemisphere  owing  to  the  presence 
of  the  inert  yolk.     In  consequence  of  these  two  factors,  the  cells  produced  in  the 
animal  hemisphere  are  smaller  and  more  numerous  than  those  of  the  vegetal 
hemisphere,  although  the  entire  egg  cleaves.     Such  cleavage  is  holoblastic  but 
unequal.     The  cells  withdraw  from  the  center,  producing  a  blastula  with  a 
somewhat  reduced  blastocoel  and  a  wall  several  layers  of  cells  thick.     The  cells 
of  the  blastula  are  of  unequal  sizes  grading  from  the  smallest  at  the  animal  pole 
to  the  largest  at  the  vegetal  pole. 

This  type  of  development  is  characteristic  of  Amphibia.  It  is  illustrated  in 
Figure  ^B;  similar  figures  are  presented  in  various  textbooks,  as  P  and  H,  K, 
and  N.  Study  also  the  models  of  the  cleavage  of  the  amphibian  egg,  provided 
in  the  laboratory.  Then  obtain  a  section  through  an  amphibian  egg  in  the 
blastula  stage  and  examine  under  the  low  power  of  the  microscope.  The  blastula 
is  a  hollow  sphere  whose  wall  is  composed  of  two  or  three  layers  of  cells.  The 
wall  of  the  animal  hemisphere  is  thin  and  consists  of  small  cells;  it  is  the  future 
dorsal  side  of  the  embryo.  The  wall  of  the  vegetal  hemisphere  is  much  thicker 
than  that  of  the  animal  hemisphere  and  is  composed  of  large  cells,  laden  with  yolk 
and  with  indistinct  boundaries;  it  is  the  future  ventral  side.  The  blastocoel  is 
smaller  than  in  the  blastula  of  Amphioxus,  and  is  displaced  dorsally,  owing  to 
the  thickness  of  the  ventral  wall.  Draw,  showing  outlines  only  of  the  cells. 


blastoderm 


blastocoel 


FIG.  4. — Cleavage  of  the  three  types  of  chordate  eggs  and  formation  of  the  blastula.  A, 
Amphioxus;  1-4,  cleavage;  5,  external  view  of  the  blastula;  6,  section  of  the  blastula.  Note  that 
the  cells  of  the  vegetal  hemisphere  are  but  slightly  larger  than  those  of  the  animal  hemisphere,  the  wall 
of  the  blastula  is  one  cell-layer  in  thickness,  and  the  blastocoel  is  large.  B,  amphibian;  1-4,  cleavage; 
5,  blastula;  6,  section  of  the  blastula.  The  cells  of  the  vegetal  hemisphere  are  considerably  larger  than 
those  of  the  animal  hemisphere,  the  wall  of  the  blastula  is  at  least  two  cell  layers  in  thickness,  and  the 
blastocoel  is  smaller  and  displaced  dorsally.  C,  reptile  or  bird  egg  with  meroblastic  cleavage;  i-j, 
cleavage;  4,  median  sagittal  section  of  the  blastula.  Only  the  germinal  disk  cleaves,  forming  a  disk 
of  cells — the  blastoderm — resting  on  the  yolk;  a  slight  slit  between  this  and  the  yolk  represents  the 
blastocoel.  (A  and  B,  1-5,  from  Parker  and  HaswelTs  Textbook  of  Zoology,  after  Hatschek,  courtesy 
of  the  Macmillan  Company;  B6  from  Prentiss  and  Arey's  Textbook  of  Embryology,  courtesy  of  the 
W.  B.  Saunders  Comoany.) 


34       LABORATORY  MANUAL  FOR  VERTEBRATE  ANATOMY 

3.  Meroblastic  cleavage. — In  eggs  containing  large  quantities  of  yolk  only 
the  small  germinal  disk  undergoes  cleavage.  This  kind  of  cleavage  is  called 
meroblastic.  As  a  result,  a  minute  disk  of  cells  is  produced  on  the  surface  of  the 
relatively  enormous  yolk.  A  slight  split  appears  between  the  disk  and  the  yolk, 
and  this  corresponds  to  the  segmentation  cavity  of  other  developing  eggs;  this 
stage  is  consequently  the  blastula  stage.  Meroblastic  cleavage  is  illustrated  in 
Figure  46".  The  disk  of  cells  produced  by  meroblastic  cleavage  is  called  the 
blastoderm;  in  the  further  development  it  expands  over  the  surface  of  the  yolk 
which  it  eventually  incloses. 

C.      FORMATION  OF   THE   GASTRULA 

i.  In  eggs  of  the  Amphioxus  type. — In  such  eggs  the  vegetal  hemisphere 
begins  to  bend  inward  and  continues  this  process  of  imagination  until  its  wall 
comes  in  contact  with  the  wall  of  the  animal  hemisphere.  An  embryo  with  a 
wall  two  cell-layers  thick  is  thus  produced.  It  is  called  a  gastrula.  The  outer 
layer  is  named  the  ectoderm  and  the  inner  layer  the  entoderm.  Because  of  their 
importance  in  the  subsequent  development,  these  layers  are  referred  to  as  the 
first  two  germ  layers.  The  hollow  tube  of  entoderm  is  called  the  archenteron  or 
primitive  intestine;  the  cavity  of  the  gastrula  is  the  cavity  of  the  archenteron  or 
gastrocoel;  and  the  opening  of  the  archenteron  to  the  exterior  is  the  blastopore. 
Note  that  the  blastocoel  is  eliminated  in  the  production  of  the  gastrula.  The 
formation  of  the  gastrula  of  Amphioxus  is  illustrated  in  Figure  5^,  and  also  in 
the  various  textbooks,  as  P  and  H,  W,  K,  and  N.  Study  further  the  models 
exhibited  in  the  laboratory. 

2.  In  eggs  of  the  amphibian  type. — In  these  eggs  gastrulation  is  somewhat 
modified  by  the  presence  of  inert  yolk  in  the  vegetal  hemisphere.  It  is  accom- 
plished partly  by  the  invagination  of  the  entoderm  particularly  at  the  dorsal  lip 
of  the  blastopore,  and  partly  by  the  expansion  of  the  ectoderm  ventrally  pushing 
the  entoderm  into  the  interior.  The  result  is  the  same  as  the  foregoing,  a  gastrula 
being  formed.  A  small  portion  of  the  inclosed  yolk-bearing  cells  commonly  re- 
mains for  some  time  protruding  through  the  blastopore,  and  is  called  the  yolk  plug. 

The  formation  of  the  amphibian  gastrula  is  illustrated  in  Figure  $B,  and  in 
P  and  H,  N,  and  other  textbooks.  Study  further  the  models  of  amphibian 
development  illustrating  this  stage,  noting  especially  the  sagittal  section  of  the 
gastrula.  Then  obtain  a  slide  bearing  a  sagittal  section  of  the  gastrula  and  study 
with  the  low  power  of  the  microscope.  The  gastrula  is  slightly  elongated  in  the 
anteroposterior  direction.  The  side  with  the  thinner  wall  is  the  dorsal  side; 
that  with  the  thick  wall,  the  ventral  side;  the  end  with  an  opening  is  the  posterior 
end;  the  opposite  end  is  anterior.  The  wall  consists  of  two  layers  each  composed 
of  more  than  one  sheet  of  cells.  The  outer  and  thinner  layer  is  the  ectoderm, 
uniform  in  width  over  the  whole  embryo.  The  inner  layer  is  the  entoderm, 
separated  from  the  ectoderm  by  a  slight  space,  and  very  thick  ventrally,  where 


GENERAL  FEATURES  OF  CHORDATE  DEVELOPMENT 


35 


its  cells  are  laden  with  yolk.     The  cavity  inclosed  by  the  entoderm  is  the  gastro- 
coel.     The  opening  of  the  archenteron  to  the  exterior  at  the  posterior  end  is 


blastopore 


arcbenteron 


archenteron 


ectoderm 


entoderm 


blastocoel 


imagination 


blastocoel 
entoderm 


entoderm  — 


yolk 


FIG.  5. — Gastrulation  and  formation  of  the  entoderm  in  the  three  types  of  chordate  eggs.  All 
shown  in  median  sagittal  section.  A,  Amphioxus;  I,  beginning  of  the  in  vagina  tion;  2,  invagination 
completed;  j,  completed  gastrula  having  a  wall  of  two  layers,  ectoderm  and  entoderm,  and  an  internal 
cavity,  the  gastrocoel.  B,  amphibian;  i,  beginning  of  the  invagination;  2,  progress  of  the  invagination 
accompanied  by  downward  growth  of  the  ectoderm;  j,  completed  gastrula,  with  very  thick  entoderm 
ventrally.  C,  reptile  or  bird;  i,  posterior  margin  of  the  blastoderm  beginning  to  turn  under;  2,  con- 
tinuation of  the  invagination;  a  slight  split  between  entoderm  and  yolk  constitutes  the  gastrocoel. 
In  A 3,  Bj,  and  €2  the  anterior  end  of  the  embryo  is  to  the  left,  posterior  end  to  the  right,  dorsal  surface 
above,  ventral  below.  (A  from  Parker  and  HaswelPs  Textbook  of  Zoology,  after  Hatschek,  courtesy  of 
the  Macmillan  Company;  B  from  Kellicott's  General  Embryology,  courtesy  of  Henry  Holt  and  Com- 
pany.) 

the  blastopore.     Ectoderm  and  entoderm  are  continuous  at  the  rim  of  the  blas- 
topore.    A  portion  of  the  entoderm,  the  yolk  plug,   protrudes  through  the 


36       LABORATORY  MANUAL  FOR  VERTEBRATE  ANATOMY 

blastopore  and  nearly  occludes  the  opening.  Make  a  diagram  of  the  section, 
coloring  ectoderm  blue  and  entoderm  yellow. 

3.  In  meroblastic  eggs. — In  such  eggs  the  formation  of  the  entoderm  will 
obviously  be  attended  by  difficulties.  It  seems  to  be  accomplished  in  two  ways: 
(a)  by  a  process  of  involution  or  imagination,  in  which  the  cells  at  the  future 
posterior  end  of  the  blastoderm  turn  under  and  grow  forward;  (b)  by  a  process 
of  denomination,  in  which  the  cells  on  the  under  side  of  the  blastoderm  arrange 
themselves  to  form  a  lower  layer.  In  different  forms  the  proportion  of  entoderm, 
formed  by  each  method  varies.  Eventually  the  two  kinds  of  entoderm  unite 
to  form  a  single  sheet  of  cells.  The  cavity  of  the  archenteron  is  very  small  in 
meroblastic  eggs,  consisting  of  a  cleft  between  the  entoderm  and  the  yolk,  and 
the  blastopore  is  reduced  to  a  slit  at  the  posterior  end  of  the  blastoderm.  After 
gastrulation  has  occurred  the  blastoderm  consists  of  two  layers,  an  outer  ectoderm 
and  an  inner  entoderm;  it  lies  on  the  surface  of  the  yolk  and  by  proliferation  at 
its  margins  gradually  spreads  out  over  the  yolk,  eventually  inclosing  it. 

The  gastrulation  of  meroblastic  eggs  by  the  invagination  process  is  illustrated 
diagrammatically  in  Figure  $C. 

D.      FORMATION  OF  THE   THIRD   GERM   LAYER,   THE   NEURAL  TUBE, 
AND   THE   NOTOCHORD 

These  processes  occur  practically  simultaneously  but  will  be  described  sepa- 
rately. 

i.  In  Amphioxus. — After  the  embryo  has  attained  the  gastrula  stage  it 
elongates  and  presents  a  flattened  dorsal  surface,  a  rounded  ventral  surface,  and 
recognizable  anterior  and  posterior  ends  (see  Fig.  5^3).  From  the  dorsolateral 
regions  of  the  entoderm,  which  it  will  be  remembered  forms  the  "inner  tube" 
of  the  gastrula,  hollow  pouches  begin  to  grow  out  in  pairs.  These  pouches 
are  called  the  coelomic  sacs  or  mesodermal  pouches.  The  walls  of  the  pouches 
constitute  the  mesoderm,  or  third  germ  layer,  which,  unlike  the  ectoderm  and 
entoderm,  consists  of  two  walls.  The  pouches  grow  laterad  and  ventrad,  filling 
the  space  between  ectoderm  and  entoderm.  The  outer  wall  of  the  pouches  in 
contact  with  the  ectoderm  is  called  the  somatic  or  parietal  mesoderm ;  the  inner 
wall  in  contact  with  the  entoderm  is  the  splanchnic  mesoderm.  The  cavity  of 
the  pouches  is  the  body  cavity  or  coelom.  Eventually  the  anterior  and  posterior 
walls  of  the  pouches  break  down  so  that  those  of  each  side  unite  to  form  a  tube. 
Thus,  the  coelom,  originally  segmented,  comes  to  consist  of  a  pair  of  continuous 
cavities,  one  on  each  side  of  the  embryo. 

Meantime,  the  ectoderm  rises  up  on  either  side  of  the  median  dorsal  line 
as  a  fold  or  ridge.  The  two  folds  meet  above  the  median  dorsal  ectoderm,  which 
is  thus  inclosed  and  becomes  the  neural  tube,  the  primordium  of  the  brain  and 
spinal  cord. 


GENERAL  FEATURES  OF  CHORDATE  DEVELOPMENT 


37 


From  the  median  dorsal  wall  of  the  entoderm  a  solid  rod  of  cells  is  elevated 
and  separated  off.  This  is  the  notochord  or  primitive  skeleton. 

These  processes  are  illustrated  in  Figure  6,  also  in  P  and  H,  page  56;  N, 
page  45;  K,  page  n;  and  particularly  well  in  W,  plates  between  pages  62  and 
63.  Study  also  the  models  of  the  development  of  Amphioxus. 

2.  In  vertebrates.— In  vertebrates  (with  the  exception  of  cyclostomes  and 
possibly  urodeles)  the  mesoderm  does  not  arise  as  pouches  from  the  entoderm. 
Instead  it  grows  out  as  a  solid  sheet  of  cells  from  the  median  dorsal  region  and 


FIG.  6. — Formation  of  the  neural  tube,  notochord,  mesoderm,  and  coelom  in  Amphioxus.  A-D, 
cross-sections;  E,  frontal  section.  A,  differentiation  of  the  medullary  plate  a,  the  notochordal  plate  d, 
the  neural  folds  b,  and  the  mesodermal  pouches  e.  B,  the  neural  folds  have  closed  across  above  the 
medullary  plate;  the  mesodermal  pouches  are  farther  evaginated.  C,  the  medullary  and  notochordal 
plates  are  beginning  to  close;  the  mesodermal  pouches/  are  completely  separated  from  the  entoderm. 

D,  the  neural  tube.;'  and  the  notochord  c  are  completed;  the  mesodermal  pouches  are  increasing  in  size. 

E,  frontal  section  to  show  the  mesodermal  pouches  /  originating  from  the  entoderm  segmentally.    a, 
medullary  plate;    b,  neural  fold;    c,  notochord;   d,  notochordal  plate;    e,  mesoderm;  /,  mesodermal 
pouches;  g,  ectoderm;  h,  entoderm  or  archenteron;  i,  coelom.    In  all  figures  the  mesoderm  is  stippled. 
(From  Parker  and  HaswelTs  Textbook  of  Zoology,  after  Hatschek,  courtesy  of  the  Macmillan  Company.) 

around  the  blastopore  and  gradually  spreads  laterally  and  ventrally  between 
ectoderm  and  entoderm.  Its  origin  cannot  be  definitely  ascribed  to  either  ecto- 
derm or  entoderm,  since  at  the  blastopore  these  two  germ  layers  are  continuous. 
As  the  mesodermal  sheets  spread,  a  central  split  appears  in  them,  dividing  them 
into  somatic  and  splanchnic  walls ;  the  split  itself  is  the  body  cavity  or  coelom.  The 
coelom  of  vertebrates  thus  consists  from  the  first  of  a  single  pair  of  cavities 


38       LABORATORY  MANUAL  FOR  VERTEBRATE  ANATOMY 

extending  along  each  side  of  the  body  axis  from  anterior  to  posterior  end,  and  is 
never  composed  of  a  series  of  cavities  as  in  Amphioxus.  The  end  result  is  the 
same  as  in  Amphioxus,  although  arrived  at  in  a  different  way. 

The  neural  tube  is  formed  from  a  pair  of  folds,  the  neural  folds,  which  appear 
in  the  median  dorsal  region,  and  fuse  together  to  produce  a  tube.  The  notochord 
is  separated  off  as  a  solid  cord  of  cells  from  the  roof  of  the  archenteron  (Amphibia, 
reptiles)  or  else  arises  from  the  somewhat  indefinite  region  where  the  mesodermal 
sheets  originate  (birds). 

The  processes  will  be  grasped  more  clearly  by  reference  to  Figure  7,  also  to 
the  figures  in  K,  page  n;  P  and  H,  page  117;  W,  page  69.  i  Obtain  a  mounted 


FIG.  7. — Diagrams  to  show  the  formation  of  the  neural  tube,  notochord,  mesoderm,  and  coelom 
in  vertebrates,  based  on  Amphibia.  Cross-sections.  A,  differentiation  of  the  notochord /in  the  roof 
of  the  entoderm;  mesodermal  plates  b  spreading  ventrally.  B,  neural  folds  h  rising  at  the  sides  of  the 
medullary  plate  g;  notochord  /  separated  from  the  entoderm;  mesodermal  plates  b  extended  farther 
ventrally  and  developing  a  central  cavity,  the  coelom  i.  C,  neural  folds  h  nearly  closed  to  form  the 
neural  tube  j;  mesodermal  plates  have  reached  the  midventral  line;  coelomic  split  i  has  extended 
ventrally.  a,  ectoderm;  b,  mesoderm;  c,  entoderm  or  archenteron;  d,  somatic  mesoderm;  e,  splanch- 
nic mesoderm;  /,  notochord;  g,  medullary  plate;  h,  neural  fold;  i,  coelom;  j,  neural  tube.  (A  from 
Hertwig-Mark's  Textbook  of  the  Embryology  of  Man  and  Mammals,  courtesy  of  the  Macmillan  Company.) 

cross-section  through  an  amphibian  embryo  at  the  stage  of  the  formation  of 
the  mesoderm,  and  examine  under  the  low  power.  The  section  is  oval  in  form; 
it  is  in  most  cases  still  surrounded  by  the  delicate  egg  membrane.  The  outer 
layer  of  the  embryo  is  the  ectoderm,  relatively  thin  and  of  the  same  width  over 
the  whole  surface.  In  the  median  dorsal  line  the  ectoderm  is  producing  or  has 
already  produced  the  neural  tube.  In  the  former  case  the  ectoderm  exhibits  a 
pair  of  neural  folds  inclosing  a  thick  plate  of  ectoderm  between  them.  In  the 
latter  case  the  folds  have  fused  across  in  the  median  line,  forming  a  tube,  the 
neural  tube,  which  is  the  oval  hollow  mass  in  the  median  dorsal  line,  just  beneath 
the  ectoderm.  The  greater  part  of  the  section  is  occupied  by  the  archenteron  or 
primitive  intestine,  composed  of  entoderm.  The  archenteron  has  a  thin  dorsal 
wall,  a  thick  ventral  wall,  whose  cells  contain  yolk,  and  incloses  the  relatively 
small  gastrocoe*.  which  occupies  its  dorsal  part.  In  the  median  dorsal  region  of 


GENERAL  FEATURES  OF  CHORDATE  DEVELOPMENT  39 

the  archenteron,  a  mass  of  cells  will  be  seen  protruding  dorsally;  or  in  some 
slides  this  mass  of  cells  may  have  separated  from  the  archenteron  and  lies  between 
the  latter  and  the  neural  tube.  This  mass  is  the  notochord.  Between  the  ecto- 
derm and  the  archenteron  on  each  side  is  a  narrow  sheet  of  cells  extending  irom 
the  sides  of  the  neural  tube  ventrally.  In  some  slides  these  sheets  will  extend 
only  a  short  distance,  while  in  others  they  reach  nearly  to  the  median  ventral 
line.  These  sheets  are  the  mesoderm.  Make  a  diagram  of  the  section,  coloring 
ectoderm  blue,  entoderm  yellow,  and  mesoderm  red. 

E.      FURTHER   HISTORY   OF   THE   MESODERM 

The  history  of  the  mesoderm  is  of  the  utmost  importance  for  the  understand- 
ing of  vertebrate  structure.  We  have  already  noted  that  the  mesoderm  splits 
into  two  layers,  an  outer  or  somatic  layer,  and  an  inner  or  splanchnic  layer,  and 
that  the  space  between  the  two  layers  is  the  body  cavity  or  coelom.  The  meso- 
derm grows  from  each  side  of  the  embryonic  axis  ventrally  to  the  median  ventral 
line,  or  in  meroblastic  eggs  grows  out  over  the  yolk,  pushing  out  between  ecto- 
derm and  entoderm. 

The  mesoderm  next  becomes  differentiated  into  three  regions:  a  dorsal  region, 
called  the  epimere,  which  lies  to  each  side  of  the  neural  tube;  a  middle  region, 
called  the  mesomere  or  nephrotome,  situated  lateral  and  ventral  to  the  epimere; 
and  a  large  ventral  region  on  each  side  of  the  archenteron,  called  the  hypomere 
or  lateral  plate.  Each  of  these  regions  has  of  course  both  somatic  and  splanch- 
nic walls  (see  Fig.  &4,  also  K,  p.  14).  The  epimere  immediately  becomes  seg- 
mented, that  is  to  say,  dorsoventral  clefts  appear  in  it  at  regular  intervals,  the 
process  beginning  at  the  anterior  end  of  the  embryo  and  proceeding  posteriorly. 
Consequently,  the  epimere  becomes  divided  up  into  a  longitudinal  row  of  blocks, 
a  row  on  each  side  of  the  neural  tube.  These  blocks  are  epimeres,  or  mesoblastic 
somites  (originally  called  provertebrae  as  it  was  erroneously  supposed  that  they 
were  primitive  vertebrae).  At  first  the  epimeres  are  still  continuous  ventrally 
and  laterally  with  the  mesomere,  but  eventually  they  are  completely  cut  off 
from  the  rest  of  the  mesoderm  (see  Fig.  SB  and  the  figure  in  K,  p.  15).  The 
mesomere  and  the  hypomere  do  not  become  segmented  and  remain  permanently 
in  close  relation  to  each  other.  Within  the  mesomere  little  tubules  appear,  which 
open  into  the  cavity  of  the  hypomere;  they  are  the  tubules  of  the  kidney  (see 
Fig.  SB).  The  hypomeres  of  each  side  fold  around  the  archenteron,  their  inner 
walls  coming  in  contact  above  and  below  the  archenteron  to  form  double-walled 
membranes,  the  dorsal  and  ventral  mesenteries  (see  Fig.  SB).  The  cavities  of 
the  two  hypomeres  become  the  coelom  of  the  adult;  the  cavity  in  each  epimere 
disappears;  and  that  of  the  mesomere  remains  as  the  cavities  of  the  tubules  of 
the  kidney. 

In  embryos  of  the  amphibian  type  the  archenteron  is  a  closed  tube  and  the 
two  hypomeres  are  closed  cavities  which  meet  below  the  archenteron.  In 


40       LABORATORY  MANUAL  FOR  VERTEBRATE  ANATOMY 

embryos  resulting  from  meroblastic  eggs,  however,  the  archenteron  is  open  below 
and  spread  out  on  the  yolk,  and  the  hypomeres  extend  out  over  the  yolk.  The 
differences  between  the  two  types  of  embryos  are  illustrated  in  Figures  8^4  and  C. 
The  embryo  in  the  case  of  meroblastic  development  is  later  constricted  from  the 


notochord 


notochord 


neural  tube 


coelo 

entoderm— 

intestine 


entoderm 


epimere 

mesomere 
dorsal  mesentery 


ectoderm 


entoderm 


FIG.  8. — Diagrams  to  show  the  differentiation  of  the  mesoderm  in  holoblastic  and  meroblastic 
types  of  development.  A  and  B,  holoblastic  type;  C  and  D,  meroblastic  type.  A ,  differentiation  of  the 
mesoderm  into  epimere,  mesomere,  and  hypomere.  B,  separation  of  the  epimere  from  the  mesomere, 
appearance  of  kidney  tubules  in  the  mesomere,  and  closure  of  hypomere  around  the  intestine  to  form 
dorsal  and  ventral  mesenteries.  C,  similar  to  A  but  in  the  meroblastic  type,  showing  entoderm  and 
mesoderm  growing  around  the  yolk;  note  how  embryo  is  spread  out  on  the  surface  of  the  yolk;  x,  indi- 
cates lines  where  embryo  is  cut  off  from  yolk  in  making  sections  for  microscopic  study.  D,  similar  to  B 
but  in  the  meroblastic  type;  the  entoderm  has  completely  surrounded  the  yolk;  the  mesoderm  has 
nearly  done  so;  the  yolk  sac  is  seen  to  be  a  part  of  the  intestine;  the  embryo  is  partly  constricted  from 
the  yolk  sac,  the  constriction  being  the  yolk  stalk.  (C  from  Wilder's  History  of  the  Human  Body, 
courtesy  of  Henry  Holt  and  Company.) 

yolk  by  the  formation  of  deep  grooves  on  all  sides.  The  yolk  then  hangs  from 
the  ventral  surface  of  the  embryo  inclosed  in  a  sac  of  blastoderm,  the  yolk  sac, 
which  is  connected  with  the  embryo  by  a  stalk,  the  yolk  stalk,  as  shown  in 


GENERAL  FEATURES  OF  CHORD  ATE  DEVELOPMENT  41 

Figure  SD.  At  the  time  of  hatching,  the  yolk  has  been  practically  used  up,  and  the 
remnant  is  withdrawn  into  the  embryo,  the  opening  in  the  body  wall  where  the  yolk 
stalk  arises  being  then  finally  closed  over. 

Obtain  a  slide  bearing  a  cross-section  through  the  trunk  of  a  chick  embryo  of 
two  days'  incubation.  As  explained  above,  the  intestine  of  the  chick  embryo 
is  open  below  on  the  yolk.  In  making  such  sections  the  embryo  is  cut  off  from 
the  yolk,  the  lines  of  section  being  indicated  in  Figure  8C  at  x.  After  under- 
standing the  relation  of  embryo  and  yolk,  examine  the  section  with  the  low 
power.  The  dorsal  boundary  of  the  section  is  a  thin  layer,  the  ectoderm,  which 
is  slightly  elevated  in  the  median  dorsal  line;  the  ventral  boundary  is  another 
thin  layer,  the  entoderm,  which  makes  a  slight  upward  bend  in  the  median 
ventral  line,  indicating  the  future  intestine.  In  the  median  dorsal  line  just 
beneath  the  ectoderm  is  the  oval  hollow  section  of  the  neural  tube.  Immediately 
ventral  to  this  is  a  small  circular  mass  of  cells,  the  notochord.  On  each  side  of 
the  neural  tube  is  a  squarish  mass,  its  cells  radiating  from  the  center.  This  is 
the  epimere  or  mesoblastic  somite.  Lateral  to  the  epimere  and  continuous  with 
it  is  a  smaller  mass,  the  mesomere  or  nephrotome,  in  which  one  or  more  tubules 
with  central  holes  are  distinguishable.  Beyond  the  nrjsomere  the  mesoderm  is 
observed  to  split  into  two  layers.  This  region  of  the  mesoderm  is  the  hypomere 
or  lateral  plate.  The  outer  or  dorsal  layer  of  the  hypomere  is  the  somatic 
mesoderm.  It  ascends  and  comes  in  contact  with  the  ectoderm,  the  two  together 
constituting  the  somatopleure  or  body  wall.  The  lower  or  ventral  wall  of  the 
hypomere  is  the  splanchnic  mesoderm;  it  descends  and  comes  in  contact  with 
the  entoderm  and  the  double  layer  thus  formed  is  the  s plane hnopleure  or  intestinal 
wall.  The  cavity  between  the  somatic  and  splanchnic  walls  of  the  hypomere  is 
the  coelom.  As  already  explained,  the  hypomere  in  such  embryos  extends  far 
out  over  the  yolk.  Observe  that  the  splanchnopleure  contains  many  holes; 
these  are  the  cross-sections  of  blood  vessels,  which  convey  the  food  from  the  yolk 
sac  to  the  embryo.  There  is  also  a  large  artery  in  the  embryo  below  each  epimere. 
Draw  the  section  in  diagram,  coloring  the  three  germ  layers  as  before. 

F.  THE   FATE   OF   THE   ECTODERM 

As  we  have  seen,  the  ectoderm  gives  rise  to  the  neural  tube,  from  which 
develop  the  brain,  spinal  cord,  and  nerves.  The  ectoderm  also  forms  the  external 
layer  of  the  skin  and  all  of  its  derivatives,  such  as  hair,  nails,  etc.  It  also  gives 
rise  to  the  sensory  part  of  all  the  sense  organs,  the  lining  membrane  of  the  nasal 
cavities,  the  mouth,  and  anus,  the  glands  and  other  outgrowths  of  the  nasal  and 
mouth  cavities,  the  glands  of  the  skin,  the  enamel  of  the  teeth,  and  the  lens  of 
the  eye. 

G.  THE   FATE    OF   THE   ENTODERM 

The  entoderm  is,  as  we  have  seen,  the  primitive  intestine.  This  intestine 
is  the  inner  lining  of  the  adult  intestine.  The  entoderm  thus  forms  the  epithelial 


42        LABORATORY  MANUAL  FOR  VERTEBRATE  ANATOMY 

lining  of  the  intestine  and  the  epithelial  lining  and  epithelial  cells  of  all  of  the 
outgrowths  of  the  intestine,  which  include  the  gill  pouches  and  gills,  the  larynx, 
windpipe,  and  lungs,  the  tonsils,  the  thyroid  and  thymus  glands,  the  liver,  the 
gall  bladder  and  bile  duct,  the  pancreas,  and  the  urinary  bladder  and  adjacent 
parts  of  the  urogenital  system.  The  student  should  note  that  only  the  epithelial 
cells  of  these  structures  arise  from  the  entoderm. 

H.     THE  FATE  OF  THE  MESODERM  AND  THE  FORMATION  OF  MESENCHYME 

i.  Mesenchyme. — In  the  further  development  of  the  mesoderm,  mesen- 
chyme plays  an  important  role.     Mesenchyme  is  not  a  germ  layer  but  a  particular 


ectoderm 


mesothelium 
coelom  of  epimere 


kidney  duct 


epimere 


mesenchyme  of  sclerotome 
forming  from  epimere 


neural  tube 


mesoderm 


notochord 


entoderm 


FIG.  9.—  Enlarged  view  of  the  epimere  of  a  chick  embryo  of  two  days'  incubation  to  show  the 
transformation  of  a  portion  of  the  epimere  into  mesenchyme.  These  mesenchyme  cells  constitute  the 
sclerotome  from  which  the  vertebral  column  arises. 


type  of  tissue.  It  is  a  primitive  kind  of  connective  tissue,  consisting  of  branched 
cells,  whose  branches  are  more  or  less  united  to  form  a  network  (see  Fig.  9). 
Nearly  all  of  the  mesenchyme  comes  from  mesoderm;  however,  it  may  arise 
from  the  other  germ  layers  also.  Hence  tissues  and  structures  which  arise 
from  mesenchyme  may  owe  their  origin  to  more  than  one  germ  layer.  Con- 
sequently, to  avoid  inaccuracy  it  is  usually  merely  stated  that  they  arise  from 
mesenchyme,  without  specifying  the  particular  germ  layer  or  layers  involved. 
When  a  germ  layer  is  about  to  produce  mesenchyme  its  cells  become  loose, 
separating  from  their  fellows,  lose  their  epithelial  form,  and  taking  on  a  branched 


GENERAL  FEATURES  OF  CHORD  ATE  DEVELOPMENT 


43 


irregular  shape  wander  away  by  amoeboid  movements  to  more  or  less  definite 
regions  where  they  give  rise  to  certain  tissues  (see  Fig.  9).  Those  parts  of  the 
mesoderm  which  do  not  become  mesenchyme  but  retain  their  epithelial  charac- 
teristics are  called  mesothelium. 

2.  The  fate  of  the  epimeres.— The  medial  wall  of  each  epimere  transforms 
into  a  mass  of  mesenchyme  cells  which  migrate  to  a  position  around  the  noto- 
chord  and  there  give  rise  to  the  vertebral  column.  This  mass  of  mesenchyme 
is  known  as  the  sclerotome  (see  Fig.  10).  The  outer  wall  of  each  epimere  trans- 
forms into  mesenchyme  cells  which  migrate  to  the  under  side  of  the  ectoderm 
and  there  give  rise  to  the  inner  layer  (dermis)  of  the  skin.  This  part  of  the 
epimere  is  called  the  dermatome  (Fig.  10).  The  remainder  of  the  epimere  persists 


neural  tube 
sclerotome  /       notochord 


neural  tube 


neural  tube 


dermatome 


intestine 


FIG.  10. — Diagram  of  cross-sections  of  vertebrate  embryos  to  show  the  differentiation  of  the  epi- 
mere into  dermatome  (skin-producer),  myotome  (muscle-producer),  and  sclerotome  (skeleton-producer). 
Dermatome  and  sclerotome  consist  of  mesenchyme,  myotome  of  mesothelium.  In  B  and  C  the  derma- 
tome  is  seen  spreading  beneath  the  ectoderm  to  form  the  dermis  of  the  skin;  the  myotomes  are  growing 
ventrally  to  form  the  muscle  layer  of  the  body  wall;  the  sclerotome  is  accumulating  around  the  noto- 
chord at  x  to  form  the  vertebrae;  and  the  hypomere  incloses  the  intestine  producing  the  dorsal  and 
ventral  mesenteries  and  the  mesoderm  of  the  intestine. 

in  place  as  mesothelium  and  is  known  as  a  myotome  or  muscle  segment.  Each 
myotome  becomes  separated  from  the  adjacent  ones  by  a  connective  tissue 
partition,  the  myocomma  or  myoseptum.  The  myotomes  give  rise  to  the  volun- 
tary muscles  of  the  body  (with  certain  exceptions).  Each  grows  from  its  original 
dorsal  position  ventrally  between  the  ectoderm  and  the  hypomere  to  the  median 
ventral  line  where  it  meets  its  fellow  from  the  opposite  side.  There  is  thus 
produced  a  complete  muscular  coat  for  the  body  (see  Fig.  10;  also  K,  pp.  15, 
17;  W,p.  64).  ' 

3.  The  fate  of  the  mesomere. — The  mesomere  gives  rise  to  the  kidneys,  the 
reproductive  organs,  and  their  ducts  (the  terminal  portions  of  the  urogenital 
ducts  may  have  ectodermal  or  entodermal  linings). 


44       LABORATORY  MANUAL  FOR  VERTEBRATE  ANATOMY 

4.  The  fate  of  the  hypomere. — The  cavity  of  the  hypomere  is  the  coelom 
of  the  adult.     The  splanchnic  walls  of  the  hypomeres  of  the  two  sides  fold 
around  the  archenteron  and  give  rise  to  mesenchyme,  from  which  are  pro- 
duced the  smooth  muscle  and  connective- tissue  coats  of  the  digestive  tract, 
and  also  the  smooth  muscle,  connective  tissue,  cartilage,  etc.,  as  the  case  may  be, 
of  all  the  derivatives  of  the  digestive  tract  mentioned  above.     The  hypomere 
also  gives  rise  to  the  linings  of  all  the  coelomic  cavities,  the  serosa  of  the  viscera, 
and  all  of  the  mesenteries.     The  splanchnic  mesoderm  of  the  hypomere  produces 
the  heart.     In  the  region  of  the  gill  slits  the  hypomere    produces    voluntary 
muscles. 

5.  The  products  of  the  mesenchyme. — The  mesenchyme  gives  rise  to  all 
of  the  connective  tissues  of  the  body,  including  cartilage  and  bone;   to  all  of  the 
involuntary  or  smooth  muscles;   to  the  blood  cells,  the  blood  vessels,  the  lymph 
vessels,  and  lymph  glands;    and  to  the  voluntary  muscles  of  the  appendages. 
It  has  already  been  stated  that  the  vast  majority  of  the  mesenchyme  is  of  meso- 
dermal  origin,  but  a  small  part  arises  from  the  other  germ  layers. 


V.     THE    COMPARATIVE    ANATOMY    OF    THE    INTEGUMENT    AND 

THE  EXOSKELETON 

A.      GENERAL  CONSIDERATIONS   ON  THE   SKELETON 

The  term  skeleton  includes  all  of  the  hardened  portions  of  the  bodies  of  animals.  The  skele- 
ton of  the  invertebrates  is  commonly  external,  forming  a  hard  covering  inclosing  the  body, 
while  that  of  the  vertebrates  is  both  external  and  internal.  In  invertebrates,  further,  the  skele- 
ton is  a  lifeless  secretion,  containing  no  cells,  while  the  vertebrate  skeleton  is  almost  invariably 
cellular,  either  being  composed  entirely  of  hardened  cells,  or  consisting  of  cells  and  intercellular 
products.  There  are  two  distinct  kinds  of  skeleton  in  vertebrates,  different  in  origin  and 
function:  (i)  The  external  skeleton,  or  exoskeleton,  derived  from  the  skin,  and  forming  a  cover- 
ing and  protective  layer  on  the  outside  of  the  body.  (2)  The  internal  skeleton,  or  endoskeleton, 
derived  chiefly  from  the  inner  wall  of  the  epimere,  and  constituting  a  support  and  framework 
for  the  body  and  a  place  of  attachment  of  the  voluntary  muscles. 

We  shall  study  the  exoskeleton  first.  However,  certain  parts  of  the  exoskele- 
ton have  become  so  closely  related  to  the  endoskeleton  that  they  will  be  considered 
with  the  latter. 

B.      THE   STRUCTURE   OF  THE   SKIN 

Since  the  exoskeleton  is  derived  from  the  skin,  a  thorough  understanding  of 
the  structure  of  the  skin  is  prerequisite  to  a  study  of  the  exoskeleton.  The  skin 
or  integument  occurs  only  in  vertebrates  and  may  be  defined  as  those  outer  layers 
of  the  body  wall  which  are  easily  separated  from  the  inner  layers.  Study  of  the 
microscopic  appearance  and  of  the  development  of  the  skin  reveals  that  it  consists 
of  two  distinct  parts:  an  outer  layer,  the  epidermis,  composed  of  epithelial  cells, 
and  an  inner  layer,  the  dermis  or  corium,  composed  of  connective  tissue  (Fig.  n). 
The  epidermis  originates  directly  from  the  surface  ectoderm  of  the  embryo,  which 
by  proliferation  produces  several  strata  of  cells  to  form  the  epidermis.  The 
dermis  is  formed  by  the  mesenchyme  of  the  dermatome.  In  the  preceding  sec- 
tion we  learned  that  the  dermatome  originates  from  the  outer  part  of  the  epimere 
(Fig.  10,  p.  43). 

i.  Microscopic  structure  of  the  frog's  skin. — Examine  under  moderate 
powers  of  the  microscope  a  cross-section  through  the  frog's  skin  and  identify 
carefully  the  following  parts: 

a)  The  epidermis:  The  outer  part  of  the  skin  consists  of  several  strata  of 
epithelial  cells.  This  is  the  epidermis.  The  outermost  of  the  cell  strata  con- 
sist of  thin  flat  cells,  which  have  become  converted  into  a  horny  material.  This 
cornified  part  of  the  epidermis  is  designated  the  stratum  corneum.  Beneath  the 
stratum  corneum  the  cells  gradually  change  from  a  flattened  to  a  rounded  and 
finally  to  a  columnar  shape.  These  lavers  of  rounded  to  columnar  cells  constitute 
the  stratum  germinativum  (also  called  stratum  mucosum  and  stratum  Malpighii) .  In 

45 


46 


LABORATORY  MANUAL  FOR  VERTEBRATE  ANATOMY 


the  frog's  skin  there  is  no  sharp  demarcation  between  the  stratum  corneum  and 
the  stratum  germinativum.  The  lowermost  layer  of  the  stratum  germinativum, 
consisting  of  tall  columnar  cells,  is  the  active  portion  of  the  epidermis  and  is  con- 
tinuously proliferating  cells  which  are  pushed  outward,  become  flat  and  horny, 
and  finally  form  part  of  the  stratum  corneum.1  The  stratum  corneum  is  shed 
at  frequent  intervals  and  is  continuously  renewed  from  below. 

b)  The  dermis  or  corium:  The  dermis  is  the  inner  part  of  the  skin.  It  con- 
sists of  connective  tissue,  which  in  the  case  of  the  frog  is  arranged  partly  in  the 
form  of  a  loose  network  located  just  beneath  the  epidermis,  and  partly  in  the  form 


gland 


FIG.  ii. — Diagrammatic  cross-section  through  the  vertebrate  skin,  based  on  mammals. 

of  layers  of  dense,  parallel,  wavy  fibers.  In  addition  to  the  connective  tissue 
fibers  the  dermis  contains :  pigment  cells,  dark,  irregular,  branching  cells  forming  a 
thin  layer  just  beneath  the  epidermis;  the  cutaneous  glands,  flask-shaped  bodies, 
produced  by  an  infolding  of  the  stratum  germinativum,  and  opening  by  a  neck 
to  the  surface;  and  at  intervals  columns  containing  smooth  muscle  cells,  blood  ves- 
sels, and  nerves,  these  columns  crossing  the  dermis  at  right  angles  to  the  surface. 
Of  these  structures  the  cutaneous  glands  are  the  most  conspicuous.  They  are 
really  parts  of  the  stratum  germLiativum,  which  have  been  evaginated  into  the 
loose  portion  of  the  dermis. 

Draw  a  small  portion  of  the  skin  to  show  the  parts  named  above. 

1  In  some  texts  only  the  proliferating  layer,  one  cell  thick,  is  called  the  stratum  germinativum, 
that  portion  of  the  epidermis  between  this  and  the  stratum  corneum  being  then  designated  the  stratum 
Malpighii.  We  shall  here  regard  the  terms  stratum  germinativum  and  stratum  Malpighii  as  synony- 
mous. 


THE  INTEGUMENT  AND  THE  EXOSKELETON  47 

C.      THE  EXOSKELETON  IN  GENERAL 

The  exoskeleton  is  derived  from  the  skin.  It  is  produced  by  hardening  processes  in  the 
epidermis  or  the  dermis  or  in  both.  Exoskeleton  derived  from  the  epidermis  is  spoken  of  as 
epidermal;  it  is  produced  by  the  activity  of  the  stratum  germinativum  and  consists  of  many  flat  horny 
cells  pressed  firmly  together  to  make  a  hard  structure.  It  will  be  seen  that  epidermal  exoskeleton 
structures  are  only  special  portions  of  the  stratum  corneum.  Exoskeleton  derived  from  the 
dermis  is  spoken  of  as  dermal;  it  nearly  always  consists  of  bone,  produced  by  the  mesenchyme  cells 
originating  from  the  dermatome.  From  the  embryological  point  of  view  epidermal  exoskeleton 
is  of  ectodermal  origin,  while  dermal  exoskeleton  is  mesodermal.  Epidermal  and  dermal  exoskele- 
ton are  different  both  morphologically  and  embryologically.  From  our  definition  of  homology 
it  follows  that  exoskeletal  structures  of  epidermal  origin  are  homologous  in  different  animals, 
and  dermal  structures  are  likewise  homologous.  It  will  be  our  purpose  to  trace  the  homology 
of  the  exoskeleton  in  the  different  vertebrate  classes. 

The  student  should  also  read  the  chapters  on  the  exoskeleton  in  the  standard  texts  of 
comparative  anatomy  as  K,  W,  and  Wd. 

D.      EXOSKELETON   OF  FISHES 

Most  fishes  are  covered  with  scales,  which  are  of  four  kinds. 

i.  The  placoid  scale. — This  type  of  scale  occurs  in  the  elasmobranch  fishes. 
It  consists  of  a  basal  plate  carrying  a  projecting  spine.  Good  examples  are  obtain- 
able from  the  skate.  Cut  out  from  the  skate  a  small  piece  of  skin  containing  one 
spine.  Clean  away  the  skin  so  as  to  expose  the  complete  scale.  It  consists  of 
a  toothed  basal  plate,  from  which  arises  a  shiny  curved  spine.  The  base  of  the 
spine  is  hollow,  the  cavity  being  known  as  the  pulp  cavity.  This  cavity  can  be  lo- 
cated by  probing  with  a  needle  point  in  the  center  of  the  under  side  of  the  basal 
plate.  The  shiny  coating  of  the  spine  is  composed  of  enamel;  the  basal  plate  and 
interior  of  the  spine  consist  of  dentine,  a  substance  similar  to  bone.  Draw  the 
scale. 

To  understand  the  structure  of  the  placoid  scale  it  is  necessary  to  consider  its  mode  of 
development.  Each  scale  originates  from  a  region  of  cell  multiplication  in  the  dermis.  The 
cells  thus  formed  heap  up  into  a  cone,  the  dermal  papilla,  which  pushes  up  against  the  under  side 
of  the  stratum  germinativum.  The  outer  cells  of  the  dermal  papilla  begin  to  secrete  dentine, 
and  produce  the  basal  plate  and  the  inside  of  the  spine.  The  stratum  germinativum  in  contact 
with  the  dermal  papilla  secretes  on  its  under  surface  the  substance  enamel,1  which  consequently 
forms  a  coat  over  the  dentine  of  the  spine.  The  spine  when  formed  breaks  through  the  sur- 
face. The  interior  cells  of  the  dermal  papilla  do  not  undergo  any  change  but  remain  as  a  soft 
pulp  occupying  the  pulp  cavity  in  the  interior  of  the  spine.  From  this  account  it  follows  that 
the  placoid  scale  is  composed  of  both  epidermal  and  dermal  constituents.  The  development 
of  the  placoid  scale  is  illustrated  in  Figure  12;  also  in  W,  page  82;  CNH,  Vol.  VII,  page 
185;  Wd,  page  40;  R,  page  4. 

Draw  an  imaginary  longitudinal  section  through  a  placoid  scale,  coloring  ecto- 
dermal parts  blue  and  mesodermal  parts  red. 

1  That  this  substance  is  really  enamel,  such  as  occurs  on  teeth,  has  been  questioned,  some  main- 
taining that  it  is  a  kind  of  dentine  (see  L,  p.  120).  We  sUall,  however,  here  take  the  usual  view  of  its 
origin. 


LABORATORY  MANUAL  FOR  VERTEBRATE  ANATOMY 


The  dogfish  is  covered  with  placoid  scales,  which  are  much  smaller  and  more 
closely  set  together  than  in  the  case  of  the  skate.  The  spines  of  the  scales  can 
be  felt  by  passing  the  hand  over  the  dogfish  skin.  By  boiling  a  piece  of  the  skin 
with  alkali,  the  scales  may  be  separated  out.  Examine  such  isolated  scales 
under  the  microscope.  They  are  similar  to  those  of  the  skate,  with  rhomboid 
basal  plates  and  a  projecting  spine. 

2.  The  homology  of  teeth  and  placoid  scales. — The  most  interesting  point 
about  the  placoid  scale  is  that  its  structure  and  mode  of  origin  are  exactly  the 


stratum 
germinativum 


derm  is 


FIG.  12. — Five  successive  stages  in  the  development  of  the  placoid  scale  of  the  dogfish.  A ,  gather- 
ing of  the  cells  of  the  dermis  to  form  the  dermal  papilla.  B,  evagination  of  the  dermal  papilla  and 
secretion  of  the  dentine  (colored  black)  by  the  outer  cells  of  the  papilla.  C,  continued  secretion  of  dentine, 
thinning  of  the  interior  of  the  papilla  to  form  the  pulp.  D,  beginning  secretion  of  the  basal  plate  and 
formation  of  the  enamel  (left  white)  by  the  under  surface  of  the  stratum  germinativum.  E,  eruption 
of  the  spine  through  the  epidermis,  completion  of  the  basal  plate  and  pulp.  (From  Goodrich  in  Part  IX 
of  Lankester's  Treatise  on  Zoology,  courtesy  of  the  Macmillan  Company.) 

same  as  those  of  the  teeth  of  all  vertebrates.  From  this  it  follows  that  teeth 
and  placoid  scales  are  homologous  structures,  and  teeth  are  merely  slightly  modi- 
fied placoid  scales.  The  homology  is  probably  due  to  the  fact  that  the  lining  of 
the  mouth  cavity  is  really  skin  turned  in,  and  hence  may  be  expected  to  give 
rise  to  structures  similar  to  those  found  in  the  skin. 

Obtain  a  longitudinal  half  of  an  ordinary  vertebrate  tooth.  Identify  the 
following  parts :  the  crown  or  shiny  upper  part,  corresponding  to  the  spine  of  the 


THE  INTEGUMENT  AND  THE  EXOSKELETON  49 

placoid  scale;  the  root,  or  dull  lower  part,  which  sets  into  the  jaw  and  corresponds 
more  or  less  to  the  basal  plate  of  the  scale;  the  pulp  cavity,  the  central  space,  filled 
in  life  with  a  dermal  papilla,  consisting  of  connective  tissue,  blood  vessels,  nerves, 
etc. ;  the  dentine,  the  bony  material  composing  most  of  the  tooth;  the  enamel,  the 
thin  shiny  outer  coating  of  the  dentine  of  the  crown.  Draw  the  specimen, 
coloring  ectodermal  part  blue  and  mesodermal  part  red. 

3.  The  ganoid  scale.— This  type  of  scale  is  characteristic  of  many  of  the 
ganoid  fishes,  such  as  the  gar  pike  and  the  sturgeon.     Examine  a  specimen  of  the 
gar  pike  (Lepidosteus)  and  note  its  complete  investment  with  hard,  shiny,  rhom- 
boid plates,  arranged  in  diagonal  rows,  fitting  closely  together.    They  are  typi- 
cal ganoid  scales.     Cut  out  a  small  piece  of  skin  containing  several  scales. 
Note  that  each  diagonal  row  is  movable  on  the  adjacent  rows  along  the  line  of 
junction  or  hinge  line,  but  the  members  of  each  row  are  immovably  joined  to  each 
other  by  a  peg-and-socket  arrangement  (visible  only  on  the  under  side  and  well 
developed  only  in  large  specimens) .     Draw  a  few  scales. 

In  the  sturgeon  note  similar  ganoid  scales,  bony  rhombic  plates  bearing  a  short 
spine.  They  are  arranged  in  five  longitudinal  rows,  with  areas  of  apparently 
naked  skin  between  the  rows.  In  some  sturgeons  (Scaphirhynchus)  the  ganoid 
scales  form  a  complete  investment  for  the  tail. 

Ganoid  scales  are  composed  of  bone  and  are  often  covered  with  a  shiny  substance  known  as 
ganoin.  They  never  bear  any  enamel.  They  are  purely  of  dermal  origin  formed  by  the  activity 
of  the  mesenchyme  cells  of  the  dermis  and  corresponding  to  the  basal  plates  of  the  placoid  scales. 
Primitively  they  clothed  the  entire  body  as  in  the  case  of  the  gar  pike,  but  this  arrangement 
obviously  hinders  movement;  hence  to  facilitate  movement  they  are  often  lost  from  some  regions 
of  the  body  as  in  the  sturgeon,  and  in  the  teleost  fishes  are  replaced  by  thinner,  more  flexible 
scales,  overlapping  each  other  like  shingles. 

4.  The  cycloid  scale. — This  is  the  earliest  form  of  the  thin,  flexible  scale 
and  occurs  in  a  few  ganoids  and  some  teleosts.     Examine  the  bowfin  (Amia), 
a  ganoid  fish,  and  note  the  thin,  rounded  scales  with  which  the  animal  is  clothed. 
These  are  cycloid  scales.     They  are  set  in  pockets  in  the  skin,  and  the  free  pro- 
jecting edges  overlap  like  shingles,  thus  allowing  greater  freedom  of  movement. 
Remove  a  scale  and  examine  under  the  microscope,  or  use  mounts  already  pre- 
pared.    Note  the  smooth  border  and  the  markings  on  the  scale.     Draw. 

5.  The  ctenoid  scale. — This  type  of  scale  is  similar  to  the  preceding,  and 
occurs  in  the  vast  majority  of  the  bony  fishes.     Note  the  arrangement  of  the 
ctenoid  scales  on  the  perch  or  other  common  fish.     The  scales  are  set  in  diagonal 
rows  in  pockets  of  the  dermis,  their  free  edges  overlapping.     Remove  one  or 
obtain  a  prepared  slide  of  one  and  examine  with  the  low  power.     The  attached 
end  of  the  scale  is  beautifully  fluted;  the  free  border  bears  several  rows  of  small 
toothlike  projections,  which  are  well  developed  in  the  perch  but  poorly  defined 
in  some  fishes;  and  the  surface  is  sculptured  with  curved  parallel  ridges.     It 


50       LABORATORY  MANUAL  FOR  VERTEBRATE  ANATOMY 

may  generally  be  noticed  that  the  epidermis  forms  a  thin  covering  over  the  free 
toothed  end.     Draw. 

Both  cycloid  and  ctenoid  scales  are  composed  of  bone  and  are  homologous  with  ganoid 
scales.  All  three  types  of  scales  are  produced  by  heaps  of  mesenchyme  cells  of  the  dermis.  The 
posterior  edge  of  each  scale  grows  outward,  carrying  the  epidermis  with  it;  the  latter  may  re- 
main as  a  thin  covering  over  the  projecting  part  of  the  scale  or  may  be  rubbed  off.  The 
scales  of  fishes  are  thus  of  dermal  origin,  with  the  exception  of  the  placoid  scale,  which  con- 
tains in  addition  to  its  dermal  part,  a  small  epidermal  contribution. 

E.      EXOSKELETON  OF  AMPHIBIA 

The  vast  majority  of  present-day  Amphibia  have  naked  skins,  that  is,  an  exoskeleton  is 
lacking.  A  few  are  provided  with  minute,  concealed  dermal  scales.  The  extinct  Amphibia 
(Stegocephala)  were  commonly  clothed  in  a  heavy  armor  of  dermal  plates  homologous  with 
ganoid  scales  and  similar  to  them  in  appearance. 


dermis 


stratum 
germinativum 


dermis 


A 


B 


FIG.  13. — Diagrammatic  longitudinal  sections  through  the  skin  of  A,  a  teleostome  fish  and  B,  a 
reptile,  to  show  the  locations  of  the  scales.  In  A,  the  scales  are  in  the  dermis,  while  in  B  they  repre- 
sent thickened  portions  of  the  stratum  corneum.  (From  Wiedersheim's  Comparative  Anatomy  of  Ver- 
tebrates, courtesy  of  the  Macmillan  Company.) 


F.      EXOSKELETON   OF   REPTILES 

The  bodies  of  reptiles  are  characteristically  clothed  with  an  exoskeleton  com- 
posed of  horny  scales.  These  scales  are  of  epidermal  origin,  representing  particu- 
larly dense,  cornified  areas  of  the  stratum  corneum.  In  the  formation  of  such  a 
horny  scale  a  dermal  papilla  first  appears  which  furnishes  nutriment  for  the  cells 
engaged  in  producing  the  scale.  The  stratum  germinativum  over  the  dermal  papilla 
begins  to  proliferate  rapidly,  producing  cells  which  become  flat  and  horny  and 
compressed  into  a  scale  (see  Fig.  13^).  In  addition  to  the  horny  scales  many 
reptiles  possess  bony  plates  of  dermal  origin  situated  beneath  the  epidermal 
scales.  These  bony  plates  originate  in  the  same  manner  as  the  scales  of  fishes. 
In  order  to  avoid  confusion,  the  epidermal  scales  will  be  designated  as  scutes  and 
the  dermal  structures  as  plates. 

i.  Exoskeleton  of  the  lizard. — Recall  the  condition  of  the  lizard,  or  re- 
examine  the  specimen.  The  body  is  clothed  with  horny  scales  of  epidermal 
origin,  overlapping  like  shingles.  Note  arrangement  and  size  of  the  scales  on 


THE  INTEGUMENT  AND  THE  EXOSKELETON  51 

various  parts  of  the  body.  The  head  may  bear  enlarged  scales  or  head  shields. 
There  is  no  dermal  exoskeleton  in  lizards. 

2.  Exoskeleton  of  the  turtle.— The  exoskeleton  of  the  turtle  is  somewhat 
complicated.  For  the  purposes  of  study,  carapaces  and  plastrons  which  have 
been  separated  by  sawing  through  the  bridges  will  be  provided.  It  is  also  de- 
sirable that  these  parts  shall  have  been  cooked  to  render  the  sutures  between  the 
plates  more  distinct.  The  following  description  is  based  on  the  exoskeleton 
of  our  common  pond  turtles;  in  other  families  of  turtles  the  arrangement  of  the 
scutes  and  plates  may  be  slightly  different. 

a)  The  carapace:  The  dorsal  surface  of  the  carapace  consists  of  large  thin 
horny  scales  or  scutes,  whose  boundaries  are  marked  by  grooves.  These  scutes 
are  of  epidermal  origin,  formed  from  the  stratum  corneum.  The  scutes  are  ar- 
ranged in  five  longitudinal  rows,  one  median,  and  two  pairs  of  lateral  rows.  The 
median  row  consists  of  five  neural  scutes;  on  each  side  of  this  are  four  costal  scutes 
and  the  margins  of  the  carapace  are  covered  by  a  number  of  small  marginal  scutes. 
The  unpaired  narrow  marginal  scute  at  the  middle  of  the  anterior  end  of  the  cara- 
pace is  termed  the  nuchal  scute;  besides  this  there  are  twelve  pairs  of  marginal 
scutes  of  which  the  median  posterior  two,  behind  the  fifth  neural  scute,  are  often 
called  pygal  scutes.  Observe  that  the  marginal  scutes  are  continued  over  the 
edge  of  the  carapace  to  cover  the  margins  of  the  under  side.  Draw  the  dorsal 
surface  of  the  carapace,  showing  accurately  the  outlines  of  the  scutes  (only  half 
need  be  filled  in). 

Turn  the  carapace  over  and  study  its  ventral  surface.  It  is  composed  of 
heavy  bony  plates  of  dermal  origin.  The  vertebral  column  with  its  ribs  occu- 
pies the  median  line,  and  both  are  firmly  fused  to  the  carapace.  The  boundaries  of 
each  plate  are  marked  by  jagged  sutures  which  should  be  located  in  identifying 
the  plates.  The  plates  like  the  scutes  are  arranged  in  five  longitudinal  rows. 
The  median  row  of  plates,  fused  to  the  dorsal  sides  of  the  vertebrae,  consists 
of  a  single  large  anterior  nuchal  plate  followed  by  eight  smaller  vertebral  or  neural 
plates,  each  attached  to  a  vertebra,  followed  by  two  postneural  or  precaudal 
plates,  not  attached  to  vertebrae.  On  each  side  of  the  median  row  is  a  row  of 
costal  plates,  eight  pairs  of  elongated  plates,  each  attached  to  a  rib.  The  margins 
are  formed  of  eleven  pairs  of  marginal  plates  and  one  single  unpaired  pygal  plate 
in  the  median  posterior  position.  Make  an  accurate  drawing  of  the  under  side 
of  the  carapace,  showing  bony  plates  and  their  relation  to  the  vertebrae  and  ribs. 
Do  the  scutes  and  plates  of  the  carapace  correspond  ? 

A  difference  of  opinion  exists  regarding  the  bony  plates  of  the  turtle's  carapace.  According 
to  one  view,  all  of  the  plates  are  of  dermal  origin  and  the  expanded  vertebrae  and  ribs  are  fused 
to  the  under  surface  of  these  dermal  plates.  According  to  the  other  view,  which  is  probably  the 
correct  one,  the  vertebral  and  costal  plates  are  formed  entirely  by  the  expansion  of  the  vertebrae 
and  ribs,  and  are  therefore  not  dermal  exoskeleton  at  all  but  parts  of  the  endoskeleton.  This 
view  also  includes  the  conception  that  turtles  originally  possessed  a  set  of  real  dermal  plates 


52        LABORATORY  MANUAL  FOR  VERTEBRATE  ANATOMY 

external  to  the  present  vertebral  and  costal  plates,  and  that  these  plates  subsequently  dissap- 
peared.  On  both  points  of  view  the  nuchal,  marginal,  postneural,  and  pygal  plates  are  dermal 
and  part  of  the  exoskeleton. 

b)  The  plastron:  The  plastron,  like  the  carapace,  consists  of  a  set  of  horny 
epidermal  scutes  covering  bony  dermal  plates.     Study  the  external  (ventral) 
surface  of  the  plastron.     It  is  covered  by  six  pairs  of  scutes  named  from  ii) 
front  backward:  gular,  humeral,  pectoral,  abdominal,  femoral,  and  anal.     Irregulai 
inframarginal  scutes  cover  the  bridges.     Study  the  internal  (dorsal)  surface, 
noting  the  large  bony  plates  united  by  jagged  sutures,  which  compose  it.    The 
small  anterior  pair  of  plates  are  named  epiplastra.     Between  them  is  a  single 
median  plate  with  a  posteriorly  projecting  point,  named  the  entoplastron  or  inter- 
clavicle.     Behind  these  are  three  pairs  of  large  squarish  plates  named  the  hyo- 
plastra,  hypoplastra,  and  xiphiplastra,  the  first  named  being  most  anterior.     Draw 
the  plastron,  showing  outlines  of  scutes  and  plates. 

c)  Other  exoskeletal  structures:  The   exoskeleton   also  includes   the  claws 
and  the  horny  beaks  which  incase  the  jaws.     Examine  these  beaks  in  a  demon- 
stration specimen  in  which  they  have  been  loosened  from  the  underlying  bones. 
Turtles  also  possess  scales  or  thickened  scalelike  areas  on  the  legs  and  tail,  and 
in  some  cases  enlarged  scutes  over  the  head.     All  of  these  structures  are  of  epi- 
dermal origin,  consisting  of  special  portions  of  the  stratum  corneum. 

G.      EXOSKELETON   OF   BIRDS 

Birds  are  clothed  in  an  exoskeleton  consisting  of  feathers  on  the  greater  part 
of  the  body,  scales  and  claws  on  the  feet,  and  horny  beaks.  All  of  these  struc- 
tures are  of  epidermal  origin,  formed  from  the  stratum  corneum.  There  are  no 
dermal  elements  of  the  exoskeleton  in  birds.  The  feathers  of  birds  are  their 
characteristic  features  and  will  be  studied  in  detail.  There  are  three  kinds  of 
feathers — down  feathers,  contour  feathers,  and  hair  feathers  or  filoplumes. 

1.  Structure  of  the  down  feather. — Down  feathers  or  plumulae  constitute 
the  fluffy  covering  of  young  birds  and  also  occur  in  adult  birds  between  the  bases 
of  the  contour  feathers.     Obtain  a  down  feather  or  a  prepared  slide  of  one. 
Identify  the  short  stem  or  quill,  the  soft  rays  or  barbs  which  spring  in  a  circle 
from  the  top  of  the  quill,  and  the  minute  side  rays  or  barbules  on  the  barbs.    Draw. 

2.  Development  of  the  down  feather. — A  down  feather  arises  from  a  papilla  of  the  skin, 
the  feather  papilla,  consisting  of  a  dermal  core,  the  pulp,  covered  by  the  epidermis.    Later  this 
papilla  sinks  into  a  pit  in  the  skin,  called  the  feather  follicle.    The  stratum  germinativum  of 
the  papilla  begins  to  proliferate,  forming  a  number  of  longitudinal  columns  which  project  into 
the  pulp  (see  Fig.  14).    These  columns  eventually  separate,  each  being  a  barb,  composed  of 
cornified  cells.    The  original  stratum  corneum  covers  the  barbs  like  a  sheath,  which  is  called 
theperiderm;  it  splits  open  and  is  shed,  releasing  the  barbs.     During  this  process  the  whole 
papilla  has  been  elongating  and  comes  to  project  above  the  surface.    The  lower  part  of  the 
papilla  does  not  split  into  barbs  but  remains  undivided  as  the  quill,  within  which  the  pulp  dries 


THE  INTEGUMENT  AND  THE  EXOSKELETON 


53 


up.  (See  further,  K,  p.  36;  P  and  H,  pp.  369-73;  Wd,  pp.  25-28.)  It  should  be  noted  that 
the  development  of  a  feather  is  similar  to  that  of  a  reptilian  scale,  involving  a  dermal  papilla  for 
nutritive  purposes  and  an  ectodermal  thickening. 

3.  Structure  of  a  contour  feather. — This  is  the  common  type  of  feather 
which  covers  the  bodies  of  birds.  Obtain  one  and  identify  the  following  parts. 
The  short,  bare,  hollow  portion  the  lower  end  of  which  is  inserted  into  the  feather 
follicle  of  the  skin  is  the  calamus  or  quill;  the  remaining  expanded  portion,  con- 
stituting most  of  the  feather,  is  the  vane.  The  quill  bears  two  openings,  the 
inferior  umbilicus  at  its  proximal  end,  which  was  inserted  in  the  skin,  and  through 
which  in  the  early  stages  of  the  feather,  the  dermal  papilla  passes;  and  the  superior 
umbilicus  at  the  junction  of  vane  and  quill  on  the  ventral  surface  of  the  feather. 
From  the  superior  umbilicus  protrudes  a  more  or  less  well-developed  accessory 


periderm 


umbilical 
groove     rachis 

\  \ 


stratum 
corneum 


FIG.  14. — Diagrams  of  the  development  of  the  down  and  contour  feathers.  A,  dermal  papilla. 
J5,  cross-section  of  a  later  stage  of  the  dermal  papilla  showing  the  thickenings  of  the  stratum  germina- 
tivum. C,  cross-section  of  a  later  stage;  each  thickening  has  separated  to  form  a  barb;  the  dermis  in 
the  center  of  the  papilla  has  degenerated  into  the  pith;  the  stratum  corneum  forms  the  periderm  or 
sheath  of  the  down  feather.  D,  section  across  a  developing  contour  feather,  showing  the  two  enlarged 
thickenings  or  barbs  which  become  the  rachis  and  the  oblique  course  of  the  other  barbs.  (A-C  from 
Wiedersheim's  Comparative  Anatomy  of  Vertebrates,  courtesy  of  the  Macmillan  Company;  D  after 
Kingsley's  Comparative  Anatomy  of  Vertebrates,  copyright  by  P.  Blakiston's  Son  and  Company.) 

feather  called  the  aftershaft,  consisting  of  only  a  few  tufts  in  some  birds  but  in 
others  of  a  complete  feather,  nearly  or  quite  as  large  as  the  primary  feather. 

The  vane  consists  of  a  central  axis  known  as  the  shaft  or  rachis  which  is  continu- 
ous with  the  quill  and  of  a  sort  of  web  or  membrane  springing  from  each  side  of  the 
rachis.  Extending  along  the  ventral  surface  of  the  rachis  is  a  groove,  the 
umbilical  groove.  The  web  or  membrane  of  the  feathers  is  obviously  composed 
of  a  large  number  of  parallel,  obliquely  placed  rays,  adhering  to  each  other. 
These  rays  are  the  barbs,  and  each  barb  bears  side  rays  called  barbules,  exactly 
as  in  the  down  feather.  The  barbules  interlock,  causing  the  barbs  to  adhere  to 
produce  an  unbroken  surface.  To  see  the  method  of  interlocking  of  the  barbules 
it  is  necessary  to  examine  a  small  piece  of  the  feather  under  the  microscope.  It 
will  then  be  noted  that  the  barbules  of  one  side  of  each  barb  (side  toward  the  quill) 


54       LABORATORY  MANUAL  FOR  VERTEBRATE  ANATOMY 

are  placed  diagonally  across  the  barbules  of  the  other  side  (side  toward  the  tip  of 
the  feather)  of  the  adjoining  barb,  and  that  the  former  are  provided  with  hooklike 
projections  which  fit  oyer  and  catch  flangelike  extensions  of  the  latter.  The 
diagonal  arrangement  permits  one  hooked  barbule  to  catch  a  number  of  flanged 
barbules.  By  pulling  on  the  barbs  one  can  unhook  the  barbules,  and  by  stroking 
the  barbs  one  can  hook  them  up  again.  The  latter  process  is  the  chief  object  of 
the  frequent  preening  of  the  feathers  habitual  with  birds.  In  the  rectrices  and 
remiges  the  booklets  are  well  developed  throughout  so  that  the  barbs  are  inter- 
locked over  the  entire  feather;  -but  in  the  coverts  the  lower  barbules  lack  the 
booklets,  and  the  lower  barbs  are  loose  and  fluffy.  In  some  birds,  as  the  ostrich, 
all  of  the  barbules  lack  hooks,  and  the  whole  feather  is  fluffy. 
Draw  a  contour  feather,  showing  its  parts. 

4.  Development  of  the  contour  feather. — Contour  feathers  are  similar  in  structure  and 
development  to  down  feathers,  except  that  in  the  latter  the  barbs  spring  directly  from  the  quill 
hi  a  circle,  while  in  the  former  they  spring  in  a  row  from  either  side  of  a  central  axis.     This  is 
due  to  the  fact  that  in  the  development  of  the  contour  feather  two  of  the  longitudinal  columns 
formed  by  the  stratum  germinativum  become  much  enlarged  and  fuse  to  produce  the  rachis, 
the  groove  between  them  remaining  as  the  umbilical  groove.    This  enlargment  of  these  two 
columns  imparts  an  oblique  position  to  the  other  columns,  which  become  the  barbs,  since  the 
rachis  grows  faster  than  the  barbs.    The  contour  feather  is  rolled  up  inside  the  periderm,  the 
future  dorsal  or  outer  surface  being  outside  next  to  the  periderm,  while  the  future  ventral  or 
inner  surface  is  inside  next  to  the  dermal  papilla.     When  the  periderm  splits,  the  feather  flattens 
out.    The  quill  is  the  base  of  the  feather  which  failed  to  split  into  barbs;    the  white  flaky 
material  noticeable  inside  the  quill  is  the  dried  remains  of  the  dermal  papilla  (see  Fig.  14; 
also  K,  p.  37,  Fig.  28). 

5.  Feather   tracts. — Birds   appear   to  be  completely  covered  by  contour 
feathers,  but  actually  the  feathers  are  borne  only  by  certain  areas  of  the  skin 
called  feather  tracts  or  pterylae,  with  featherless  areas,  aplerylae,  between  them. 
Observe  the  feather  tracts  on  a  demonstration  specimen  of  a  young  bird  and  con- 
sult also  K,  Figure  26,  page  35,  or  P  and  H,  Figure  1037,  page  372. 

6.  Structure  of  the  filoplume. — The  filoplumes  are  the  "  hairs"  visible  on  a 
plucked  bird.     Examine  a  prepared  slide  or  remove  a  filoplume  from  a  plucked 
bird,  mount  on  a  slide  in  a  drop  of  water,  and  examine  under  the  low  power.     It 
consists  of  a  main  axis  bearing  a  few  terminal  barbs.     It  is  a  miniature  degener- 
ated contour  feather.     Draw. 

H.      EXOSKELETON   OF  MAMMALS 

The  exoskeleton  of  mammals  consists  primarily  of  hair,  found  in  no  other 
vertebrates.  Some  mammals  are  provided  with  scales  in  addition. 

i.  Structure  of  hair. — By  examining  your  own  skin  determine  that  each 
hair  springs  from  a  pit  in  the  skin,  known  as  a  hair  follicle.  Remove  a  fine  hair 
from  the  under  side  of  the  forearm  and  examine  with  high  powers  of  the  micro- 
scope. Observe  by  focusing  on  the  surface  of  the  hair  the  irregular  wavy 


THE  INTEGUMENT  AND  THE  EXOSKELETON  55 

outlines  of  the  cells  of  which  it  is  composed.  Examine  with  the  microscope  a 
cross-section  through  the  skin  containing  growing  hairs,  and  study  a  hair  follicle. 
Each  is  a  deep  pit  in  the  skin  and  is  lined  by  epidermis.  At  the  bottom  of  the 
follicle  the  dermis  forms  a  small  bulb-shaped  enlargement,  the  hair  papilla.  The 
stratum  germinativum  over  this  papilla  is  seen  proliferating  a  conical  heap  of  cells 
from  which  the  hair  arises.  This  cone  extends  up  the  follicle  and  after  a  short 
distance  a  split  is  seen  separating  a  central  shaft,  the  root  of  the  hair,  from  the 
walls  of  the  follicle.  The  transition  from  the  ordinary  epithelial  cells  of  the  pro- 
liferating area  to  the  horny  cells  of  the  root  of  the  hair  is  readily  observable.  The 
lining  of  the  follicle  is  also  somewhat  cornified,  and  is  known  as  the  outer  root 
sheath.  It  is  the  white  coat  which  clings  to  the  roots  of  hairs  when  they  are 
pulled  from  their  follicles.  From  this  account  it  follows  that  hairs  are  epidermal 
structures  produced  by  the  activity  of  the  stratum  germinativum.  Draw  a  hair 
follicle,  showing  its  structure.  Further  details  will  be  found  in  textbooks  of 
histology. 

2.  Scales  of  mammals. — A  number  of  mammals  possess  horny  scales  like 
those  of  reptiles.  These  may  cover  the  body,  as  in  the  scaly  anteater  (Manis) ,  fig- 
ured in  P  and  H,  page  485,  and  N,  page  375,  but  commonly  occur  on  the  tails  only, 
as  in  rats,  beavers,  and  a  number  of  other  mammals.  Such  scaly  parts  are  also 
provided  with  scanty  hairs.  Examine  a  rat's  tail  and  observe  the  scales  and 
hairs  upon  it. 

The  armadillos  are  the  only  living  mammals  which  possess,  like  the  turtle,  an 
armor  composed  of  both  epidermal  scutes  and  dermal  plates.  Obtain  a  dried 
armadillo  armor  and  examine  the  external  surface.  It  is  made  up  of  an  anterior 
shield  of  small  polygonal  scales,  a  middle  region  composed  of  nine  movable  bands 
with  bare  areas  of  skin  between  them,  and  a  posterior  shield  similar  to  the  anterior 
shield.  The  outer  surface  of  the  armor  consists  of  thin  horny  epidermal  scales  or 
scutes,  polygonal  on  the  shields,  triangular  on  the  bands.  The  triangular  scutes  are 
alternately  reversed  in  position,  so  that  in  half  of  them  the  apex  points  anteriorly, 
and  in  half  posteriorly.  The  former  bear  hairs  at  their  posterior  margins.  How 
many  hairs  to  each  scute  ?  Draw  some  scutes  and  hairs  to  show  their  relation. 
Turn  the  armor  over  and  study  the  internal  surface.  It  is  composed  of  bony 
plates  of  dermal  origin,  polygonal  on  the  shields,  rectangular  on  the  bands. 
With  a  knife  point  scrape  off  some  of  the  epidermal  scutes;  note  relative  thickness 
of  scutes  and  plates,  differences  in  the  materials  of  which  they  are  composed — one 
of  horn,  the  other  of  bone — and  the  impressions  left  on  the  plates  by  the  scutes. 

Read  W,  pages  87-97,  on  the  relation  of  scales  to  hair  and  to  the  friction  ridges. 

I.     SUMMARY 

1.  The  vertebrate  skin  consists  of  an  outer  epidermis,  ectodermal  in  origin  and  composed  of 
epithelial  cells,  and  an  inner  dermis,  mesodermal  in  origin  and  composed  of  connective  tissue. 

2.  Exoskeletal  structures  are  produced  by  the  skin,  either  by  the  epidermis  or  the  dermis 
or  both. 


56        LABORATORY  MANUAL  FOR  VERTEBRATE  ANATOMY 

3.  Epidermal  exoskeletal  structures  consist  of  horn  and  are  composed  of  numerous  flat- 
tened dead  cornified  cells  pressed  together.    These  cells  arise  by  proliferation  of  the  lower  layers 
of  the  epidermis  (stratum  germinativum).     Of  such  nature  are  the  superficial  scales  of  reptiles, 
birds,  and  mammals,  feathers,  hair,  claws,  nails,  beaks,  hoofs,  horns  such  as  those  of  cattle. 

4.  Dermal  exoskeletal  structures  consist  of  bone  secreted  by  the  mesenchyme  cells  of  the 
dermis.     Of  such  nature  are  the  scales  of  fishes — in  whole  or  large  part,  the  bony  plates  of 
reptiles  and  mammals,  the  horns  of  deer  and  antelope,  the  fin  rays  of  fishes,  the  plates  of  recent 
and  extinct  Amphibia. 

5.  Certain  exoskeletal  structures  contain  both  dermal  and  epidermal  constitutents  inextri- 
cably fused.     Such  are  the  placoid  scales  of  the  elasmobranch  fishes  and  the  teeth  of  all  verte- 
brates.   By  the  identity  of  their  structure  and  mode  of  development  these  two  structures  are 
shown  to  be  homologous. 

6.  In  the  formation  of  exoskeletal  parts  a  dermal  papilla  is  involved,  which  furnishes 
nutrition  for  the  proliferating  or  secreting  cells.     In  the  case  of  dermal  structures  the  cells  of  the 
papilla  give  rise  to  the  structure.     In  the  case  of  epidermal  exoskeleton  the  papilla  takes  no 
part  in  the  formation  of  the  structure  but  simply  brings  a  blood  supply  to  the  developing  part 
which  arises  solely  by  proliferation  and  subsequent  cornification  of  epidermal  cells. 

7.  From  the  foregoing  account  and  from  the  definition  of  homology  it  follows  that  all  exo- 
skeietal  structures  of  epidermal  origin  are  homologous,  and  that  similarly  all  exoskeletal 
structures  of  dermal  origin  are  homologous,  whatever  their  shape  or  form  or  function. 

8.  Epidermal  structures  exhibit  more  modifications  and  a  greater  complexity  of  structure 
than  do  dermal  parts. 

9.  There  has  been  no  definite  progressive  evolution  of  the  exoskeleton  throughout  the 
vertebrate  groups,  but  each  group  is  provided  with  exoskeletal  structures  correlated  with  its 
habits  and  mode  of  life.    There  is,  however,  a  certain  tendency  for  the  higher  vertebrates  to 
develop  complicated  epidermal  structures. 


VI.     THE  ENDOSKELETON:  THE  COMPARATIVE  ANATOMY  OF 
THE  VERTEBRAL  COLUMN  AND  RIBS 

A.      GENERAL  CONSIDERATIONS   ON  THE  ENDOSKELETON 

1.  The  parts  of  the  endoskeleton. — We  have  already  defined  the  endoskeleton  as  the 
internal  skeleton  of  the  body.    The  notochord  is  the  first  endoskeleton  of  the  chordates  and 
the  principal  endoskeleton  of  the  lower  chordates.    In  the  vertebrates  the  notochord  is  always 
more  or  less  replaced  by  a  skull  and  vertebral  column,  and  the  vertebrates  possess  in  addition 
other  components  of  the  endoskeleton  in  association  with  the  gills  and  paired  appendages. 
The  parts  of  the  endoskeleton  of  vertebrates  are:  the  skull  in  the  head;  the  visceral  skeleton 
composed  of  gill  arches  supporting  the  gills;  the  vertebral  column  occupying  the  median  dorsal 
region;   the  ribs,  projecting  from  the  vertebrae,  one  pair  to  each  vertebra  primitively;   the 
sternum,  occupying  the  median  ventral  region  of  the  anterior  part  of  the  trunk;  the  pectoral 
girdle  supporting  the  anterior  paired  appendages;   the  pelvic  girdle,  supporting  the  posterior 
paired  appendages;    and  the  skeleton  of  the  appendages.     The  four  parts  first  named  con- 
stitute the  axial  skeleton,  while  the  other  parts  constitute  the  appendicular  skeleton. 

2.  The   skeletogenous   regions. — The   endoskeleton  develops  from  mesenchyme.    The 
mesenchyme  accumulates  in  certain  regions  known  as  the  skeletogenous  regions  where  skeleton 
is  to  be  formed.     The  arrangement  of  these  skeletogenous  regions  is  to  some  extent  depend- 
ent on  the  disposition  of  the  myotomes.     As  we  have  already  learned,  the  myotomes  or  muscle 
segments,  which  consist  of  those  portions  of  the  epimeres  remaining  after  the  formation  of 
mesenchyme,  grow  down  between  the  skin  and  the  digestive  tract,  so  as  to  form  the  muscular 
layer  of  the  body  wall.    (Review  Fig.  10,  p.  43.)     Each  myotome  is  separated  from  the  adjacent 
ones  by  a  transverse  partition  or  plate  of  mesenchyme,  called  the  myoseptum  or  myocomma. 
Each  myotome  is  further  divided  into  a  dorsal  and  a  ventral  half  by  a  horizontal  partition, 
the  horizontal  skeletogenous  septum,  which  extends  from  the  notochord  to  the  level  of  the 
lateral  line  on  the  sides  of  the  body.    The  notochord  and  neural  tube  are  also  surrounded  by 
mesenchyme  which  extends  from  the  neural  tube  to  the  median  dorsal  line,  forming  the 
dorsal  skeletogenous  septum,  and  from  the  notochord  to  the  median  ventral  line  (in  the  tail) 
forming  the  ventral  skeletogenous  septum.    In  the  trunk  region  the  ventral  skeletogenous 
septum  is  naturally  split  into  two  septa,  ventrolaterally  situated,  on  account  of  the  inter- 
vention of  the  coelom.    The  horizontal,  dorsal,  and  ventral  septa  are,  it  is  to  be  understood, 
continuous  longitudinal  septa,  extending  the  length  of  the  body.    The  skeletogenous  septa 
are  illustrated  in  Figure  15,  also  in  K,  Figure  33,  page  41,  and  W,  Figure  34,  page  127.  As  their 
name  implies,  the  skeletogenous  septa  are  regions  of  skeleton  formation.    At  the  intersection  of 
every  myoseptum  with  the  medially  placed  mesenchyme  of  the  dorsal  and  ventral  septa  and  thai 
surrounding  the  notochord  and  neural  tube  a  vertebra  arises.    As  the  myosepta  are  segmentally 
repeated,  owing  to  the  primary  segmentation  of  the  myotomes,  it  follows  that  the  vertebrae 
are  also  segmentally  arranged  and  that  the  vertebrae  alternate  with  the  myotomes. 

3.  Cartilage  and  membrane  bones. — The  mesenchyme  in  the  process  of  forming  endo- 
skeleton first  produces  cartilage.  All  of  the  endoskeleton  proper  is  first  composed  of  cartilage, 
and  in  the  lower  vertebrates  the  endoskeleton  may  remain  wholly  or  partly  cartilaginous. 
The  endoskeleton  of  elasmobranchs,  for  example,  is  composed  entirely  of  cartilage.  Such 
cartilage  may  be  and  often  is  stiffened  by  the  deposition  within  it  of  calcium  salts.  In  such 
cases  the  cartilage  is  said  to  be  calcified.  In  most  vertebrates,  however,  the  cartilage  is  more 

57 


58        LABORATORY  MANUAL  FOR  VERTEBRATE  ANATOMY 

or  less  replaced  during  development  by  bone,  secreted  by  bone-forming  cells.  The  skeleton 
is  then  said  to  be  ossified.  Bone  produced  in  this  manner  by  replacement  of  pre-existing 
cartilage  is  known  as  cartilage  bone. 

Investigation  of  the  internal  skeleton  of  vertebrates  shows  that  not  all  of  their  bones 
arise  in  this  manner,  but  some  of  them  develop  directly  from  the  mesenchyme  without  pass- 
ing through  a  cartilage  stage.  Such  bones  are  called  dermal,  membrane,  or  investing  bones. 
They  are  really  derived  from  the  dermis  of  the  skin  and  are  therefore  dermal  plates  homologous 
to  ganoid  scales  and  to  the  plates  of  the  turtle's  armor.  They  are  consequently  parts  of  the 
exoskeleton.  They  have  sunk  inward  from  their  original  position  in  the  skin  and  have 
attached  themselves  to  the  endoskeleton  with  which  they  are  now  so  closely  associated  that 
they  must  be  considered  a  part  of  it. 

The  student  should  particularly  understand  that  cartilage  and  membrane  bones  look 
exactly  alike;  they  have  the  same  histological  structure  and  chemical  composition;  they  are 
both  completely  formed  bone;  it  is  not  possible  to  distinguish  them  by  examining  them. 
It  is  only  their  manner  of  origin  that  is  different.  In  order  to  determine  which  bones  of  the 


P   h      g 


FIG.  15. — Diagrams  to  show  the  skeleton-forming  septa  in  At  the  tail  region,  and  B,  the  trunk  region 
of  a  vertebrate,  a,  skin;  b,  neural  tube;  c,  notochord;  d,  blood  vessel;  e,  dorsal  skeletogenous  septum; 
/,  ventral  skeletogenous  septum;  g,  horizontal  skeletogenous  septum;  h,  myoseptum;  i,  epaxial  part  of 
the  myotome;  j,  hypaxial  part  of  the  myotome;  k,  coelom;  /,  intestine;  m-p,  cartilages  from  which 
the  vertebrae  are  formed — m,  basidorsal;  n,  interventral;  o,  basiventral;  p,  interdorsal — q,  inter- 
muscular  rib;  r,  subperitoneal  rib.  In  B  note  positions  of  the  vertebral  cartilages  and  ribs  with  respect 
to  the  skeletogenous  septa.  (A  after  Kingsley's  Comparative  Anatomy  of  Vertebrates,  copyright  by 
P.  Blakiston's  Son  and  Company;  B  from  Goodrich  in  Part  IX  of  Lankester's  Treatise  on  Zoology, 
courtesy  of  the  Macmillan  Company.) 

endoskeleton  are  cartilage  bones  and  which  are  membrane  bones  it  is  necessary  to  study  their 
embryonic  development.  This  has  been  done  for  the  majority  of  the  bones  of  the  skeleton. 
In  order  to  trace  the  homology  of  the  parts  of  the  skeleton  in  different  vertebrates  it  is  abso- 
lutely essential  to  know  which  bones  are  cartilage  bones  and  which  are  dermal.  This  informa- 
tion is  given  in  the  following  pages.  For  the  present  we  may  state  that  dermal  bones  occur 
in  connection  with  the  skull,  jaws,  and  pectoral  girdle  (these  parts  also  contain  cartilage  bones, 
of  course).  All  other  parts  of  the  endoskeleton  are  composed  wholly  of  cartilage  bone. 


B.      THE   EMBRYONIC   ORIGIN   OF  THE  VERTEBRAE  AND  RIBS 

i.  The  development  of  the  sclerotome. — The  axis  of  the  vertebrate  skeleton  is  the  vertebral 
column  or  backbone,  which  is  composed  of  a  longitudinal  series  of  bones,  each  of  which  is  called 
a  vertebra.  The  vertebrae  arise  from  the  sclerotomes.  We  have  already  learned  that  a 
sclerotome  is  a  mass  of  mesenchyme  originating  from  the  medial  wall  of  each  epimere  (see 


THE  ENDOSKELETON:  VERTEBRAL  COLUMN  AND  RIBS 


59 


Fig.  10,  p.  43).  The  sclerotome  migrates  to  the  median  region  of  the  embryo  and  surrounds  the 
notochord  and  neural  tube.  It  then  gives  rise  to  cartilage,  which  is  laid  down  around  the 
notochord.  The  vertebrae  may  remain  permanently  in  this  cartilaginous  state,  as  in  elasmo- 
branchs,  being  stiffened  by  calcification,  or  they  later  transform  into  bone,  as  in  the  majority 
of  vertebrates.  The  student  should  understand  clearly  that  the  vertebral  column  does  not 
come  from  the  notochord  but  is  formed  around  it.  The  notochord  is  inclosed  inside  the 
vertebral  column,  and  is  readily  located  in  this  position  in  the  more  primitive  fishes;  but  in 
most  vertebrates  it  is  gradually  squeezed  down  and  disappears  except  for  remnants  between 
the  vertebrae. 

A  sclerotome  does  not,  as  might  be  supposed,  produce  a  vertebra  as  one  piece.  The 
sclerotomes  are  segmentally  arranged,  there  being  a  sclerotome  medial  to  each  myotome.  It 
has  already  been  stated,  however,  that  the  vertebrae  alternate  with  the  myotomes  and  arise 
at  the  intersection  of  myosepta  with  the  median  skeletogenous  region.  This  result  is  achieved 
as  follows.  Each  sclerotome  soon  becomes  divided  by  a  vertical  split  into  an  anterior  or 
cranial  half,  posterior  to  the  plane  of  the  preceding  myoseptum,  and  a  posterior  or  caudal  half, 
anterior  to  the  plane  of  the  succeeding  myoseptum.  A  vertebra  arises  from  the  fusion  of  the 


ectoderm 


anterior  half 
of  sclerotome 


hord 


^  intersegmental 
artery 


A  B 

FIG.  1 6. — Two  stages  in  the  development  of  the  vertebrae  from  the  sclerotomes,  only  one  side  of 
the  body  being  shown.  A,  division  of  the  sclerotome  into  anterior  and  posterior  halves.  B,  union  of 
the  posterior  half  of  one  sclerotome  with  the  anterior  half  of  the  succeeding  sclerotome  to  form  a 
vertebra.  (From  Prentiss  and  Arey's  Textbook  of  Embryology,  courtesy  of  the  W.  B.  Saunders 
Company.) 

posterior  half  of  one  sclerotome  with  the  anterior  half  of  the  succeeding  sclerotome.  The 
anterior  half  of  one  side  of  the  vertebra  comes  from  the  posterior  half  of  a  sclerotome  and 
the  posterior  half  of  one  side  of  the  vertebra  from  the  anterior  half  of  the  sclerotome.  (It 
should  be  understood  that  the  sclerotomes  of  the  two  sides  of  the  body  axis  co-operate  in  the 
formation  of  each  vertebra.)  It  thus  happens  that  the  center  of  the  vertebra  intersects  a 
myoseptum  and  that  the  vertebrae  alternate  with  the  myotomes  (see  Fig.  16). 

2.  The  arcualia. — In  the  formation  of  a  vertebra  definite  paired  cartilages  known  as 
arcualia  arise  in  the  sclerotome.  Theoretically  there  are  four  pairs  of  arcualia  to  each  vertebra . 
The  posterior  halves  of  the  two  sclerotomes  of  each  segment,  which  form  the  anterior  half  of 
a  vertebra,  produce  two  pairs  of  arcualia — a  dorsal  pair  above  the  notochord,  called  the 
basidorsals,  and  a  ventral  pair  below  the  notochord,  called  the  basivenlrals.  Similarly 
the  anterior  halves  of  the  two  sclerotomes  of  each  segment,  which  together  form  the 
posterior  half  of  a  vertebra,  produce  a  dorsal  pair  of  arcualia,  the  interdorsals,  and 
a  ventral  pair,  the  interventrals  (see  Fig.  17).  There  may  be  other  elements  in  addition 
above  and  below  these,  such  as  the  supradorsals  and  infraventrals,  but  these  are  not 
of  general  occurrence.  The  members  of  the  chief  pairs  of  arcualia  tend  to  fuse  together 
medially  producing  arches  or  pieces  which  straddle  the  notochord  above  and  below.  Thus 


6o 


LABORATORY  MANUAL  FOR  VERTEBRATE  ANATOMY 


basidorsal 


notochord 


basidorsal 


basiventral 


neural  arch 

neural  canal 


notochord 


haemal  canal 


basiventral 


interventral 


FIG.  17. — Diagrams  to  show  the  arcualia  from  which  the  vertebrae  are  formed.  A,  side  view, 
showing  the  four  arcualia  of  one  side  to  each  segment.  B,  cross-section  through  the  basiventrals  and 
basidorsals,  showing  their  relation  to  the  notochord.  C,  later  stage  of  the  same  cross-section  as  in  B, 
showing  union  of  the  basidorsals  to  form  the  neural  arch  and  of  basiventrals  to  form  the  haemal  arch. 


•g 


E  F 

FIG.  18. — Diagrammatic  sections  through  developing  vertebrae  to  show  formation  of  the  arches 
and  the  centra.  A,  early  stage,  showing  the  skeletogenous  regions  b  around  the  notochord.  B-D, 
formation  of  the  chordal  type  of  centrum:  B,  appearance  of  the  arcualia/  and  the  sheath  of  the  noto- 
chord e;  C,  invasion  of  skeleton-forming  cells  from  the  arcualia  into  the  sheath  of  the  notochord,  as  at.;'; 
D,  completion  of  the  chordal  centrum  in  the  sheath  of  the  notochord,  completion  of  the  arches  by  fusion 
of  the  arcualia,  and  union  of  centrum  and  arches.  E  and  F,  formation  of  the  perichordal  or  arch  type 
of  centrum:  E,  appearance  of  the  arcualia/  and  the  sheath  of  the  notochord  e;  F,  formation  of  the 
centrum  and  the  arches  by  the  fusion  of  the  arcualia  around  and  above  and  below  the  notochord. 
a,  notochord;  b,  skeletogenous  regions;  c,  neural  canal;  d,  haemal  canal;  e,  sheath  of  the  notochord; 
/,  arcualia;  g,  neural  arch;  h,  centrum;  »",  haemal  arch.  (From  Wiedersheim's  Comparative  Anatomy 
of  Vertebrates,  courtesy  of  the  Macmillan  Company  ) 


THE  ENDOSKELETON:    VERTEBRAL  COLUMN  AND  RIBS 


61 


the  basidorsals  extend  dorsally  around  the  neural  tube  and  fuse  to  form  the  neural  arch  inclos- 
ing the  neural  tube;  the  interdorsals  similarly  form  an  intercalary  arch,  the  basiventrals,  a 
haemal  arch,  and  the  interventrals,  an  interhaemal  arch  (see  Fig.  17).  Thus  primitively  each 
segment  of  the  vertebrate  body  is  provided  with  two  dorsal  arches,  one  anterior  and  one 
posterior,  and  two  similarly  placed  ventral  arches.  Such  a  condition  actually  occurs  in  the 
adults  of  some  of  the  lower  vertebrates.  This  tendency  for  the  production  of  two  vertebrae 
to  each  segment  is  known  as  diplospondyly  (see  K,  pp.  51-53).  In  most  vertebrates,  however, 
there  is  a  single  vertebra  to  each  segment  (alternating  with  the  segment);  this  condition 
arises  through  the  loss  of  some  of  the  arcualia  or  their  fusion.  The  manner  of  formation  of 
the  vertebrae  from  the  arcualia  is  different  in  different  vertebrate  classes  and  is  described  in 
connection  with  each  group. 

3.  The  formation  of  the  centrum. — The  completed  vertebra  consists  not  only  of  the 
arches  arising  from  the  arcualia  but  also  of  a  central  mass,  the  body  or  centrum  of  the  vertebra. 
The  centrum  is  produced  in  two  ways  in  vertebrates:  (a)  by  developing  within  the  sheath  of 


neural  spin* 


neural  splnt 


transverse  proem 


haemal  spine 


FIG.  19. — Diagrams  of  typical  vertebrae.  A,  tail  vertebra  of  a  teleost  fish.  B,  trunk  vertebra 
of  the  same,  showing  opening  of  the  haemal  arch  to  form  the  transverse  processes.  C,  vertebra  of  a 
land  vertebrate,  showing  particularly  relation  of  ribs  to  the  centrum.  (After  Kingsley's  Comparative 
Anatomy  of  Vertebrates,  copyright  by  P.  Blakiston's  Son  and  Company.) 

the  notochord  and  becoming  secondarily  fused  to  the  arcualia;  (b)  from  the  arcualia,  either 
by  fusion  of  the  bases  of  the  arches,  or  from  entire  arcualia  (see  Fig.  18).  The  first  type  of 
centrum  is  called  a  chordal  centrum  and  is  characteristic  of  elasmobranchs;  the  second  type 
is  named  a  perichordal  or  arch  centrum  and  is  found  in  practically  all  other  vertebrates.  The 
centrum  in  both  cases  is  formed  around  the  notochord  which  is  inclosed  within  it. 

In  the  embryonic  development  of  a  vertebra  the  dorsal  arcualia  arise  first,  the  ventral 
arcualia  next,  and  the  centrum  last.  The  same  progression  is  observable  in  the  phylogeny 
of  vertebrates.  In  primitive  vertebrates  such  as  the  cyclostomes  only  the  dorsal  arcualia  are 
present,  straddling  a  large  persistent  notochord  (see  Fig.  2iA).  In  many  ganoids  all  of  the 
arcualia  appear  surrounding  the  large  notochord,  but  the  centrum  has  not  yet  developed; 
such  vertebrae  are  designated  as  acentrous  (Fig.  21  B).  In  the  higher  fishes  and  all  land 
vertebrates  well-developed  centra  are  present,  and  the  notochord  is  reduced  or  absent. 

4.  The  parts  of  a  typical  vertebra. — A  typical  completed  vertebra  consists  of  a  centrum 
inclosing  the  notochord,  a  dorsal  neural  arch  inclosing  the  neural  tube,  and  a  ventral  haemal 
arch  inclosing  blood  vessels.  Neural  and  haemal  arches  are  commonly  prolonged  dorsally 


62 


LABORATORY  MANUAL  FOR  VERTEBRATE  ANATOMY 


neural  spine 


and  ventrally,  respectively,  into  neural  and  haemal  spines.  In  addition  the  centrum  commonly 
presents  various  projecting  processes,  known  as  apophyses,  serving  for  articulations  with  adjoin- 
ing vertebrae  or  with  ribs  or  for  the  attachment  of  muscles.  In  lower  forms  there  are  not 
uncommonly  two  centra  or  arches,  one  anterior  and  one  posterior,  to  each  metamere,  but 
usually  only  one  of  these  structures  is  present  to  each  segment,  owing  to  the  loss  or  fusion  of 
the  others.  Figure  19  illustrates  a  typical  vertebra  with  its  parts. 

5.  The  origin  of  ribs. — Each  vertebra  is  theoretically  provided  with  a  pair  of  ribs  which 
project  out  from  the  centrum  into  the  body  wall  and  serve  to  strengthen  the  latter.  The 
ribs,  like  other  parts  of  the  axial  skeleton,  arise  in  the  skeletogenous  septa.  There  are  two 
kinds  of  ribs,  both  of  which  arise  in  the  myosepta  and  consequently  are  segmental  in  arrange- 
ment. One  type  of  rib  is  formed  at  the  intersection  of  each  myoseptum  with  the  horizontal 
skeletogenous  septum.  Since  the  horizontal  septum  divides  the  myotomes  into  dorsal  and  ven- 
tral halves  (see  Fig.  i$A,p.  58)  such  ribs  lie  between  muscles  and  hence  are  named  intermus- 
cular  ribs.  They  are  also  known  as  dorsal  or  true  ribs.  Intermuscular  ribs  are  characteristic  of 

the  majority  of  vertebrates.  The 
second  type  of  rib  arises  at  the 
points  of  intersection  of  the  myo- 
septa with  the  ventral  skeletoge- 
nous septum  or  its  derivatives.  It 
will  be  recalled  that  in  the  trunk 
region  the  ventral  septum  is  split 
into  two  lateral  septa,  owing  to 
the  intervention  of  the  coelom  and 
its  contents  on  the  ventral  side  of 
the  body  (see  Fig.  1 5$) .  The  ribs 
under  consideration  appear  at  the 
points  of  intersection  of  the  myo- 
septa with  these  ventrolateral 
septa.  They  lie  just  outside  the 
coelomic  lining,  between  the  coe- 
lomic  wall  and  the  muscle  layer. 
They  are  designated  as  subperi- 
toneal  ribs;  also  as  ventral  or  false 
ribs.1  Subperitoneal  ribs  are  char- 
acteristic of  the  bony  fishes.  Both 
types  of  ribs  may  be  regarded  as 

extensions  of  the  basiventral  arcualia,  their  position  in  the  myoseptum  being  shifted  dorsally 
or  ventrally  in  different  vertebrates.  In  many  fishes  both  kinds  of  ribs  are  simultaneously 
present  on  each  vertebra.  The  two  kinds  of  ribs  and  their  morphological  relations  to  the 
body  wall  are  illustrated  in  Figure  20. 

Ribs,  like  vertebrae,  appear  first  as  collections  of  mesenchyme  which  later  transforms 
into  cartilage.  The  ribs  are  permanently  cartilaginous  in  some  fishes  but  in  other  vertebrates 
are  ossified,  at  least  in  part. 

For  the  comparative  anatomy  of  the  vertebral  column  and  ribs,  the  student  should  con 
suit  K,  W,  and  Wd. 

1  The  term  pleural  as  applied  to  ribs  is  here  avoided,  owing  to  the  fact  that  it  is  differently  employed 
in  different  texts.  K  and  W  use  the  term  pleural  as  synonymous  with  intermuscular,  while  L  uses  it  as 
synonymous  with  subperitoneal.  The  latter  usage  appears  to  be  correct. 


.neural  tube 
neural  arch 
of  vertebra 
centrum  of 
vertebra 

epaxial  muscles 

notochord 

horizontal  skeletogenous 
septum 

intermuscular  rib 
subperitoneal  rib 
hypaxial  muscles 
dorsal  mesentery 


coelom 

intestine 

peritoneum 


FIG.  20. — Diagrammatic  cross-section  through  the  trunk 
of  a  vertebrate  showing  the  relation  of  the  ribs  to  the  skele- 
togenous regions  and  the  positions  of  the  two  kinds  of  ribs. 
(From  Parker  and  HaswelTs  Textbook  of  Zoology,  courtesy  of 
the  Macmillan  Company.) 


THE  ENDOSKELETON:   VERTEBRAL  COLUMN  AND  RIBS  63 

C.      SOME  PRIMITIVE  VERTEBRAL  COLUMNS 

It  has  already  been  stated  that  in  some  living  and  in  many  extinct  forms  the 
vertebrae  are  embryonic  in  structure,  consisting  of  separate  arcualia  and  without 
a  centrum.  In  other  cases  diplospondyly  is  evident.  We  may  glance  at  a  few 
of  these. 

i.  The  vertebral  column  of  the  sturgeon.— In  the  sturgeon  the  vertebral  col- 
umn is  very  primitive — acentrous,  and  consisting  of  separate  arcualia.  Examine 


B 


A 


FIG.  21. — Some  primitive  vertebral  columns.  A,  cyclostomes;  the  vertebral  column  consists  of  a 
row  of  arches,  two  arches  to  each  segment,  resting  on  the  dorsal  surface  of  the  notochord.  B,  sturgeon; 
the  vertebral  column  consists  of  separate  arcualia  partially  surrounding  the  notochord.  C,  tail  region 
of  Amia,  showing  the  two  centra,  k  and  /,  to  each  segment,  illustrating  diplospondyly.  D,  vertebrae 
of  an  extinct  amphibian  belonging  to  the  Stegocephala,  showing  the  arcualia  around  the  notochord. 
a,  segmental  blood  vessels;  b,  nerve;  c,  neural  tube;  d,  notochord;  e,  basidorsal  or  neural  arch;  /,  basi- 
ventral;  g,  interdorsal;  h,  interventral;  i,  neural  spine  or  supradorsal;  j,  rib;  k,  posterior  centrum; 
/,  anterior  centrum,  for  each  segment;  m,  haemal  arch;  n,  haemal  spine;  o,  blood  vessels.  (A  and  B 
after  Goodrich  in  Part  IX  of  Lankester's  Treatise  on  Zoology,  courtesy  of  the  Macmillan  Company, 
C  from  Zittell,  D  from  Kingsley's  Comparative  Anatomy  of  Vertebrates,  after  Zittell,  copyright  by 
P.  Blakiston's  Son  and  Company.) 

the  demonstration  specimens  and  compare  with  Figure  2iB.    The  large  noto- 
chord traverses  the  center  of  the  vertebral  column  and  is  covered  dorsally  and 


64       LABORATORY  MANUAL  FOR  VERTEBRATE  ANATOMY 

ventrally  by  the  cartilaginous  arcualia.  On  the  dorsal  side  are  the  large  basi- 
dorsals,  united  to  form  a  neural  arch  topped  by  a  neural  spine  (supradorsal) 
Between  the  bases  of  the  basidorsals  are  the  small  inter  dor  sals.  The  ventro- 
lateral  regions  of  the  notochord  are  covered  on  each  side  by  the  large  basiventrals 
each  of  which  bears  a  projecting  process.  Note  that  the  basiventrals  of  the  two 
sides  do  not  meet  below  the  notochord.  Between  the  ventral  portions  of  the 
basiventrals  are  the  small  inlerventrals. 

2.  The  tail  vertebrae  of  the  bowfin. — Examine  a  demonstration  specimen 
of  the  tail  of  the  bowfin.     Compare  with  Figure  2iC.     Note  that  the  notochord 
is  not  visible,  being  completely  inclosed  by  the  vertebrae.     In  the  latter  there 
is  an  alternation  of  a  vertebra  bearing  dorsal  and  ventral  arches,  with  one  devoid 
of  arches.     The  former  consists  of  the  fused  basidorsals  and  basiventrals,  form- 
ing neural  and  haemal  arches  and  a  central  centrum.     The  latter  consists  of  the 
fused  interdorsals  and  interventrals.     There  are  thus  two  vertebrae  to  each 
segment  of  the  body,  a  condition  not  uncommon  in  primitive  vertebrates. 

3.  Vertebrae  of  Stegocephala. — Study  Figure  2iD,  or  K,  Figure  4&B,  page  53. 
These  are  representations  of  the  vertebrae  of  extinct  Amphibia.     Each  vertebrae 
consists  of  six  pieces  of  which  four  are  visible  from  each  side.     There  is  a  dorsal 
arch  composed  of  the  fused  basidorsals,  a  large  ventral  piece  composed  of  the 
fused  basiventrals,  and  two  small  posterior  pieces  on  each  side,  the  dorsal  one 
being  the  interdorsal,  the  ventral  one,  the  interventral.     The  large  notochord 
is  inclosed  by  these  arcualia. 

D.   THE  VERTEBRAL  COLUMN  OF  THE  DOGFISH 

i.  Cross-section  of  the  tail. — Obtain  a  cross-section  through  the  tail  of  the 
dogfish  and  study  the  cut  surface.  Be  sure  that  you  have  a  section  passing 
through  the  junction  of  successive  vertebrae  and  not  through  the  center  of  a 
vertebra.1  The  center  of  the  section  contains  the  vertebra,  composed  of  carti- 
lage— a  clear,  relatively  soft  material.  Outside  of  the  vertebra  is  the  layer  of 
voluntary  muscles,  consisting  of  a  number  of  leaves,  each  separated  from  its 
neighbor  by  a  plate  of  connective  tissue,  the  myoseptum  or  myocomma.  Each 
leaf  is  a  myotome  or  muscle  segment.  The  myotomes  appear  in  whorls  in  the 
section  because  they  are  zigzag  in  form  like  those  of  Amphioxus.  Each  cross- 
section  will  consequently  cut  a  number  of  myotomes.  The  muscle  segments  are 
somewhat  indistinctly  divided  into  dorsal  and  ventral  portions  by  a  partition 
which  extends  from  the  central  region  of  the  vertebra  to  the  skin  where  it  meets 
a  line,  the  lateral  line,  running  along  the  outer  surface  of  the  skin.  This  connec- 
tive tissue  partition  is  the  horizontal  skeletogenous  septum.  The  muscles  above 
it  are  the  dorsal  or  epaxial  muscles,  those  below  it,  the  ventral  or  hypaxial 
muscles. 

1  When  the  section  passes  through  the  center  of  the  vertebra,  four  rays  forming  a  sort  of  cross  will 
be  seen  in  the  vertebra.  These  are  areas  of  calcification. 


THE  ENDOSKELETON:    VERTEBRAL  COLUMN  AND  RIBS  65 

Turning  now  to  the  vertebra  itself,  we  note  that  it  consists  of  a  central 
circular  concave  portion,  the  centrum  or  body;  dorsal  to  this  an  arch,  the  neural 
arch,  which  incloses  a  cavity,  the  neural  canal,  in  which  the  spinal  cord,  a  soft 
white  body,  is  located;  ventral  to  the  centrum,  another  arch,  the  haemal  arch, 
which  incloses  a  cavity,  the  haemal  canal,  containing  the  caudal  artery  and  vein. 
The  neural  arch  terminates  in  a  point,  the  neural  spine,  and  the  haemal 
arch  similarly  terminates  in  the  haemal  spine.  Observe  the  connective  tissue 
partitions  which  extend  from  the  neural  spine  to  the  median  dorsal  line  and 
from  the  haemal  spine  to  the  median  ventral  line.  These  are  the  dorsal  and 
ventral  skeletogenous  septa  and  they,  together  with  the  horizontal  skeletogenous 
septum  already  mentioned,  mark  the  chief  sites  of  skeleton  formation. 

Draw  the  section,  labeling  all  parts. 

2.  Sagittal  section  of  the  tail. — Obtain  or  make  a  median  sagittal  section 
through  a  piece  of  the  tail  of  a  dogfish.     The  vertebrae  form  a  row  in  the  center 
of  the  section.     Identify  the  centra  in  the  section.     Each  consists  of  two  some- 
what triangular  pieces,  apparently  separate,  the  rounded  apexes  of  the  triangles 
directed  toward  each  other,  the  whole  shaped  somewhat  like  an  hourglass.     The 
two  ends  of  the  centra  are  concave,  so  that  diamond-shaped  spaces  are  present 
between  successive  centra.     These  spaces  are  filled  with  a  soft,  gelatinous 
material,  the  notochord,  which  also  fills  the  canal  which  runs  through  the  center 
of  the  centrum,  between  the  apexes  of  the  triangular  halves  of  the  centra.     Centra 
of  this  shape,  concave  at  each  end,  are  known  as  amphicoelous  centra.     Above 
each  centrum  identify  the  neural  arch,  arching  over  the  neural  canal.     Between 
successive  neural  arches,  and  lying  therefore  dorsal  to  the  diamond-shaped  spaces 
between  the  centra,  observe  an  extra  arch,  inverted,  however,  with  apex  inclos- 
ing the  neural  canal.     This  is  the  intercalary  arch.     Below  the  centrum  is  the 
haemal  canal,  its  sides  formed  by  the  haemal  arches,  rectangular  in  section. 
Draw  the  section. 

The  centrum  of  the  dogfish  vertebrae  (and  of  the  vertebrae  of  all  the  elasmobranch 
fishes)  arises  within  the  sheath  of  the  notochord.  The  cells  which  produce  the  centra  migrate 
into  the  sheath  from  the  arcualia.  Centra  of  this  type  are  chordal  centra,  and  vertebrae  having 
such  centra  are  known  as  chordocentrous  vertebrae.  The  neural  arch  is  the  fused  basidorsals; 
the  intercalary  arch,  the  fused  interdorsals;  the  haemal  arch  is  the  fused  basiventrals;  inter- 
vent  rals  are  lacking  (Fig.  2  2^4).  The  vertebrae  of  elasmobranchs  are  permanently  cartilagi- 
nous with  the  notochord  persistent  in  the  center  of  the  centra. 

3.  Cross-section  of  the  trunk  region. — In  a  cross-section  of  the  anterior 
part  of  the  trunk  region  of  the  dogfish  identify  the  following  parts.     The  muscle 
segments  are  arranged  as  in  the  tail  region,  their  division  into  dorsal  and  ventral 
masses  being  well  marked  by  the  horizontal  skeletogenous  septum.     The  dorsal 
or  epaxial  muscles  above  the  septum  are  thick  masses,  but  the  ventral  or  hypaxial 
muscles  below  the  septum  form  a  thin  layer  inclosing  a  large  cavity,  the  body 
cavity  or  coelom,   lined  by  a  smooth  membrane,   the  pleuro peritoneum.     The 


66 


LABORATORY  MANUAL  FOR  VERTEBRATE  ANATOMY 


coelom  incloses  the  viscera,  some  of  which  will  be  observed  to  be  suspended  by  a 
delicate  membrane,  the  dorsal  mesentery,  from  the  median  dorsal  line  of  the  coe- 
lomic  wall.  Study  the  vertebra,  which  appears  in  the  middle  of  the  myotomes. 
It  consists  of  centrum  and  neural  arch,  similar  in  appearance  to  those  of  tail 
vertebrae;  but  the  haemal  arch  appears  to  be  absent.  It  is  represented  by  a 
pair  of  small  cartilages  at  the  sides  of  the  ventral  part  of  the  centrum.  These 
cartilages,  known  as  transverse  processes,  represent  the  stumps  of  the  haemal  arch, 


B 


n 


A  A:: 


V 

FIG.  22. — Diagrams  to  show  the  composition  of  the  vertebrae  in  the  different  vertebrate  classes, 
according  to  Gadow.  A,  elasmobranch,  vertebra  consisting  of  chordal  centrum  a,  basidorsals  b,  inter- 
dorsals  c,  and  basiventrals  d.  B,  teleost,  vertebra  composed  of  all  of  the  arcualia  fused  together. 
C,  pseudocentrous  vertebra  of  a  urodele;  I,  early  stage  showing  the  arrangement  of  the  arcualia; 
2,  adult  stage,  showing  fusion  of  the  arcualia  along  the  dotted  lines;  note  splitting  of  the  interdorsals 
and  interventrals  to  form  the  ends  of  the  vertebra.  D,  notocentrous  vertebra  of  anurans;  I,  early 
stage,  showing  the  arcualia  which  form  the  vertebra;  2,  adult  stage,  arcualia  fused  along  the  dotted 
lines;  note  end  of  the  procoelous  centrum  formed  by  the  interdorsal.  E,  gastrocentrous  vertebra  of 
reptiles,  birds,  and  mammals,  centrum  formed  of  the  interventral  e,  the  basiventral  d  functioning  as 
the  intervertebral  cartilage.  F,  composition  of  the  atlas  and  axis  of  the  alligator,  according  to  Gadow; 
the  centrum  of  the  atlas  z  functions  as  the  odontoid  process  of  the  axis,  a,  chordal  centrum;  b,  basi- 
dorsal  or  neural  arch;  c,  interdorsal  (intercalary  arch  in  A);  d,  basiventral  or  haemal  arch;  e,  inter- 
ventral  (centrum  in  E  and  F);  x,  atlas;  y,  axis;  z,  odontoid  process  (interventral  of  atlas).  (F  after 
Gadow  in  the  Cambridge  Natural  History,  courtesy  of  the  Macmillan  Company.) 

which  may  be  regarded  as  having  opened  out  and  shifted  to  a  more  lateral  posi- 
tion, the  arch  portion  disappearing.  Examine  the  horizontal  skeletogenous  sep- 
tum carefully  and  find  within  it,  by  picking  away  the  muscles  if  necessary,  a 
slender  cartilage  on  each  side,  which  articulates  with  the  transverse  process. 
These  cartilages  are  ribs,  and  since  they  are  located  in  the  horizontal  skeletoge- 
nous septum  they  are  intermuscular  ribs.  There  is,  of  course,  a  pair  of  such  ribs 
to  each  vertebra.  Draw  the  section. 


THE  ENDOSKELETON:    VERTEBRAL  COLUMN  AND  RIBS  67 

E.      VERTEBRAL   COLUMN   OF   TELEOSTS 

1.  The  tail  vertebrae. — Examine  a  separate  dried  tail  vertebra  of  any  bony 
fish.     Note  that  the  vertebra  is  very  much  harder  and  more  opaque  than  the 
dogfish  vertebrae,  owing  to  the  fact  that  it  is  composed  of  bone.     Identify  the 
same  parts  as  already  seen  in  the  dogfish  vertebrae:  the  biconcave  or  amphicoe- 
lous  centrum  bearing  a  minute  canal  in  its  center  for  the  notochord,  the  neural 
arch  terminating  in  a  very  long  sharp  neural  spine,  the  haemal  arch  terminating 
in  a  similar  haemal  spine.     The  neural  canal  or  space  inclosed  within  the  neural 
arch  is  generally  smaller  than  the  haemal  canal  inclosed  by  the  haemal  arch;  in  this 
way  the  dorsal  and  ventral  sides  of  the  vertebrae  may  be  distinguished.     The 
spines  are  directed  posteriorly.     In  some  fish  there  are  two  neural  spines  to  each 
vertebra,  an  anterior  and  a  posterior  one;  the  second  one  probably  corresponds 
to  the  intercalary  arch  of  the  dogfish.     Draw  a  vertebra. 

2.  The  trunk  vertebrae. — Obtain  a  separate  dried  trunk  vertebra  of  a  bony 
fish.     Identify  as  before  the  centrum  and  the  neural  arch  and  neural  spine. 
The  haemal  arch  and  spine  appear  to  be  missing.     Instead  there  is  a  pair  of 
projections  at  the  sides  of  the  base  of  the  centrum  to  each  of  which  a  long  slender 
rib  is  articulated.     These  stumps  or  transverse  processes  are  the  opened  bases  of 
the  haemal  arch.     Draw  a  vertebra  with  ribs. 

The  centra  of  teleost  vertebrae  are  arch  centra  formed  by  the  fusion  of  the  bases  of  the 
basidorsals  and  basiventrals  chiefly,  although  interdorsal  and  interventral  elements  may  be 
added  to  the  ends  of  the  centra  (Fig.  22^).  The  neural  arch  is  formed  by  the  basidorsals, 
the  haemal  arch  by  the  basiventrals. 

3.  Section  through  the  trunk  of  a  bony  fish. — In  such  a  section  identify 
the  parts  already  described  for  a  similar  cross-section  of  the  dogfish.     Note  the 
muscle  segments,  the  centrum  and  neural  arch  and  spine  of  the  vertebra,  and 
the  coelom  with  its  lining.     Find  the  ribs  located  just  outside  of  the  coelomic 
lining.     The  ribs  are  in  a  different  position  than  those  of  the  dogfish,  which  are 
in  the  horizontal  skeletogenous  septum.    The  ribs  of  teleosts  are  situated  just 
outside  of  the  coelomic  walls.     They  are  false  or  subperitoneal  ribs.     Make  a 
diagram  of  the  section,  showing  the  position  of  the  ribs. 

4.  Further  study  of  ribs. — Some  fishes  have  two  (or  more)  pairs  of  ribs 
simultaneously  on  each  vertebra.     Examples  are  Polypterus,  a  crossopterygian 
ganoid  from  Africa,  and  many  teleosts,  including  members  of  the  salmon,  herring, 
and  pike  families.     Examine  the  skeleton  of  Polypterus  and  note  two  pairs  of 
ribs  attached  to  each  vertebra.     The  dorsal  pair  attached  to  the  transverse 
processes  of  the  vertebrae  are  the  true  or  intermuscular  ribs;   the  ventral  pair 
loosely  attached  to  the  ventral  surface  of  the  centrum  are  the  false  or  subperito- 
neal ribs.     Vertebrae  of  fishes  like  the  salmon  may  also  be  examined,  or  sections 
through  the  trunks  of  such  fishes.     Note  the  ventral  or  subperitoneal  ribs,  and 
the  dorsal  or  intermuscular  ribs  (also  called  epipleurals)  located  in  the  horizontal 


68       LABORATORY  MANUAL  FOR  VERTEBRATE  ANATOMY 

skeletogenous  septum.  Additional  ribs  may  also  be  present,  articulated  with 
centrum  or  neural  arch  and  extending  out  into  the  myosepta  between  myotomes. 
It  appears  that  in  teleosts  ribs  may  be  formed  at  almost  any  level  of  the 
myosepta.  Make  a  diagram  showing  the  vertebra  and  its  ribs. 

5.  The  vertebral  column  as  a  whole. — Study  the  entire  mounted  skeleton 
of  a  bony  fish.  Note  that  the  axis  and  strongest  part  of  the  column  consists  of 
the  centra,  which  are  jointed  together  to  form  a  longitudinal  series.  The 
vertebral  column  is  divisible  into  two  regions,  a  trunk  region  and  a  tail  region. 
In  the  former  the  haemal  arches  are  reduced  to  stumps,  the  transverse  processes, 
to  the  ends  of  which  the  long  slender  ribs  are  articulated.  In  the  tail  region 
ribs  are  lacking  and  haemal  arches  are  present.  Observe  the  transition  between 
trunk  and  tail  regions.  In  the  posterior  part  of  the  trunk  there  is  a  gradual 
elongation  of  the  transverse  process  and  a  reduction  of  the  ribs.  At  the  beginning 
of  the  tail  region  the  reduced  ribs  finally  vanish,  and  the  elongated  transverse 
processes  fuse  to  form  the  haemal  arch.  Note  that  the  neural  canals  of  successive 
vertebrae  together  form  a  continuous  canal  which  in  life  contains  the  spinal 
cord.  Similarly  the  haemal  canals  of  the  tail  region  form  a  continuous  cavity 
inclosing  in  life  blood  vessels.  At  the  end  of  the  tail  observe  the  enlargement 
and  flattening  of  the  haemal  arches  forming  what  are  called  hypural  bones,  which 
support  the  tail  fin.  The  last  centrum  (probably  consisting  of  several  centra) 
forms  an  elongated  slender  bone  (the  urostyle)  which  turns  sharply  upward, 
producing  an  asymmetry;  but  the  symmetry  of  the  tail  is  restored  by  the  arrange- 
ment of  the  hypural  bones.  Tails  of  this  kind  are  called  homocercal. 

F.      VERTEBRAL  COLUMN   OF  AMPHIBIA 

i.  The  vertebral  column  of  urodeles. — Whole  skeletons  of  urodeles  such 
as  Necturus  and  Cryptobranchus  are  needed  for  this  study.  With  such  a  skeleton 
before  you  note  the  parts  of  the  vertebrae  of  different  regions.  The  tail  vertebrae 
are  similar  to  the  tail  vertebrae  of  fishes.  Each  possesses  a  low  neural  arch,  a 
haemal  arch,  and  a  centrum  bearing  transverse  processes.1  In  the  trunk  region 
the  haemal  arch  is  absent,  and  the  transverse  processes  bear  ribs,  separated  from 
them  by  a  suture.  The  vertebrae  are  articulated  to  each  other  by  processes 
known  as  zygapophyses.  These  consist  of  a  projection  on  each  side  of  the  neural 
spine  which  fits  over  a  similar  projection  arising  from  the  anterior  end  of  the 
succeeding  vertebra.  Thus,  each  vertebra  has  a  pair  of  prezygapophyses  on  its 
anterior  end  whose  articulating  surfaces  face  upward,  and  a  pair  of  post- 
zygapophyses  on  its  posterior  end  whose  articular  surfaces  face  downward. 
These  zygapophyses  yoke  the  vertebrae  together.2 

1  In  fishes  the  transverse  processes  were  stated  to  be  the  stumps  of  the  haemal  arches.  It  seems 
probable  that  the  transverse  processes  on  the  vertebrae  of  land  vertebrates  are  not  of  this  kind  but  are 
lateral  outgrowths  of  the  centrum. 

3  Zygapophyses  are  poorly  developed  or  absent  in  fishes. 


THE  ENDOSKELETON:    VERTEBRAL  COLUMN  AND  RIBS  69 

The  vertebral  column  of  urodeles  consists  of  four  regions,  not  very  sharply 
marked  off  from  each  other.  The  first  region,  the  cervical  or  neck  region,  con- 
sists of  one  vertebra,  the  first  vertebra  or  atlas,  just  behind  the  skull  and  serving 
as  a  support  for  the  skull.  The  atlas  lacks  ribs.  Following  the  cervical  region 
is  the  long  trunk  region  of  similar  rib-bearing  vertebrae.  There  is  next  a  region 
consisting  of  but  one  vertebra.  This  is  the  vertebra  to  whose  ribs  the  hind  legs 
are  attached.  It  is  called  the  sacral  vertebra  and  its  ribs  are  known  as  sacral 
ribs.  This  region  of  the  vertebral  column  is  the  sacral  region  or  sacrum.  Pos- 
terior to  the  sacrum  is  the  caudal  or  tail  region,  composed  of  vertebrae  which 
lack  ribs  and  bear  haemal  arches  in  most  cases.  By  moving  the  vertebrae  apart 
note  that  the  ends  of  the  centra  are  concave,  that  is,  the  centra  are  amphicoelous 
as  in  most  fishes. 

The  attached  ends  of  the  ribs  (Necturus)  are  forked  into  two  processes  or 
heads,  a  dorsal  tubercular  head  and  a  ventral  capitular  head  which  articulate 
with  similar  but  less  marked  dorsal  and  ventral  projections  of  the  transverse 
processes.  The  ribs  are  intermuscular  ribs. 

The  vertebrae  of  the  urodele  Amphibia  differ  from  those  of  all  other  land  vertebrates  in 
the  manner  in  which  the  centra  are  formed.  In  the  tail  region  the  centra  are  produced  by 
the  fusion  of  the  bases  of  basidorsals  and  basiventrals,  the  former  giving  rise  also  to  the  neural 
arches,  the  latter  to  the  haemal  arches.  In  the  trunk  region  the  centra  and  neural  arches 
are  formed  from  the  basidorsals,  the  basiventrals  having  disappeared.  Interdorsals  and 
interventrals  are  absent  or  represented  by  the  well-developed  intervertebral  cartilages,  form- 
ing pads  between  successive  centra.  In  some  urodeles  these  cartilages  split,  half  fusing  to 
the  vertebra  in  front  and  half  to  the  vertebra  behind.  The  vertebrae  of  urodeles  are  desig- 
nated as  pseudocentrous  vertebrae,  as  the  centrum  does  not  correspond  to  that  of  the  majority 
of  land  vertebrates  (see  Fig.  22C,  p.  66). 

2.  The  vertebral  column  of  an  anuran  amphibian,  the  frog. — The  vertebral 
column  of  the  frog  is  specialized  in  that  the  caudal  vertebrae  are  all  united 
into  one  piece,  the  urostyle  or  coccyx.  There  is,  as  in  Necturus,  one  cervical 
vertebra,  the  atlas.  This  is  followed  by  seven  trunk  vertebrae.  These  have 
low  neural  arches,  no  haemal  arches,  and  centra  bearing  long  transverse  processes. 
Ribs  are  apparently  absent,  but  embryological  studies  show  that  the  distal  part 
of  the  transverse  process  is  a  rib,  which  has  become  indistinguishably  fused  to 
the  process.  There  is  one  sacral  vertebra  supporting  the  hind  legs  and  beyond 
this  the  long  urostyle  already  mentioned.  The  vertebrae  are  yoked  together 
by  zygapophyses  as  in  Necturus.  In  single  vertebrae  from  the  frog  note  that 
the  posterior  end  of  the  centrum  is  rounded  like  a  ball,  while  the  anterior  end  is 
concave.  Such  centra  are  procoelous  and  fit  together  in  a  ball-and-socket  manner. 

The  centra  of  the  vertebrae  of  Anura  are  formed  by  the  union  of  the  basidorsals  and 
basiventrals  as  in  urodeles,  with  the  difference  that  the  interdorsals  are  fused  to  either  the 
anterior  or  posterior  end  of  the  centrum.  In  the  frog  the  interdorsals  are  fused  to  the  posterior 
end  forming  the  ball-shaped  projection.  Interventrals  are  lacking.  Vertebrae  of  this  type 
are  called  notocentrous  (see  Fig.  22D,  p.  66). 


70  LABORATORY  MANUAL  FOR  VERTEBRATE  ANATOMY 

G.      VERTEBRAL   COLUMN   OF   REPTILES 

The  vertebral  column  of  reptiles  reaches  a  degree  of  differentiation  consider- 
ably in  advance  of  that  of  the  Amphibia  and  fishes. 

i.  The  vertebral  column  of  the  alligator. — Study  the  whole  mounted  skeleton 
of  the  alligator.  Observe  that  the  column  is  differentiated  into  five  regions: 
the  cervical  or  neck  region,  much  longer  than  in  Amphibia;  the  thoracic  region 
bearing  long  ribs;  the  lumbar  region,  without  ribs;  the  sacral  region,  composed 
of  two  fused  vertebrae,  supporting  the  hind  limbs;  and  the  caudal  region.  The 
tail  vertebrae  are  the  most  primitive  in  form  and  will  be  studied  first*.  Each 
consists  of  a  large  centrum,  a  neural  arch  with  a  high  neural  spine,  prominent 
transverse  processes  directed  straight  out  from  the  centrum,  and  a  haemal  arch, 
missing  on  the  first  caudal  vertebra.  The  haemal  arches  are  relatively  small  and 
situated  at  the  posterior  end  of  the  centrum.  They  are  often  referred  to  in 
texts  as  chevron  bones.  Toward  the  end  of  the  tail  the  various  processes  project- 
ing from  the  vertebrae  tend  to  become  reduced  and  finally  vanish  altogether,  so 
that  the  last  vertebrae  consist  of  centra  only.  The  sacral  region  consists  of  two 
vertebrae,  each  bearing  a  stout  sacral  rib,  to  which  the  supports  of  the  hind 
limbs  are  articulated.  The  sacral  vertebrae  have  high  neural  spines  but  lack  the 
haemal  arches.  The  lumbar  vertebrae,  anterior  to  the  sacrum,  are  five  in 
number,  possess  high  neural  spines  and  broad  transverse  processes.  There  are 
ten  thoracic  vertebrae,  similar  in  form  to  the  lumbar  vertebrae,  but  bearing 
long  ribs,  which  reach  the  median  ventral  line.  There  are  nine  cervical  vertebrae, 
of  which  the  two  first,  known  as  the  atlas  and  the  axis,  are  somewhat  different 
from  the  other  seven.  These  latter  have  strong  neural  arches  with  long  neural 
spines  and  short  transverse  processes  bearing  ribs.  Most  of  these  cervical  ribs 
are  of  peculiar  form,  being  V-shaped  and  attached  to  the  cervical  vertebrae  by 
the  two  ends  of  the  V.  Cervical,  thoracic,  lumbar,  and  sacral  vertebrae  lack 
haemal  arches,  but  the  cervical  and  anterior  thoracic  vertebrae  possess  short 
ventral  projections,  the  hypapophyses,  on  the  centra.  All  of  the  vertebrae  are 
provided  with  well-developed  pre-  and  postzygapophyses.  By  moving  the 
vertebrae  apart  note  that  the  centra  are  procoelous,  i.e.,  concave  in  front,  con- 
vex behind. 

The  first  two  cervical  vertebrae,  the  atlas  and  the  axis,  are  very  remarkable 
and  deserve  further  attention.  They  are  remarkable  in  that  the  parts  which 
compose  the  primitive  vertebra  are  retained  in  these  two  vertebrae  almost 
unchanged  (Fig.  22^).  The  atlas  or  first  cervical  vertebra  is  composed  of  four 
separate  pieces,  which  together  form  a  ring.  The  ventral  side  of  the  ring  is 
composed  of  a  median  piece,  the  fused  basiventrals.  It  bears  a  pair  of  long 
movable  ribs.  The  sides  of  the  ring  are  curved  bones,  the  basidorsals.  The 
ring  is  completed  above  by  the  neural  spine.  The  atlas  apparently  has  no 
centrum  but  the  centrum  is  really  present  and  has  become  attached  to  the  anterior 
end  of  the  second  cervical  vertebra  or  axis.  This  centrum  is  composed  of  the 


THE  ENDOSKELETON:    VERTEBRAL  COLUMN  AND  RIBS  71 

fused  interventrals.  It  projects  forward  into  the  ring  formed  by  the  atlas  as  a 
pointed  projection,  the  odontoid  process.  The  atlas  therefore  possesses  all  of 
the  parts  of  a  primitive  Vertebra  except  the  interdorsal.  The  axis  (also  called 
the  epistropheus)  has  a  large  centrum,  composed  of  the  fused  interventrals  and 
bears  at  its  anterior  end  the  odontoid  process,  which,  as  already  explained,  is 
really  the  centrum  of  the  atlas.  The  place  of  union  of  the  odontoid  process  with 
the  centrum  of  the  axis  represents  the  reduced  basiventral  of  the  axis.  The  axis 
has  a  strong  neural  arch  (basidorsals)  and  broad  neural  spine.  It  bears  a  pair 
of  ribs,  which,  curiously  enough,  have  moved  forward  so  that  they  are  attached 
to  the  odontoid  process.  Hence  it  might  be  said  that  the  atlas  has  two  pairs  of 
ribs  and  the  axis  none. 

The  vertebrae  of  the  alligator  are  composed  in  general  of  the  following  arcualia.  The 
neural  arch  consists  of  the  fused  basidorsals;  the  centrum  of  the  fused  interventrals;  basi- 
ventrals  are  present  only  in  the  atlas,  the  axis,  and  the  caudal  vertebrae,  where  they  form  the 
chevron  bones;  but  they  are  probably  represented  throughout  by  the  intervertebral  cartilages 
(absent,  of  course,  on  dried  skeletons).  The  basiventrals  are  present  as  distinct  bones,  the 
intercentra,  wedged  in  between  the  vertebrae  in  the  primitive  reptile  Sphenodon,  which  is 
sometimes  called  a  "living  fossil"  on  account  of  its  primitive  characteristics,  in  some  lizards 
and  in  many  extinct  reptiles.  Interdorsals  are  absent.  Vertebrae  like  those  of  the  alligator 
in  which  the  centrum  is  formed  of  the  interventrals  are  named  gastrocentrous  vertebrae. 
Gastrocentrous  vertebrae  are  characteristic  of  reptiles,  birds,  and  mammals  (Fig.  2222,  p.  66). 

2.  Study  of  the  ribs. — The  ribs  may  be  studied  on  the  cervical  or  first 
thoracic  vertebrae.  Observe  that  each  rib  is  attached  to  the  vertebra  by  two 
processes,  which  are  called  heads.  The  upper  head,  or  tuberculum,  is  attached 
to  a  short  blunt  transverse  process  arising  from  the  neural  spine.  The  lower 
head,  or  capitulum,  is  attached  to  the  side  of  the  anterior  end  of  the  centrum.  A 
large  opening  is  naturally  inclosed  by  the  two  heads  of  the  ribs.  The  successive 
openings  form  a  canal  called  the  vertebrarterial  canal  in  which  blood  vessels  to 
the  head  are  located.  The  ribs  of  the  third  to  seventh  cervical  vertebrae  are 
short  and  blunted,  those  of  the  eight  and  ninth  cervicals  considerably  longer, 
and  those  of  the  first  eight  thoracic  vertebrae  very  long  and  curved,  reaching 
to  the  ventral  side.  Observe  in  the  series  of  the  thoracic  vertebrae  that  the 
capitular  head  of  the  rib  gradually  moves  dorsally  until  it  finally  comes  to  be 
attached  to  a  little  projection  on  the  transverse  process.  The  little  smooth 
places  on  the  vertebrae  to  which  the  heads  of  the  ribs  are  articulated  are 
known  as  facets,  and  rib-bearing  vertebrae  can  always  be  recognized  by  such 
facets. 

The  thoracic  ribs  consist  of  three  parts.  The  upper  third  which  has  the 
heads  is  composed  of  bone  and  is  known  as  the  vertebral  rib.  The  middle  third 
is  partly  cartilaginous  and  is  called  the  intermediate  rib.  The  lower  third  is 
also  partly  cartilaginous  and  is  the  sternal  rib  or  costal  cartilage.  The  last  two 
thoracic  ribs  consist  of  vertebral  ribs  only. 


72        LABORATORY  MANUAL  FOR  VERTEBRATE  ANATOMY 

On  the  ventral  side  of  the  lumbar  region  will  be  noted  a  series  of  riblike  bones 
not  attached  to  any  other  part  of  the  skeleton.  These  are  the  so-called  abdomi- 
nal ribs.  They  are  membrane  bones  and  therefore  belong  to  the  exoskeleton, 
being  in  fact  homologous  with  the  dermal  plates  of  the  turtle's  plastron.  They 
are  likely  to  occur  in  long-bellied  animals. 

Draw  a  cervical  or  anterior  thoracic  vertebra  of  the  alligator  with  all  of  its 
parts,  showing  particularly  the  relation  of  the  ribs. 

3.  The  vertebral  column  of  the  turtle. — In  our  study  of  the  carapace  of 
the  turtle  we  already  noted  certain  peculiarities  of  the  vertebral  column  of  these 
animals.  These  peculiarities  arise  from  the  circumstance  that  part  of  the  verte- 
bral column  is  fused  to  the  exoskeleton.  The  vertebral  column  of  the  turtle 
consists  of  cervical,  trunk,  sacral,  and  caudal  regions.  As  usual,  the  caudal 
region  is  the  most  primitive.  The  caudal  vertebrae  have  neural  and  haemal 
arches  and  transverse  processes,  of  which  the  most  anterior  are  ribs,  as  shown  by 
the  suture  at  their  bases.  The  first  caudal  vertebra  is  fused  to  the  sacrum,  which 
consists  of  two  sacral  vertebrae  bearing  sacral  ribs.  The  first  caudal,  the  two 
sacral,  and  the  ten  trunk  vertebrae  are  all  fused  to  the  carapace.  As  already 
explained  in  connection  with  the  exoskeleton,  the  neural  arches  of  these  ver- 
tebrae are  broadened  to  form  the  vertebral  plates  of  the  carapace,  and  the  ribs 
of  the  trunk  vertebrae  are  similarly  expanded  to  form  the  costal  plates  of  the 
carapace.  The  first  and  second  pairs  of  ribs  are  fused  distally.  The  ribs  of  the 
turtle  possess  a  single  head,  the  capitular  head,  which  articulates  at  or  near  the 
boundary  between  two  successive  centra.  There  are  eight  cervical  vertebra  which 
are  devoid  of  ribs  and  which  are  very  flexibly  articulated  by  ball-and-socket 
joints.  The  first  two  are  differentiated  as  in  the  alligator  into  atlas  and  axis. 
The  odontoid  process,  which  is  the  centrum  of  the  atlas,  is  the  large  mass 
attached  to  the  anterior  end  of  the  axis. 

H.      VERTEBRAL   COLUMN   OF   BIRDS 

The  vertebral  column  of  birds,  like  their  entire  structure,  is  highly  special- 
ized in  relation  to  the  flight  habit.  With  a  prepared  skeleton  of  the  bird- 
chicken  or  pigeon — before  you,  note  the  following  points.  The  vertebral  column 
is  divided  into  cervical,  thoracic,  lumbar,  sacral,  and  caudal  regions,  but  all  of 
these  except  the  first  are  fused  together  in  order  to  strengthen  the  back.  The 
cervical  vertebrae  are  numerous  (sixteen  in  the  chicken,  thirteen  or  fourteen  in 
the  pigeon)  and  have  very  flexible  articulations,  birds  being  the  only  vertebrates 
that  can  turn  their  heads  halfway  round.  This  flexibility  is  due  to  the  shape 
of  the  centra.  On  isolated  vertebrae  or  by  slightly  separating  some  of  the  cer- 
vical vertebrae  on  the  mounted  skeleton  note  that  the  ends  of  the  centra  are 
saddle  shaped;  this  type  of  centrum  is  heterocoelous.  The  first  and  second 
cervical  vertebrae  are  the  atlas  and  the  axis;  the  former  is  a  small  ring-shaped 
bone,  the  latter  bears  an  odontoid  process  as  usual.  Behind  the  axis  are  the 


THE  ENDOSKELETON:  VERTEBRAL  COLUMN  AND  RIBS       73 

typical  cervical  vertebrae.  They  have  low  neural  arches  and  spines,  well- 
developed  zygapophyses,  and  ribs.  The  ribs  are  the  masses  attached  to  the 
sides  of  the  vertebrae  and  bearing  ventrally  sharp  spines  directed  posteriorly. 
The  ribs,  as  in  reptiles,  articulate  with  the  vertebrae  by  two  heads,  forming 
thereby  a  vertebrarterial  canal.  The  ribs  of  the  last  two  cervical  vertebrae  are 
longer  than  the  others.  The  thoracic  vertebrae  are  those  whose  ribs  extend 
completely  to  the  ventral  side.  They  have  high  neural  spines  and  well- 
developed  transverse  processes;  spines,  centra,  transverse  processes,  and  zyga- 
pophyses are  immovably  fused  to  one  another.  The  last  thoracic,  the  lumbar, 
the  sacral,  and  the  first  few  caudal  vertebrae  are  fused  into  one  continuous  piece, 
the  so-called  sacrum  or  synsacrum,  which  is  separated  by  a  suture  from  the 
broad  hip  bones  which  extend  on  either  side  of  it.  By  examining  the  ventral 
side  of  the  synsacrum  the  individual  vertebrae  of  which  it  is  composed  can  be 
distinguished  by  their  separate  transverse  processes.  It  should  be  noted  that 
only  certain  of  these  vertebrae  are  the  true  sacral  vertebrae,  corresponding  to 
those  of  reptiles,  the  vertebrae  in  front  and  behind  these  having  been  secondarily 
fused  to  them  to  furnish  additional  support  for  the  hind  limbs.  Posterior  to 
the  synsacrum  are  a  few  free  caudal  vertebrae  ending  in  an  enlarged  piece,  the 
pygostyle,  which  represents  several  fused  vertebrae.  Haemal  arches  are  absent 
from  all  of  the  vertebrae.  The  ribs  of  birds  are  divisible  into  an  upper  verte- 
bral rib,  composed  of  bone,  and  a  lower  sternal  rib,  or  costal  cartilage,  partly 
cartilaginous.  The  vertebral  ribs  bear  backwardly  directed  processes,  the  un- 
cinate  processes,  characteristic  of  birds  and  serving  apparently  to  lend  greater 
firmness  to  the  ribs.  The  ribs  of  birds  are  true  or  intermuscular  ribs. 

The  vertebrae  of  birds  are,  like  those  of  reptiles,  gastrocentrous,  that  is,  they  consist  of  a 
centrum  composed  of  the  interventrals  and  a  neural  arch  composed  of  the  basidorsals.  As 
haemal  arches  are  absent  on  the  vertebrae,  it  is  evident  that  the  basiventral  elements  are 
lacking  (except  presumably  in  atlas  and  axis). 

I.      VERTEBRAL   COLUMN   OF  MAMMALS 

The  vertebral  column  of  mammals  is  markedly  differentiated  into  the  usual 
five  regions:  cervical,  thoracic,  lumbar,  sacral,  and  caudal;  and  the  vertebrae 
of  these  regions  are  so  distinct  from  each  other  as  to  be  readily  identifiable  when 
isolated.  Study  whole  mounted  skeletons  of  the  cat  or  rabbit  and  isolated 
vertebrae. 

i.  The  cervical  vertebrae. — There  are,  as  in  almost  all  other  mammals,  seven 
cervical  vertebrae.  The  first  two  are  differentiated  as  the  atlas  and  the  axis. 
The  atlas  is  very  different  from  the  others;  it  is  ring  shaped  and  has  wide  wing- 
like  transverse  processes,  which  are  perforated  by  a  pair  of  openings,  the  verte- 
brarterial canals.  The  low,  flat  neural  arch  of  the  atlas  is  also  perforated  by  a 
pair  of  holes  for  the  passage  of  the  spinal  nerves.  The  anterior  end  of  the  atlas 
bears  a  pair  of  large,  curved  articulating  surfaces  which  support  the  skull,  and 


74       LABORATORY  MANUAL  FOR  VERTEBRATE  ANATOMY 

its  posterior  end  has  likewise  two  surfaces  which  articulate  with  the  axis.  As  in 
the  case  of  the  alligator,  the  ventral  side  of  the  atlas  ring  is  the  basiventral; 
the  sides  and  top,  the  basidorsals;  parts  of  the  transverse  processes  are  ribs; 
and  the  centrum  (interventral)  is  attached  to  the  axis.  Draw  the  atlas  from  the 
front. 

The  second  cervical  vertebra,  the  axis  or  epistropheus,  has  a  very  long  and 
broad  neural  arch,  provided  with  a  neural  spine  which  projects  forward  over  the 
atlas.  Its  centrum  bears  at  its  anterior  end  a  pointed  projection,  the  odontoid 
process,  which  fits  into  the  ring  of  the  atlas,  allowing  the  turning  of  the  head. 
The  odontoid  process  is  in  reality  the  centrum  of  the  atlas.  The  axis  bears 
laterally  the  so-called  transverse  processes,  but  only  the  dorsal  part  of  this  is  the 
real  transverse  process,  the  lower  half  being  a  rudimentary  rib.  This  rib,  as  is 
usually  the  case,  is  united  with  the  vertebra  and  the  transverse  process  in  such 
a  manner  as  to  leave  an  opening,  the  vertebrarterial  canal.  The  anterior  end 
of  the  centrum  of  the  axis  has  two  articulating  surfaces  for  the  atlas,  its  posterior 
end,  one  for  the  next  vertebra.  Its  neural  arch  has  a  pair  of  postzygapophyses. 
Draw  the  axis  from  the  side. 

The  remaining  cervical  vertebrae  are  more  or  less  similar.  They  have  well- 
developed  neural  arches  and  spines,  the  latter  increasing  in  height  toward  the 
thoracic  region.  All  have  pre-  and  postzygapophyses  and  transverse  processes. 
The  latter  are  in  reality  in  part  composed  of  a  rib,  resulting  as  before  in  the 
formation  of  the  vertebrarterial  canal.  Draw  a  cervical  vertebra  from  the 
front.1 

2.  The  thoracic  vertebrae. — These  vertebrae  are  recognizable  through  the 
fact  that  they  bear  long  ribs,  most  of  which  extend  to  the  ventral  side.  In  the 
isolated  vertebrae  the  smooth  costal  facets  where  the  ribs  were  attached  always 
serve  to  identify  them  as  thoracic.  There  are  thirteen  thoracic  vertebrae  in 
the  cat,  generally  twelve  in  the  rabbit,  various  numbers  in  other  mammals  (man 
has  twelve).  The  majority  of  the  thoracic  vertebrae  have  very  tall  neural  spines, 
directed  caudad,  short  centra,  small  pre-  and  postzygapophyses,  and  short,  stout 
transverse  processes,  bearing  at  their  ends  a  facet  for  articulation  with  the 
upper  head  of  the  rib.  The  centrum  bears  at  its  anterior  and  posterior  ends 
half-facets  tor  the  lower  head  of  the  rib.  The  last  thoracic  vertebrae  differ 
somewhat  from  the  others.  They  are  longer  and  stouter,  with  short  neural 
spines,  more  prominent  zygapophyses,  and  much  reduced  transverse  processes. 
They  bear  but  one  facet  for  the  ribs,  a  small  depression  near  the  anterior 
end  of  the  centrum.  Below  the  postzygapophysis  in  these  last  thoracic  verte- 
brae is  a  posteriorly  directed  process,  the  accessory  process,  conspicuous  in 
the  cat. 

1  Note  that  the  anterior  and  posterior  ends  of  vertebrae  can  be  recognized  as  follows:  the  smooth 
articulating  surfaces  or  facets  of  the  prezygapophyses  face  upward  or  forward  while  the  facets  of  the 
postzygapophyses  face  downward  or  backward. 


THE  ENDOSKELETON:  VERTEBRAL  COLUMN  AND  RIBS       75 

3.  The  ribs. — The  ribs  of  mammals  consist  of  a  dorsal,  bony  part,  the  ver- 
tebral rib,  and  a  ventral  cartilaginous  portion,  the  sternal  rib  or  costal  carti- 
lage.    The  vertebral  rib  is  articulated  to  the  vertebra  by  two  heads,  an  upper 
tuberculum,  whose  facet  engages  the  costal  facet  on  the  under  surface  of  the 
transverse  process;    and  the  lower  capitulum,  attached  between  two  centra  to 
demifacets,  i.e.,  half  a  facet  on  the  posterior  end  of  one  centrum  and  half  a  facet 
on  the  anterior  end  of  the  succeeding  centrum.     The  tuberculum  is  diminished 
on  the  more  posterior  ribs  and  wanting  on  the  last  three  ribs  which  are  provided 
with  capitular  heads  only.     The  narrow  part  of  a  rib  between  tuberculum  and 
capitulum  is  called  the  neck;  the  point  of  greatest  curvature,  just  a  little  beyond 
the  tuberculum,  the  angle;  the  remainder  of  the  rib,  the  body.     Those  ribs  which 
reach  to  the  median  ventral  line  and  are  independently  attached  to  the  median 
ventral  structure  (the  breastbone)  are  known  as  true  ribs;  those  which  are  not 
so  attached  to  the  breastbone  are  called  false  ribs;   and  those  false  ribs  whose 
ventral  ends  are  free  are  floating  ribs.1     The  cat  has  nine  true  ribs  and  four 
false  ribs,  of  which  the  last  one  is  floating;   the  rabbit  has  seven  true  ribs  and 
five  false  ribs,  of  which  the  last  three  are  floating;  in  man  there  are  seven  true 
ribs  and  five  false  ribs,  the  last  two  floating.     The  ribs  of  mammals  are  inter- 
muscular  ribs.     Draw  one  of  the  typical  thoracic  vertebrae  with  its  ribs,  from 
the  front. 

4.  The  lumbar  vertebrae. — There  are  seven  lumbar  vertebrae  in  the  cat 
and  rabbit.     They  are  large  and  stout  vertebrae  with  prominent  neural  spines, 
conspicuous   zygapophyses,   and   long   transverse   processes   directed   craniad. 
Below  the  postzygapophysis  is  a  pointed  projection,  directed  caudally,  called 
the  accessory  process.     Draw  a  lumbar  vertebra  from  the  side. 

5.  The  sacrum. — The  sacrum  is  composed  of  a  variable  number  of  vertebrae 
fused  together  for  articulation  with  the  hind  limbs.     There  are  three  sacral 
vertebrae  in  the  cat,  generally  four  in  the  rabbit  (of  which  only  the  first  two  really 
contribute  to  the  attachment),  five  in  man.     The  boundaries  between  the  fused 
sacral  vertebrae  are  readily  made  out  by  means  of  the  openings  between  them 
through  which  the  spinal  nerves  pass  out,  and  by  means  of  the  number  of  neural 
spines,  zygapophyses,  etc.     The  first  sacral  vertebra  assumes  the  greater  part 
of  the  task  of  transmitting  the  support  of  the  hind  limbs  to  the  vertebral  column; 
for  this  purpose  it  has  large  lateral  expansions  bearing  articular  surfaces  for  the 
insertion  of  the  bony  structure  which  supports  the  hind  limb.    These  lateral 
expansions  consist  in  part  of  transverse  processes  and  in  part  of  sacral  ribs, 
indistinguishably  fused  to  the  vertebra. 

6.  The  caudal  vertebrae.— The  caudal  vertebrae  are  variable  in  number 
in  mammals.     Neural  arches,  transverse  processes,  and  zygapophyses  diminish 

1  This  use  of  the  terms  true  and  false  should  not  be  confused  with  the  usage  previously  given  in 
which  the  expression  true  rib  refers  to  intermuscular  ribs  and  false  rib  to  subperitoneal  ribs.  In  this 
comparative  sense  all  mammalian  ribs  are  "true"  ribs. 


76       LABORATORY  MANUAL  FOR  VERTEBRATE  ANATOMY 

caudally,  the  last  vertebrae  consisting  of  centra  only.  Very  small  haemal 
arches  (chevron  bones)  are  present  in  the  tail  vertebrae  of  the  cat,  but  are  missing 
on  prepared  skeletons.  Man  has  three  to  five  caudal  vertebrae  fused  into  a 
single  piece,  the  urostyle  or  coccyx. 

7.  General  remarks  on  the  whole  column. — The  ends  of  the  centra  of  most 
mammalian  vertebrae,  as  can  be  seen  by  inspection  of  isolated  vertebrae,  are 
more  or  less  flat.  Such  centra  are  called  amphiplatyan.  Between  the  ends  of 
the  vertebrae  are  found  in  life  cartilaginous  disks,  the  iniervertebral  cartilages. 
On  the  mounted  skeleton  observe  an  opening  on  each  side  between  successive 
centra.  Through  these  openings,  the  intervertebral  foramina,  the  spinal  nerves 
pass  out  from  the  spinal  cord.  The  spinal  cord  occupies  the  continuous  canal 
formed  by  the  neural  arches.  Haemal  arches  (basiventrals)  are  absent  except 
in  the  atlas  and  axis  and  in  the  tails  of  some  mammals,  where  they  form  chevron 
bones.  The  vertebrae  of  mammals  are  gastrocentrous. 

J.      SUMMARY    OF   THE   VERTEBRAL   COLUMN  AND  RIBS 

1.  The  vertebrae  arise  at  the  intersection  of  the  myosepta  with  the  mesenchyme 
surrounding  the  notochord  and  neural  tube.     Each  vertebra  is  produced  by  the  union  of  the 
posterior  halves  of  the  two  sclero tomes  of  one  segment  with  the  anterior  halves  of  the  two 
sclerotomes  of  the  succeeding  segment.     Owing  to  this  manner  of  origin  the  vertebrae  alter- 
nate with  the  myotomes. 

2.  The  vertebrae  and  ribs  are  first  formed  in  cartilage,  produced  by  the  activity  of  the 
mesenchyme.    In  the  elasmobranch  fishes  they  remain  permanently  in  the  cartilage  stage. 
In  most  other  vertebrates  they  ossify  during  development.    They  belong,  consequently,  to 
the  category  of  cartilage  bones. 

3.  Each  vertebra  begins  as  four  pairs  of  cartilages  or  arcualia  surrounding  the  notochord: 
dorsally  an  anterior  pair  of  basidorsals  and  a  posterior  pair  of  interdorsals;  ventrally  an 
anterior  pair  of  basiventrals  and  a  posterior  pair  of  interventrals. 

4.  In  primitive  vertebrae  (occurring  in  some  fish,  extinct  Amphibia  and  reptiles)  these 
pieces  remain  more  or  less  separate  in  the  adult.     In  the  majority  of  vertebrates,  however, 
some  of  them  are  lost  and  the  remainder  fuse  into  a  single  structure. 

5.  The  parts  of  a  single  vertebra  thus  formed  by  the  fusion  of  originally  separate  pieces 
are:   the  centrum  or  main  body  of  the  vertebra,  which  incloses  the  notochord;   a  dorsally 
directed  arch,  the  neural  arch,  inclosing  the  spinal  cord;  and  a  ventrally  directed  arch,  the 
haemal  arch,  inclosing  blood  vessels.     The  neural  arch  consists  of  the  fused  basidorsals,  the 
haemal  arch,  of  the  fused  basiventrals;  the  centrum  is  of  different  origin  in  different  groups. 

6.  The  centrum  in  the  elasmobranch  fishes  is  formed  within  the  sheath  of  the  notochord; 
it  is  called  a  chordal  centrum.     The  centra  in  nearly  all  other  vertebrates  are  produced  by 
the  fusion  of  certain  of  the  arcualia  and  are  known  as  perichordal  or  arch  centra.     In  different 
groups  of  vertebrates  different  arcualia  contribute  to  the  centra. 

7.  The  ends  of  the  centra  are  variously  shaped:    amphicoelous,  or  concave  at  each  end 
(fishes,  urodeles);  procoelous,  concave  in  front,  convex  behind  (Amphibia,  reptiles);  opistho- 
coelous,  convex  in  front,  concave  behind  (Amphibia,  reptiles);  heterocoelous,  saddle  shaped 
(birds);   and  amphiplatyan,  flat  at  each  end  (mammals). 

8.  In  addition  to  centrum  and  arches,  vertebrae  commonly  bear  projecting  processes 
or  apophyses.     The  most  common  of  these  are  the  transverse  processes  and  the  zygapophyses. 


THE  ENDOSKELETON:  VERTEBRAL  COLUMN  AND  RIBS       77 

The  former  serve  for  the  articulation  of  the  ribs  and  are  probably  not  homologous  in  different 
vertebrates.    The  zygapophyses  yoke  successive  vertebrae  together. 

9.  The  haemal  arches  tend  to  disappear  or  become  reduced  in  vertebrates.    They  persist 
chiefly  in  the  tail  region  and  may  also  contribute  to  the  axis  and  the  atlas. 

10.  The  vertebral  column  in  fishes  is  divided  into  trunk  and  tail  regions  only.    In 
Amphibia  a  very  short  cervical  region  and  a  sacral  region  are  added.     In  reptiles,  birds,  and 
mammals,  the  cervical  region  is  longer,  and  the  trunk  region  is  divided  into  an  anterior  thoracic 
region,  bearing  long  ribs,  and  a  posterior  lumbar  region  with  reduced  or  no  ribs.     The  differ- 
ences between  the  vertebrae  of  the  different  regions  become  more  and  more  marked  the  higher 
one  ascends  in  the  vertebrate  scale. 

11.  Originally  each  vertebra  was  provided  with  a  pair  of  ribs,  but  in  the  higher  verte- 
brates these  are  reduced  or  absent  except  in  the  trunk  or  thoracic  regions.     Reduced  ribs 
are  generally  present  on  the  cervical  vertebrae  and  always  on  the  sacral  vertebrae. 

12.  Ribs  are  of  two  kinds:  those  that  arise  at  the  intersection  of  the  myosepta  with  the 
horizontal  skeletogenous  septum,  known  as  true  or  intermuscular  ribs;  and  those  that  arise 
at  the  intersection  of  myosepta  with  the  ventral  skeletogenous  septum  or  its  derivatives, 
known  as  false  or  subperitoneal  ribs.    The  latter  are  characteristic  of  teleosts,  the  former  of 
all  other  vertebrates.     Some  fishes  possess  both  kinds  of  ribs  simultaneously  and  may  also 
develop  additional  ribs  at  other  levels  of  the  myosepta. 

13.  Ribs  typically  articulate  with  the  vertebrae  by  two  heads,  the  space  between  the 
heads  forming  a  vertebrarterial  canal  for  the  passage  of  blood  vessels. 


VII.     THE    ENDOSKELETON:      THE    COMPARATIVE    ANATOMY    OF 

THE  GIRDLES,  THE  STERNUM,  AND  THE  PAIRED 

APPENDAGES 


median  fin  fold 


A.      GENERAL  CONSIDERATIONS 

i.  Definitions. — The  girdles  are  crescent-shaped  or  arch-shaped  portions  of  the  endo- 
skeleton  which  function  for  the  support  of  the  paired  appendages.  The  center  of  the  arch  is 
directed  ventrally,  the  points  dorsally.  The  girdles  are  composed  of  cartilage  in  the  lower 
forms;  this  is  partly  or  completely  ossified  in  the  higher  ones.  The  pectoral  girdle  supports  the 
anterior  appendages;  the  pelvic  girdle  the  posterior  appendages. 

The  sternum  or  breastbone  is  an  elongated  structure  lying  in  the  median  ventral  line  of  the 
anterior  part  of  the  trunk  region.    It  is  commonly  composed  of  a  chain  of  cartilages  or  bones,  or 

both.  The  ribs  and 
the  pectoral  girdle  us- 
ually articulate  with 
the  sternum.  Such 
arrangements  streng- 
then the  anterior 
part  of  the  trunk  in 
relation  to  the  air- 
median  fins  breathing  habit  and 
the  presence  of  lungs. 
The  paired  append- 
ages consist  of  fins  in 
the  fishes  and  limbs  in 
all  of  the  land  verte- 
brates. The  anterior 
or  pectoral  append- 
ages articulate  with 
the  pectoral  girdle, 
and  their  support  is 


•paired  fins 

FIG.  23. — Diagrams  to  illustrate  the  theory  of  the  origin  of  the  median 
and  paired  fins  through  the  persistence  of  certain  regions  of  originally  con- 
tinuous median  and  lateral  fin  folds.  A,  early  stage  showing  the  median 
dorsal  fin  fold  and  the  two  lateral  fin  folds  uniting  at  the  anus.  B,  later 
stage  illustrating  persistence  of  certain  regions  of  the  fin  folds  as  the  median 
and  paired  fins,  and  disappearance  of  the  remainder  of  the  fin  folds,  as 
indicated  by  dotted  lines.  (From  Wilder's  History  of  the  Human  Body, 
courtesy  of  Henry  Holt  and  Company.) 


transmitted  to  the 
body  by  means  of  this 
girdle.  Similarly  the 
posterior  or  pelvic 

appendages  are  articulated  to  the  pelvic  girdle  and  transmit  their  support  through  this  girdle. 
As  the  support  of  the  posterior  appendages  is  generally  the  more  important,  the  pelvic 
girdle  is  commonly  stronger  and  more  massive  than  the  pectoral  girdle.  This  is  particularly 
true  of  biped  vertebrates.  The  appendages  possess  an  internal  skeleton. 

2.  The  origin  of  the  paired  appendages  and  the  girdles. — The  origin  of  these  structures 
in  vertebrates  is  obscure.  According  to  the  most  probable  theory,  the  fin-fold  theory,  the 
ancestral  vertebrate  possessed  a  pair  of  continuous  folds,  one  running  along  each  side  of  the  trunk. 
These  fused  behind  the  anus  to  a  single  median  fin  which  extended  around  the  tail  and  along 
the  median  dorsal  line  (see  Fig.  23^).  This  hypothetical  condition  resembles  that  actually 
occurring  in  Amphioxus,  with  its  paired  ventral  metapleural  folds  and  median  caudal  and  dorsal 

78 


THE  ENDOSKELETON:    GIRDLES,  THE  STERNUM,  AND  APPENDAGES     79 

fin.  These  fin  folds  were  supported  by  cartilaginous  fin  rays.  The  paired  fins  of  present 
fishes  are  supposed  to  have  arisen  through  the  persistence  of  certain  regions  of  the  paired  fin 
folds,  and  the  median  unpaired  fins  through  the  persistence  of  particular  regions  of  the  median 
fin  fold.  The  remaining  portions  of  the  fin  folds  have  vanished  (see  Fig.  23$).  This  theory 
is  supported  by  the  facts  that  the  unpaired  and  paired  fins  of  fishes  are  identical  in  structure,  that 
in  the  young  stages  of  some  forms  continuous  paired  folds  are  present  from  which  the  paired  fins 
arise,  and  that  in  the  extinct  shark  Cladoselache  (see  K,  Fig.  121,  p.  115)  the  arrangement  of 
the  fin  rays  in  the  paired  fins  is  such  as  to  suggest  strongly  the  origin  of  the  paired  fins  from 
continuous  folds. 

The  girdles  arose  later  than  the  paired  fins,  since  extinct  forms  possess  the  latter  without 
the  former.  The  fins  as  already  stated  are  supported  by  cartilaginous  fin  rays,  which  may 
occur  in  several  rows.  It  is  suggested  that  the  most  medial  and  anterior  of  these  fin  rays 
of  the  fins  fused  across  the  midventral  line  to  form  median  plates  or  bars  of  cartilage.  Such 
plates  or  bars  represent  primitive  girdles.  Later  ossification  occurs  in  the  cartilage,  result- 
ing in  the  bony  girdles  composed  of  several  bones,  as  found  in  the  higher  vertebrates. 
Read  W. 

In  the  evolution  of  fish  into  Amphibia  the  paired  fins  transformed  into  limbs.  Stages 
in  this  transformation  are  very  imperfectly  known  and  difficult  to  conceive.  It  is  supposed 
that  the  fin  rays  of  the  paired  fins  were  gradually  reduced  in  number  either  by  loss  or  fusion. 
A  few  enlarged  fin  rays  arranged  similarly  to  the  bones  of  the  limbs  persisted.  Such  reduc- 
tion and  enlargement  of  fin  rays  really  occurs  in  the  crossopterygian  ganoids  living  and 
extinct,  and  the  arrangement  of  the  fin  rays  in  these  forms  is  strongly  suggestive  of  the 
arrangement  of  the  bones  in  the  vertebrate  limb.  (See  further,  N,  p.  176,  Fig.  102,  and 
p.  178,  Fig.  103.) 

Read  further  on  the  comparative  anatomy  of  the  sternum,  girdles,  and  paired  append- 
ages in  K,  W,  and  Wd. 

B.      THE   PELVIC   GIRDLE  AND  THE   POSTERIOR  PAIRED  APPENDAGES 

1.  The  parts  of  a  typical  girdle  and  hind  limb. — Before  beginning  the  study  of  the 
comparative  anatomy  of  these  parts  it  may  be  well  to  describe  the  generalized  girdle  and 
limb.    The  bony  pelvic  girdle  arises  from  the  ossification  of  a  cartilaginous  arch  or  plate. 
Three  centers  of  ossification  appear  in  each  half,  resulting  in  the  production  of  three  pairs  of 
cartilage  bones.     Of  these  there  are  on  each  side  a  dorsal  bone — the  ilium — -an  anterior 
ventral  bone — the  pubis — and  a  posterior  ventral  bone — the  ischium  (Fig.  24).     The  two 
pubes  and  ischia  commonly  meet  in  the  median  ventral  line.    The  ilia  are  articulated  to  the 
sacral  ribs. 

The  hind  limb  typically  consists  of  three  regions:  a  proximal  segment,  the  thigh,  containing 
a  single  long  bone,  the  femur;  a  middle  segment,  the  shank,  containing  two  long  bones:  a  pre- 
axial  tibia  and  a  posta.xia.1  fibula;  and  a  distal  segment,  the  foot  or  pes,  including  the  ankle,  sole, 
and  toes,  containing  a  number  of  small  bones.  The  generalized  ankle  is  composed  of 
nine  or  ten  bones  in  three  rows:  a  proximal  row  of  three  bones — a  preaxial  tibiale,  a  medial 
intermedium,  and  a  postaxi&lfibulare;  a  middle  row  of  one  or  two  centrales;  and  a  distal  row  of 
five  tar  sales.  The  sole  of  the  foot  is  composed  of  five  bones,  the  metatarsals,  arranged  in  a  trans- 
verse row.  The  toes  or  digits  contain  a  series  of  small  bones,  the  phalanges,  of  which  in 
primitive  feet  there  are  two  to  the  first  digit  and  three  to  all  of  the  others.  Figure  24  illus- 
trates the  bones  of  the  typical  pelvic  girdle  and  hind  limb. 

2.  The  pelvic  girdle  and  pelvic  fin  of  elasmobranchs. — For  this  purpose 
study  preserved  skeletons  of  the  dogfish  or  other  elasmobranch  fish.     The  pelvic 


8o 


LABORATORY  MANUAL  FOR  VERTEBRATE  ANATOMY 


ilium 


ischium 


pubis 


tibiale 

intermedium 
tarsales 

metatarsals yy 

phalanges ff 


fibulare 
centralia 


girdle  is  the  straight  or  slightly  curved  bar  of  cartilage  placed  across  the  ventral 
side  at  the  end  of  the  trunk  region  between  the  anterior  ends  of  the  pelvic  fins. 
This  bar  is  called  the  ischiopubic  bar  and  represents  a  primitive  unossified  pelvic 
girdle.  In  many  elasmobranchs  (including  the  dogfish)  the  two  ends  of  the  bar 
bear  slight  processes  projecting  outward;  these  are  the  iliac  processes. 

Attached  to  the  pelvic  girdle,  articulating  to  it  at  the  base  of  the  iliac  pro- 
cesses, are  the  pelvic  fins.     The  skeleton  of  the  basal  part  of  the  fins,  which  is 

imbedded  in  the  body  wall,  consists  of  a  num- 
ber of  cartilaginous  pieces,  the  cartilaginous  fin 
rays;  the  skeleton  of  the  free  external  part  of  the 
fin  is  composed  of  a  number  of  slender  parallel 
dermal  fin  rays.  As  the  latter  are  part  of  the 
exoskeleton,  they  will  not  be  considered  further. 
The  cartilaginous  fin  rays  are  arranged  in  two 
series,  a  medial  series,  consisting  of  one  or  two 
(in  some  forms  three  or  five)  much  enlarged  carti- 
lages, and  a  lateral  or  outer  series,  composed  of 
a  number  of  small  cartilages  disposed  in  one  or 
more  rows.  The  inner  series  of  fin  rays  are  called 
basalia  or  basals,  the  outer  series,  radialia  or 
radials.  There  is  usually  present  a  single  basal, 
the  metapterygium,  an  elongated  curved  cartilage 
forming  the  whole  medial  border  of  the  fin,  but 
in  some  forms  there  is  an  additional  basal,  the 
propterygium,  situated  at  the  anterior  end  of  the 
metapterygium.  The  radials  usually  consist  of  a 
row  of  rod-shaped  cartilages,  their  long  axes  at 
right  angles  to  the  axis  of  the  metapterygium. 

In  male  specimens  the  posterior  radial  is  greatly  enlarged  to  form  the  cartilage 
of  the  clasper.  The  basals  have  probably  arisen  through  the  fusion  of  a  number 
of  smaller  cartilages. 

Draw  the  pelvic  girdle  and  fin  of  an  elasmobranch. 

3.  The  pelvic  girdle  and  hind  limb  of  urodeles. — Study  these  parts  in  dried 
or  preferably  preserved  specimens  of  Necturus  or  Cryptobranchus.  The  urodele 
girdle  is  in  a  condition  intermediate  between  the  cartilaginous  girdle  of  elasmo- 
branchs and  the  bony  girdle  of  reptiles.  It  is  situated  on  the  ventral  side  at  the 
end  of  the  trunk  between  the  hind  limbs.  Its  ventral  portion  has  the  form  of  a 
flattened  plate,  which  may  be  designated  the  ischiopubic  plate  or  pelvic  plate.  The 
anterior  part  of  this  plate  is  cartilaginous  and  is  called  the  pubic  cartilage.  In  the 
posterior  part  of  the  plate  a  pair  of  opaque  (i.e.,  bony),  rounded  areas  may  be 
seen.  Each  of  these  areas  is  a  bone,  the  ischium,  which,  as  already  explained,  is 
the  posterior  ventral  bone  of  typical  girdles.  The  ischia  are  produced  by  ossifi- 
cation in  the  pelvic  plate.  From  each  side  of  the  pelvic  plate  a  rod  of  bone  extends 


v 


FIG.  24. — Diagram  of  the  bones 
of  the  typical  pelvic  girdle  and  hind 
limb.  (From  Parker  and  Haswell's 
Textbook  of  Zoology,  courtesy  of  the 
Macmillan  Company.) 


THE  ENDOSKELETON:    GIRDLES,  THE  STERNUM,  AND  APPENDAGES     81 

dorsally  and  is  firmly  articulated  to  the  end  of  the  sacral  rib.  This  bone  is  the 
ilium  and  represents  an  ossification  in  the  iliac  process  of  elasmobranch  girdles. 
At  the  point  of  junction  of  pubic  cartilage,  ischium,  and  ilium,  there  is  a  depres- 
sion, the  acetabulum,  into  which  the  proximal  end  of  the  hind  limb  is  inserted  by 
a  ball-and-socket  joint. 

The  hind  limbs  of  urodeles  like  Necturus  are  exceedingly  primitive.  This 
matter  was  already  discussed  in  Section  III  of  this  manual.  The  limb  is  divided 
into  a  proximal  segment — the  thigh,  consisting  of  a  single  bone,  the  femur,  a 
middle  segment — the  shank,  consisting  of  two  parallel  bones,  a  preaxial  bone, 
the  tibia,  and  a  postaxial  bone,  the  fibula,  and  a  distal  segment — the  pes,  consist- 
ing of  ankle  and/00/.  The  ankle  consists  of  several  small  bones,  the  tarsals,  which 
are,  unfortunately,  impossible  to  make  out  on  most  skeletons.  The  foot  has  four 
elongated  bones,  the  metatarsals,  which  bear  the  toes.  Each  toe  consists  of  two 
or  three  small  bones,  the  phalanges,  arranged  in  a  row.  The  first  toe  is  missing. 

Draw  the  pelvic  girdle  and  hind  limb  from  above. 

4.  The  pelvic  girdle  and  hind  limb  of  the  turtle. — In  the  reptiles  the  ossi- 
fication of  the  pelvic  girdle  is  complete;  very  little  cartilage  remains,  and  the  full 
number  of  bones  characteristic  of  the  pelvic  girdle  is  present.  The  pelvic  girdle 
of  the  turtle  is  a  very  generalized  and  representative  girdle.  Isolated  girdles  may 
be  studied,  but  the  position  and  attachment  of  the  girdle  should  be  noted  on 
the  entire  mounted  skeleton.  The  girdle  consists  of  three  pairs  of  stout  bones, 
two  pairs  ventral  in  position,  one  pair  lateral  and  dorsal.  The  ventral  bones 
consist  of  an  anterior  pair — the  pubes,  which  meet  in  the  median  ventral  line  form- 
ing the  pubic  symphysis,  and  a  posterior  pair — the  ischia,  united  similiarly  to 
form  the  ischial  symphysis.  These  symphyses  are  composed  of  cartilage,  generally 
missing  from  dried  skeletons.  The  lateral  and  dorsal  bones  of  the  girdle  are 
the  ilia,  which  are  articulated  at  their  dorsal  ends  to  the  ends  of  the  two  sacral 
ribs.  Note  on  the  mounted  skeleton  the  inverted .  arch  or  U  formed  by  the 
pelvic  girdle.  The  arch  is  completed  dorsally  by  the  sacral  vertebrae  and  sacral 
ribs.  Through  the  arch  of  the  pelvic  girdle  pass  the  terminal  portions  of  diges- 
tive and  urogenital  systems.  Between  the  pubis  and  ischium  of  each  side  is  a 
large  opening,  the  obturator  foramen,  through  which  in  the  fresh  condition  nerves 
and  blood  vessels  pass.  The  two  foramina  are  completely  separated  in  life  by 
a  cartilage  which  bridges  the  space  between  the  pubic  and  ischial  symphyses 
and  continues  as  the  cartilage  of  these  symphyses.  Attached  to  the  anterior 
extremity  of  the  pubic  symphysis  is  a  cartilage,  the  epipubis.  Each  pubis  has 
a  prominent  lateral  process,  the  pectineal  process,  projecting  forward.  Pubis, 
ischium,  and  ilium  meet  at  the  place  where  the  hind  limb  articulates,  and  share 
equally  in  the  formation  of  a  concave  depression,  the  acetabulum,  into  which  the 
convex  head  of  the  femur  is  inserted.  Draw  the  pelvic  girdle  (one  side  is  suffi- 
cient). 

The  parts  of  the  hind  limb  are  the  same  as  in  the  urodeles,  but  the  limb  no 
longer  retains  its  primitive  orientation  with  regard  to  the  body.  The  middle 


82       LABORATORY  MANUAL  FOR  VERTEBRATE  ANATOMY 

segment  is  directed  ventrally,  forming  an  angle  with  thigh  and  foot,  and  there 
has  also  been  some  slight  torsion.  See  further  on  these  points  hi  Section  III. 
The  thigh  consists  of  one  large  bone,  the  femur,  which  fits  into  the  acetabulum 
by  a  prominent  knob,  the  head  of  the  femur.  The  shank  is  composed  of  tibia  and 
fibula,  the  latter  being  the  smaller.  The  ankle  is  made  up  of  five  (or  in  some 
turtles  six)  bones.  At  the  bases  of  the  tibia  and  fibula  is  a  large,  transversely 
elongated  bone,  which  is  in  reality  composed  of  four  bones  fused  (tibiale, 
intermedium,  centrale,  fibulare).  In  some  turtles  the  fibulare,  at  the  base  of  the 
fibula,  is  separate.  Distal  to  this  compound  bone  is  a  row  of  four  bones,  the 
four  tarsales,  numbered  from  the  preaxial  (tibial)  side  to  the  postaxial  (fibular) 
side.  The  apparent  fourth  tarsale,  the  largest  of  the  four,  is  really  the  fused 
fourth  and  fifth  tarsales.  Distal  to  the  tarsales  are  the  five  metatarsals,  and 
beyond  these  the  digits,  composed  of  bony  joints  or  phalanges,  terminating  in 
horny  claws.  The  number  of  phalanges  in  the  digits  is  that  regarded  as  primi- 
tive for  vertebrates,  namely,  two  phalanges  to  the  first  digit,  and  three  to 
all  of  the  others  (see  Fig.  24).  It  is  worthy  of  note  that,  in  the  turtles  and 
reptiles  in  general,  the  movement  of  the  foot  upon  the  leg  occurs  between 
the  two  rows  of  tarsal  bones,  i.e.,  it  is  an  intratarsal  joint.  Figures  of  the  tarsal 
bones  of  turtles  are  given  in  Wd,  K,  and  R. 

5.  The  pelvic  girdle  and  hind  limb  of  birds. — These  structures,  like  the 
remainder  of  the  skeleton,  are  highly  modified  in  birds,  although  consisting  of 
the  same  parts  as  in  other  vertebrates.  Examine  isolated  backbones  with  the 
pelvic  girdles  attached,  or  study  the  whole  mounted  specimens.  The  pelvic 
girdle  consists  of  three  pairs  of  bones  as  in  rep  tiles,  i.e.,  ilium,  ischium,  and  pubis. 
All  three  are  fused  on  each  side  to  form  a  continuous  broad  bone,  the  innominate 
bone.  The  ilium  is  the  largest  and  most  dorsal  part  of  the  innominate  bone. 
It  forms  an  elongated  thin  plate,  concave  in  front,  convex  behind,  extending  from 
the  last  thoracic  vertebra  to  the  tail  region.  It  is  fused  along  its  entire  length 
with  the  synsacrum,  the  boundary  between  ilium  and  synsacrum  generally 
being  marked  by  a  suture.  (In  the  bird  embryo  the  ilium  is  articulated  to 
only  two  vertebrae  which  are  the  true  sacral  vertebrae.)  The  side  of  each 
innominate  bone  is  composed  of  the  ischium,  the  only  boundary  mark  between 
this  and  the  ilium  being  a  large  oval  opening,  the  ilioischiac  foramen.  The  pubis 
is  the  long  slender  bone  along  the  ventral  border  of  the  ischium,  from  which  it  is 
separated  by  a  more  or  less  distinct  suture,  and  the  slitlike  obturator  foramen 
which  may  be  divided  into  two  or  more  openings.  Ilium,  ischium,  and  pubis 
contribute  to  the  formation  of  the  acetabulum.  The  anterior  end  of  the  pubis 
is  situated  anterior  to  the  acetabulum,  the  normal  position  of  the  pubis,  but 
during  development  the  pubis  turns  posteriorly  and  comes  to  project  beyond 
the  posterior  end  of  the  ischium.  In  the  embryonic  development  of  birds, 
the  three  components  of  the  innominate  bone,  ilium,  ischium,  and  pubis,  orig- 
inate separately.  Observe  that  neither  pubic  nor  ischial  symphyses  are  present, 


THE  ENDOSKELETON :    GIRDLES,  THE  STERNUM,  AND  APPENDAGES     83 

but  the  two  innominate  bones  are  widely  separated  ventrally.  This  is  probably 
associated  with  the  habit  of  laying  large  eggs. 

The  hind  limb  offers  several  peculiarities.  The  femur  has  a  large  head 
fitting  into  the  acetabulum  and  a  prominent  projection  lateral  to  the  head, 
called  the  great  trochanter.  The  distal  end  of  the  femur  is  shaped  like  a  pulley, 
consisting  of  a  central  depression  with  curved  ridges — the  condyles — on  either 
side.  Over  the  joint  between  thigh  and  shank  is  an  extra  small  bone,  the  patella  or 
kneecap,  not  found  in  the  lower  vertebrates.  The  patella  is  a  sesamoid  bone,  that 
is,  a  bone  developed  in  a  tendon.  Such  sesamoid  bones  are  quite  common 
in  the  limbs  of  higher  vertebrates.  The  shank  is  composed  of  two  bones,  a 
medial  large  one  and  a  lateral  short  rudimentary  bone.  The  large  bone  is  the 
tibiotarsus.  It  consists  of  the  tibia  fused  at  its  distal  end  with  the  proximal 
tarsal  bones.  The  proximal  end  of  the  tibiotarsus  has  two  condyles  for  articula- 
tion with  the  condyles  of  the  femur,  and  bears  in  front  two  diverging  elevations 
or  crests  for  muscle  attachments.  The  small  bone  of  the  shank  is  the  fibula, 
whose  distal  portion  is  atrophied.  The  distal  end  of  the  tibiotarsus  has  a 
pulley-like  surface  for  articulation  with  the  succeeding  bone,  the  raised  articular 
surfaces  being  named  malleoli.  Beyond  the  tibiotarsus  is  a  long  stout  bone, 
the  tarsometatarsus,  evidently  formed  by  the  fusion  of  three  bones,  as  shown 
by  the  three  ridges  on  its  distal  end.  The  three  fused  bones  are  the  meta- 
tarsals  (second,  third,  and  fourth) ;  in  addition  the  tarsometatarsus  includes 
in  its  proximal  portion  the  distal  ankle  bones.  It  will  thus  be  seen  that  the  ankle 
bones  do  not  exist  separately  in  adult  birds,  but  the  proximal  ankle  bones  are 
fused  to  the  lower  end  of  the  tibia,  while  the  distal  ones  are  fused  to  the  upper 
ends  of  the  metatarsals.  The  ankle  joint  is,  therefore,  as  in  reptiles,  an  intratarsal 
joint.  The  three  metatarsals  which  are  fused  to  form  the  tarsometatarsus  are 
the  second,  third,  and  fourth,  but  a  remnant  of  the  first  metatarsal  is  present  as 
a  small  projection  on  the  medial  side  of  the  distal  end  of  this  compound  bone. 
Each  metatarsal  articulates  with  its  respective  digit,  composed  of  phalanges  and 
terminating  in  claws.  Fifth  metatarsal  and  digit  are  quite  wanting.  The  gait 
of  birds  is  digitigrade. 

Draw  the  pelvic  girdle  and  hind  limb  from  the  side. 

6.  The  pelvic  girdle  and  hind  limb  of  mammals. — The  pelvic  girdle  of 
mammals  is  very  similar  to  that  of  reptiles  in  shape,  and  similar  to  that  of  birds 
in  that  the  pelvic  bones  are  fused  into  innominate  bones.  The  girdle  consists 
of  the  usual  three  pairs  of  bones,  pubis,  ischium,  and  ilium,  the  three  of  each 
side  being  indistinguishably  fused  into  an  innominate  bone  or  hip  bone.  The 
ilium  is  the  most  dorsal,  most  anterior,  and  largest  of  the  three  components  of  the 
innominate  bone.  It  articulates  with  the  sacral  vertebrae  and  terminates  ante- 
riorly and  dorsally  in  a  curved  border,  known  as  the  crest  of  the  ilium.  The  ilium 
extends  as  far  posteriorly  as  the  acetabulum;  the  dorsal  part  of  the  girdle  posterior 
to  the  acetabulum  is  the  ischium  which  is  continuous  with  the  ilium.  The 


84 


LABORATORY  MANUAL  FOR  VERTEBRATE  ANATOMY 


ilium 


pubis 


posterior  ends  of  the  two  ischia  form  prominent  projections  in  the  rabbit  or  a 
curved  rough  surface  in  the  cat,  called  the  ischial  tuberosity,  and  extend  toward 
the  median  line  as  the  rami  (singular,  ramus)  of  the  ischium,  meeting  in  the 
median  ventral  line  to  form  the  ischial  symphysis.  The  anterior  ventral  part 
of  each  innominate  bone  is  formed  by  the  pubis.  Each  pubis  sends  a  projection, 
the  ramus,  toward  the  median  ventral  line,  the  two  rami  uniting  as  the  pubic 
symphysis.  Both  ischial  and  pubic  symphyses  are  in  life  composed  of  cartilage. 
Between  the  rami  of  the  ischium  and  pubis  is  the  large  obturator  foramen.  Ilium, 

ischium,  and  pubis  meet  at  the  acetabulum 
and  take  part  in  the  formation  of  its  walls. 
Draw  the  pelvic  girdle  from  in  front. 
On  a  demonstration  specimen  of  half  of 
the  girdle  of  a  kitten  note  complete  separa- 
tion and  boundaries  of  ilium,  ischium,  and 
pubis.  Note  further  the  presence  in  the 
acetabulum  of  a  small  bone,  the  acetabular 
bone,  which  forms  that  part  of  the  acetabu- 
lum which  would  otherwise  be  occupied  by 
the  pubis.  In  the  young  of  all  mammals 
the  three  bones  of  the  pelvic  girdle  are  sepa- 
rate, as  in  the  adults  of  reptiles  (Fig.  25). 

The  hind  limb  is  fairly  typical.  The 
femur  has  a  head,  a  greater  trochanter 
lateral  to  the  head  (which  in  the  rabbit 
continues  posteriorly  terminating  in  a  small 
projection,  the  third  trochanter),  and  a 
lesser  trochanter,  situated  below  the  head. 
These  trochanters  serve  for  muscle  attach- 
ments. The  large  articulating  surfaces  at 
the  distal  end  of  the  femur  are  condyles  (medial  and  lateral),  and  they  bear 
additional  elevations  or  roughened  areas,  the  epicondyles.  At  the  knee  joint 
a  patella  is  present.  The  shank  is  composed  of  a  stout  tibia  and  slender  fibula, 
the  latter  in  the  rabbit  fused  with  the  tibia  for  the  greater  part  of  its  length. 
The  anterior  face  of  the  tibia  presents  a  crest;  its  proximal  articulating  surfaces 
are  known  as  condyles;  its  distal  ones  as  malleoli.  The  bones  of  the  ankle  are 
identical  with  those  of  the  human  ankle  and  are  designated  by  the  same  names, 
which  are,  unfortunately,  somewhat  fanciful  and  not  based  upon  comparative 
anatomy.  The  name  derived  from  comparative  anatomy  is  given  in  paren- 
thesis after  the  name  derived  from  human  anatomy.  The  ankle  consists  of  seven 
bones  (cat),  or  six  (rabbit).  The  largest  and  most  conspicuous  of  these,  which 
projects  backward  as  the  heel,  is  the  calcaneus  (fibulare) .  Articulating  with  the 
malleoli  of  the  tibia  and  fibula  is  the  astragalus  or  talus  (tibiale).  Directly  in 


acetabulum 


ischium 


FIG.  25. — Half  of  the  human  pelvic 
girdle  at  birth,  showing  the  three  bones  of 
which  it  is  composed.  Stippled  regions 
represent  cartilage  which  later  ossifies, 
obliterating  the  boundaries  between  the 
bones.  (From  a  specimen  loaned  by  the 
anatomy  department.) 


THE  ENDOSKELETON:    GIRDLES,  THE  STERNUM,  AND  APPENDAGES    85 


front  of  the  astragalus  is  the  navicular  or  scaphoid  (centrale),  a  curved  bone 
reaching  to  the  medial  side  of  the  foot.  Directly  in  front  of  the  calcaneus  is  the 
cuboid  (fourth  and  fifth  tar  sales  fused),  which  articulates  with  the  fourth  and  fifth 
metatarsals.  Medial  to  the  cuboid  is  the  third  or  lateral  cuneiform  (third 
tar  sale),  articulating  with  the  third  metatarsal. 
Medial  to  this  is  the  second  or  intermediate 
cuneiform  (second  tar  sale),  articulating  with  the 
second  metatarsal.  In  the  cat  there  is  a  first  or 
medial  cuneiform  (first  tarsale)  along  the  medial 
border  of  the  anterior  part  of  the  ankle  in  front 
of  the  navicular.  It  articulates  with  the  small 
rudimentary  first  metatarsal  which  lies  directly  in 
front  of  it.  In  the  rabbit  the  first  cuneiform  is 
fused  to  the  proximal  end  of  the  second  meta- 
tarsal. The  homology  of  these  ankle  bones  with 
those  given  in  the  primitive  vertebrate  plan  (Fig. 
24)  is  quite  evident.  The  sole  consists  of  four 
long  metatarsals  and  one  rudimentary  one  (the 
first)  on  the  medial  or  ventral  side  of  the  proximal 
end  of  the  second  metatarsal.  The  terminal 
phalanges  of  the  digits  are  curiously  beak  shaped, 
for  the  support  of  the  horny  claws,  and  in  the  cat 
have  sheaths  at  their  bases  into  which  the  bases 
of  the  horny  claws  fit. 

The  joint  between  foot  and  shank  in  mam- 
mals is  between  the  ankle  bones  and  the  malleoli 
of  the  tibia  and  fibula,  unlike  the  condition  seen 
in  birds  and  reptiles,  where  the  joint  lies  between 
the  distal  and  proximal  ankle  bones.  The  gait 
of  the  cat  and  rabbit  is  chiefly  digitigrade,  al- 
though the  hind  legs  assume  the  plantigrade  pos- 
ture when  the  animal  sits  down. 

Attention  was  called  in  Section  HI  of  the 
manual  to  the  torsion  which  the  mammalian 
hind  limb  has  undergone  with  the  result  that 
the  toes,  originally  pointing  laterally  as  in  Nec- 

turus  now  point  anteriorly.     The  statements  made  at  that  place  should  be 
reviewed  here. 

C.     THE  PECTORAL  GIRDLE,   THE   STERNUM,   AND  THE  ANTERIOR 

PAIRED  APPENDAGES 

i.  The  origin  of  the  sternum. — Since  the  origin  of  the  pectoral  girdle  and  appendages  is  the 
same  as  that  of  the  pelvic  girdle  and  appendages,  it  remains  to  discuss  the  formation  of  the 


sternum 


FIG.  26. — Diagrams  to  illustrate 
the  theory  of  the  origin  of  the  ster- 
num from  the  pectoral  girdle.  A, 
pectoral  girdle  of  an  elasmobranch. 
B,  central  portion  of  the  girdle 
beginning  to  separate.  C,  central 
portion  of  the  girdle  completely 
separated,  forming  the  sternum. 


86 


LABORATORY  MANUAL  FOR  VERTEBRATE  ANATOMY 


scapula 


clavicle 
procoracoid 


coracoid 


glenoid  fossa 


humerus 


radius 


sternum.  The  sternum  occurs  only  in  lung-breathing  vertebrates,  that  is  to  say,  it  is  absent 
in  fishes.  There  is  some  doubt  as  to  the  origin  of  the  sternum.  According  to  the  prevailing 
view,  given  in  K  and  W,  the  sternum  of  the  higher  vertebrates  is  produced  by  the  fusion  of  the 
ventral  ends  of  the  ribs,  i.e.,  of  the  costal  cartilages.  But  since  in  the  lower  vertebrates  the  ribs 
are  short  and  do  not  reach  to  the  ventral  side,  this  theory  necessitates  the  invention  of  a  different 
theory  for  the  origin  of  the  sternum  of  Amphibia.  The  sternum  of  Amphibia  is  consequently 
supposed  to  have  arisen  through  the  fusion  of  certain  cartilages  present  in  the  ventral 
myosepta  of  urodeles.  See  W,  pages  140-43,  for  details  of  this  idea.  Recently,  however, 
evidence  has  been  brought  forward  in  favor  of  the  conception  that  the  sternum  has  the  same 
origin  in  all  of  the  vertebrates  and  that  its  origin  is  independent  of  the  ribs.  According  to  this 

theory  the  sternum  is  the  median  ventral  portion  of 
the  pectoral  girdle;  it  has  separated  from  the  girdle 
and  extended  posteriorly;  it  may  or  may  not  con- 
nect secondarily  with  the  ribs1  (see  Fig.  26). 

2.  The  parts  of  a  typical  girdle  and  fore 
limb. — The  pectoral  girdle  is  somewhat  more  com- 
plicated in  structure  than  the  pelvic  girdle.  It 
arises  in  part  from  the  ossification  of  a  cartilagi- 
nous bar  or  plate.  In  this  plate  as  in  the  case  of 
the  pelvic  plate  three  pairs  of  bones — two  ventral 
pairs  and  one  dorsal  pair — arise.  These  bones  are 
on  each  side:  a  dorsal  bone,  the  scapula,  an  anterior 
ventral  bone,  the  procoracoid  or  precoracoid,  and  a 
posterior  ventral  bone,  the  coracoid  (see  Fig.  27). 
The  scapula  is  analogous  to  the  ilium,  the  procora- 
coid to  the  pubis,  and  the  coracoid  to  the  ischium. 
These  bones  like  the  pelvic  bones  are  cartilage 
bones  since  they  arise  in  cartilage.  In  the  case  of 
the  pectoral  girdle,  unlike  the  pelvic,  only  two  of  the 
three  cartilage  bones  persist  in  the  vertebrate  series. 
Of  these  the  scapula  is  always  present.  Of  the  two 
ventral  pairs  of  elements  only  one  pair  is  present  in 
the  majority  of  living  groups  of  reptiles,  in  birds, 
and  in  mammals.  It  is  commonly  supposed  that 
the  element  which  persists  is  the  coracoid  and  that 
the  procoracoid  is  missing.  If  this  theory  be  cor- 
rect, there  are  hardly  any  living  vertebrates  with 

a  separate  ossified  procoracoid.  However,  there  is  considerable  evidence  that  the  per- 
sistent element  is  really  the  procoracoid  in  present  reptiles,  and  that  in  this  group  the  coracoid 
is  missing.  In  mammals,  however,  the  matter  is  disputed,  some  regarding  the  persistent 
element  as  the  procoracoid,  others  as  the  coracoid  (Fig.  28).  This  persistent  ventral  cartilage 
bone  of  the  pectoral  girdle  of  reptiles,  birds,  and  mammals  is  called  the  coracoid,  and  this  name 
will  be  retained  here,  with  the  understanding  that  in  reptiles  it  is  almost  certainly  homologous 
with  the  procoracoid.  In  nearly  all  mammals  the  coracoid  is  rudimentary,  the  only  cartilage 
bone  of  the  girdle  which  retains  its  full  importance  being  the  scapula  or  shoulder  blade.  Dia- 
grams illustrating  the  hoinology  and  parts  of  the  girdles  of  various  forms  are  given  in 
Figure  29. 

In  addition  to  the  cartilage  bones  present  in  the  girdle  some  of  the  original  cartilage  is 
likely  to  persist  unossified.    The  cartilages  of  most  common  occurrence  are:  the  suprascapula, 

1 F   B.  Hanson,  American  Journal  of  Anatomy,  XXVI  (1919),  41. 


metacarpals 


phalanges 


FIG.  27. — Diagram  of  the  bones  of  the 
typical  pectoral  girdle  and  fore  limb.  The 
cartilage  bones  of  the  girdle  are  stippled, 
the  membrane  bone  (clavicle)  left  blank. 
All  of  the  limb  bones  are  cartilage  bones. 
(From  Parker  and  Haswell's  Textbook  of 
Zoology,  courtesy  of  the  Macmillan  Com- 
pany.) 


procoracoid 


coracoid 


scapula 


procoracoid 


coracoid 


scapula 


THE  ENDOSKELETON:    GIRDLES,  THE  STERNUM,  AND  APPENDAGES    87 

situated  along  the  dorsal  border  of  the  scapula  and  sometimes  partially  ossified,  the  epicora- 
coids,  cartilages  anterior  or  medial  to  the  coracoid  (procoracoid)  element  of  reptilian  and  mam- 
malian pectoral  girdles;  and  cartilages  between  the  medial  ends  of  the  coracoids  (also  but 
confusingly  named  epicoracoids  in  texts,  and  in  this  manual  left  unnamed). 

The  pectoral  girdle  is  still  further  complicated  by  the  addition  of  membrane  bones  to  the 
cartilage  bones  and  cartilages  described  above.  As  previously  stated ,  such  membrane  bones  are 
homologous  with  the  dermal  plates  of  the  exoskeleton 
and  have  secondarily  become  associated  with  the 
endoskeleton.  It  was  stated  in  the  introduction 
to  the  endoskeleton  that  such  membrane  bones 
occur  in  relation  to  certain  parts  of  the  endo- 
skeleton, and  we  meet  them  here  for  the  first 
time,  in  association  with  the  pectoral  girdle.  In 
teleosts  there  are  a  number  of  such  dermal 
elements  added  to  the  pectoral  girdle,  as  shown  in 
Figure  30;  they  form  a  sort  of  ventral  incase- 
ment  of  the  girdle.  Of  these  there  persist  in  the 
most  primitive  land  vertebrates  (extinct)  five 
elements,  a  median  single  inter  clavicle,  paired 
lateral  clavicles,  and  lateral  to  the  clavicles  paired 
cleithra  (singular,  clei thrum)  (see  Fig.  29^).  The 
interclavicle  (also  called  episternum)  is  situated 
immediately  ventral  to  the  sternum,  the  clavicles 
invest  the  procoracoids  which  are  often  concealed 
within  or  dorsal  to  them,  while  the  cleithra  are 
situated  along  the  borders  of  the  scapulae.  The 
cleithra  are  lost  early  in  the  evolution  of  land 
vertebrates  and  do  not  occur  in  living  forms,  but 
the  interclavicles  and  clavicles  are  characteristic 
of  many  reptiles,  of  birds,  and  of  monotremes.  In 
placental  mammals  the  interclavicle  is  missing, 
and  in  some  of  them  the  clavicles  are  also  reduced 
or  absent.  There  has  thus  occurred  in  the  evolu- 
tion of  the  pectoral  girdle  a  gradual  reduction  in 
the  number  of  membrane  bones,  as  will  be  seen  by 
reference  to  Figure  29. 

The  pectoral  girdle  thus  consists  of  a  complex 
of  cartilage  and  membrane  bones.  The  cartilage 
bones  may  articulate  ventrally  with  the  sternum 
but  rarely  have  any  articulation  or  fusion  with 
the  vertebral  column  (exceptions:  skates,  ptero- 
saurs). In  this  respect  the  pectoral  girdle  stands 

in  marked  contrast  to  the  pelvic  girdle,  which,  as  we  have  seen,  always  has  in  land  verte- 
brates a  firm  articulation  with  the  sacrum.  In  teleosts  the  membrane  bones  of  the  girdle 
may  articulate  with  the  skull.  The  cartilage  bones  of  the  girdle  always,  and  the  membrane 
bones  never,  take  part  in  the  articulation  with  the  fore  limb.  This  is  one  of  the  simplest  ways 
of  distinguishing  the  two  kinds  of  components  in  the  girdle. 

The  skeleton  of  the  fore  limb  is  composed  of  parts  similar  to  those  already  described  for  the 
hind  limb.    There  is  a  proximal  segment — the  upper  arm  or  brachium,  consisting  of  a  single 


procoracoid 


coracoid 


scapula 


FIG.  28. — Diagrams  to  illustrate  two 
theories  of  the  homology  of  the  cartilage  bones 
of  the  pectoral  girdle  of  amniotes.  A,  carti- 
lage bones  of  the  primitive  pectoral  girdle; 
there  are  three  bones  in  each  half  of  the 
girdle,  a  condition  found  only  in  extinct 
forms.  B,  one  theory  of  the  composition  of 
the  pectoral  girdle  of  present  amniotes,  show- 
ing disappearance  of  the  procoracoid  and 
retention  of  the  coracoid.  C,  alternative 
theory,  showing  retention  of  the  procoracoid 
(called,  however,  coracoid)  and  disappearance 
of  the  coracoid. 


88 


LABORATORY  MANUAL  FOR  VERTEBRATE  ANATOMY 


bone,  the  humerus,  a  middle  segment — the  forearm  or  antibrachium,  containing  two  bones: 
a  preaxial  radius  and  a  postaxial  ulna,  and  a  distal  segment — the  hand  or  manus.  The  hand 
is  subdivided  into  three  parts:  the  wrist,  the  palm,  and  the  fingers.  The  wrist  is  primitively 
composed  of  nine  or  ten  bones  in  three  rows:  a  proximal  row  of  three,  named  the  radiate,  the  inter- 
medium, and  the  ulnare;  a  middle  row  of  one  or  two  centrales;  and  a  distal  row  of  five 


\       <?      * 


FIG.  29. — Pectoral  girdles  of  some  vertebrates.  A,  side  view  of  half  of  the  pectoral  girdle  of  an 
extinct  amphibian,  Cacops,  belonging  to  the  Stegocephala;  note  presence  of  the  cleithrum  b.  B,  ventral 
view  of  both  halves  of  the  pectoral  girdle  of  a  very  ancient  extinct  reptile,  Seymour ia,  belonging  to  the 
Cotylosauria,  illustrating  the  complete  generalized  pectoral  girdle;  note  presence  of  both  coracoid  and 
procoracoid.  C,  ventral  view  of  both  halves  of  the  pectoral  girdle  of  a  modern  reptile,  a  lizard;  note 
loss  of  one  of  the  coracoid  bones.  D,  ventral  view  of  both  halves  of  the  pectoral  girdle  of  a  monotreme 
mammal,  the  duckbill,  Ornithorhynchus,  illustrating  the  most  primitive  pectoral  girdle  found  among  present 
mammals;  note  persistence  of  the  coracoid  and  interclavicle,  similar  to  reptiles.  E,  ventral  view  of 
one-half  of  the  pectoral  girdle  of  an  ape;  note  absence  of  the  interclavicle  and  reduction  of  the  coracoid 
to  the  coracoid  process/.  Cartilage,  close  stippling;  cartilage  bones,  open  stippling;  membrane  bones, 
blank,  a,  coracoscapula;  b,  cleithrum;  c,  clavicle;  d,  interclavicle;  e,  scapula;  /,  coracoid  (coracoid 
process  in  £);  g,  procoracoid;  h,  epicoracoid  cartilage;  i,  glenoid  fossa;  j,  spine  of  the  scapula.  (A 
from  Willis  ton's  Water  Reptiles  of  the  Past  and  Present,  University  of  Chicago  Press;  B,  after  Williston; 
C  from  Reynolds'  The  Vertebrate  Skeleton,  courtesy  of  the  Macmillan  Company;  D  from  Wiedersheim's 
Comparative  Anatomy  of  Vertebrates,  courtesy  of  the  Macmillan  Company.) 

car  pales.    The  palm  is  composed  of  five  elongated  metacarpals  and  the  fingers  of  phalanges 
whose  numbers  are  primitively  the  same  as  in  the  case  of  the  toes  (see  Fig.  27). 

3.  The  pectoral  girdle  and  pectoral  fin  of  elasmobranchs. — In  these  fish 
we  find  the  pectoral  girdle  in  a  primitive  condition,  resembling  that  of  the  pelvic 
girdle.  It  is  a  curved  cartilage  almost  completely  encircling  the  anterior  part  of 


THE  ENDOSKELETON:    GIRDLES,  THE  STERNUM,  AND  APPENDAGES    80 

the  trunk.  The  median  ventral  portion  between  the  bases  of  the  two  fins  is 
called  the  coracoid  bar;  the  long  processes  extending  dorsally  beyond  the  articula- 
tions of  the  fins  are  the  scapular  processes;  the  ends  of  the  scapular  processes 
commonly  consist  of  separate  pieces,  the  suprascapular  cartilages.  The  pectoral 
fins  are  similar  to  the  pelvic.  They  are  supported  at  their  bases  by  several  series 
of  cartilaginous  fin  rays  and  externally  by  dermal  fin  rays.  The  cartilaginous 
fin  rays  consist  of  a  proximal  row  of  enlarged  basals  and  several  distal  rows  of 


f  posttemporal 

.  supraclavicle 


upper  postclavicle 


metapterygium 
clavicle  mesopterygial  cartilage 


radials 


FIG.  30. — Pectoral  girdle  of  a  crossopterygian  fish,  Polypterus,  to  show  the  large  number  of  mem- 
brane bones  occurring  in  the  pectoral  girdle  of  teleostome  fishes.  Viewed  from  the  inside  (dorsal  view) 
Membrane  bones,  blank;  cartilage  bones,  close  stippling;  cartilage,  open  stippling.  Note  also  the 
arrangement  of  the  basals  and  radials  in  the  "stalk"  of  the  fin,  the  external  appearance  of  which  is 
shown  in  Figure  i.  (After  Goodrich  in  Part  IX  of  Lankester's  Treatise  on  Zoology,  courtesy  of  the 
Macmillan  Company.) 

smaller  radials.  There  are  generally  three  basals:  an  inner  one,  the  largest,  the 
metapterygium;  a  middle  one,  the  mesopterygium;  and  an  outer  one,  the  prop- 
terygium.  There  is  no  trace  of  a  sternum  as  such,  but  the  median  ventral 
portion  of  the  coracoid  bar  may  be  regarded  as  the  primordium  of  the  sternum 
on  the  basis  of  the  theory  of  the  origin  of  the  sternum  presented  at  the  beginning 
of  this  section  (see  Fig.  26,  p.  85).  Draw  the  pectoral  girdle  and  one  pectoral  fin. 
4.  The  pectoral  girdle  and  fore  limb  of  urodeles. — In  urodeles  the  girdle  is 
still  in  a  primitive  condition.  Necturus  or  Cryptobranchus  may  be  studied.  On 
the  ventral  side  the  girdle  forms  paired,  flat  cartilaginous  plates,  which  may  be 
designated  the  coracoid  plates.  The  anterior  part  of  these  plates  may  be 
regarded  as  a  procoracoid  region;  this  projects  forward  in  Necturus  as  an 


go       LABORATORY  MANUAL  FOR  VERTEBRATE  ANATOMY 

elongated  process.  The  posterior  part  is  the  coracoid  region,  but  no  ossification 
has  occurred  in  either  of  the  regions.  Dorsally  above  the  joint  with  the  fore 
limb  is  a  bone,  the  scapula — the  only  bone  in  the  girdle.  Attached  to  the  dorsal 
border  of  the  scapula  is  the  suprascapular  cartilage.  There  is  no  sternum 
(although  two  or  three  pairs  of  small  cartilages  found  in  the  ventral  wall, 
located  in  the  myosepta,  are  regarded  as  the  sternum  by  some  authors).  Note 
that  the  pectoral  girdle  has  no  connection  with  the  vertebral  column. 

The  fore  limb  is  similar  to  the  hind  limb.  The  upper  arm  is  composed  of  the 
humerus;  the  forearm  of  the  preaxial  radius  and  the  postaxial  ulna;  the  elements 
of  the  carpus  or  wrist  are  difficult  to  distinguish  (there  are  six  or  seven  cartilages 
in  three  rows);  there  are  four  metacarpals  in  the  palm  followed  by  phalanges. 
The  middle  segment  of  the  limb  is  bent  and  directed  ventrally,  thus  elevating 
the  animal  slightly  above  the  ground. 

Draw  the  pectoral  girdle  and  one  fore  limb. 

5.  The  pectoral  girdle  and  sternum  of  the  frog. — In  the  frog  we  find  that 
the  cartilages  present  in  the  urodele  girdle  have  been  partially  replaced  by  bone. 
A  sternum  is  further  present  in  the  median  ventral  line  between  the  ventral  ends 
of  the  halves  of  the  pectoral  girdle.     Study  the  girdle  and  sternum.     The  sternum 
consists  of  two  bones  and  two  cartilages  arranged  in  a  longitudinal  series  and 
separated  into  two  groups  by  the  pectoral  girdle.     The  anterior  group,  projecting 
cranially  from  the  girdle,  consists  of  a  terminal  rounded  cartilage,  the  episternum, 
posterior  to  which  is  a  bone,  the  omosternum.     The  posterior  group,  projecting 
caudad  from  the  girdle,  is  composed  of  an  anterior  bone,  the  sternum  proper,  and 
a  terminal  rounded  cartilage,  the  xiphisternum.    The  ventral  part  of  the  pectoral 
girdle  consists  of  two  pairs  of  bones,  their  ventral  ends  meeting  in  the  median 
ventral  line  between  the  two  parts  of  the  sternum.     The  anterior  pair  of  bones 
is  the  clavicles.    The  clavicles  are  membrane  bones  which  cover  and  conceal 
the  procoracoid  cartilages;    these  cartilages  remain  unossified  in   Anura.    The 
posterior  ventral  bones  of  the  girdle  are  the  coracoids,  ossified  from  the  coracoid 
cartilages.     The  medial  ends  of  the  coracoids  remain  in  the  cartilaginous  condi- 
tion (forming  cartilages  which  are  designated  in  texts  as  the  epicoracoids,  but 
this  term  is  confusing  as  it  is  also  used  in  another  connection) .     The  dorsal  part  of 
the  girdle  consists  of  two  bones  on  each  side:   one  next  to  the  articulation  of  the 
fore  limb,  the  scapulay  and  a  large,  thin,  flat  bone  medial  to  the  scapula,  the 
suprascapula,  having  a  cartilaginous  border.     The  girdle  is  not  connected  with 
the  vertebral  column.     The  depression  which  receives  the  head  of  the  humerus 
is  the  glenoid  fossa;  scapula  and  coracoid  take  part  in  the  formation  of  the  glenoid 
fossa,  but  the  clavicle  does  not.     This  indicates  that  the  clavicle  is  of  different 
origin  from  the  other  bones  of  the  girdle.     Draw  the  girdle  and  sternum  of  the 
frog. 

6.  The  pectoral  girdle,  sternum,  and  fore  limb  of  reptiles. — On  the  skeleton 
of  the  turtle  observe  that  a  pectoral  girdle  is  present  but  a  sternum  is  absent. 


THE  ENDOSKELETON:    GIRDLES,  THE  STERNUM,  AND  APPENDAGES    91 

Each  half  of  the  girdle  consists  of  two  ventral  parts  and  one  dorsal  part.  The 
anterior  ventral  part  is  not  an  independent  bone  but  a  projection  from  the 
scapula,  and  is  hence  named  the  proscapular  process  (erroneously  referred  to  in 
some  texts  as  the  procoracoid  bone).  The  posterior  ventral  bone,  the  larger  of 
the  two,  is  the  coracoid  (which  as  already  explained  is  probably  in  reality  the 
procoracoid).  The  dorsal  bone,  an  elongated  bone  reaching  to  the  carapace,  is 
the  scapula.  All  of  these  are  cartilage  bones.  In  addition  there  are  certain 
bones  of  the  pectoral  girdle  which  are  included  in  the  plastron  and  are  there- 
fore membrane  bones.  These  are  the  paired  clavicles  (epiplastra)  and  the 
median  inter  clavicle  (entoplastron) .  Refer  to  your  drawings  of  the  turtle 
plastron. 

The  pectoral  girdle  of  the  alligator  consists  of  a  stout  dorsal  bone,  the  scapula; 
a  stout  ventral  bone,  the  coracoid ;  and  a  long,  slender  dagger-shaped  bone  in  the 
median  ventral  line,  the  inter  clavicle.  Clavicles  are  lacking.  A  sternum  is 
present.  It  is  composed  of  a  plate  of  cartilage  between  the  ventral  ends  of 
the  coracoids  and  just  above  the  interclavicle;  it  is  drawn  out  posteriorly  into 
long  curved  cartilages,  the  xiphisternal  horns.  Observe  that  the  ribs  are  attached 
to  the  sternum  by  means  of  their  sternal  ribs,  a  condition  first  met  with  in 
reptiles. 

The  bones  of  the  fore  limbs  are  the  same  as  those  of  Necturus  in  general.  The 
bones  of  the  carpus  or  wrist  of  the  turtle  are  remarkably  primitive  in  number 
and  position,  and  deserve  further  attention.  If  not  sufficiently  clear  on  the  speci- 
men, expose  them  by  cautiously  picking  away  the  dried  ligaments  covering  them. 
At  the  base  of  the  ulna  are  two  bones,  an  outer  ulnare,  and  an  inner  intermedium. 
The  center  of  the  carpus  is  occupied  by  a  long  bone  which  is  the  fused  radiate  (end 
at  the  base  of  the  radius)  and  the  centrale  (larger  end).  Distal  to  this  is  a  row 
of  five  carpales,  one  for  each  metacarpal.  This  arrangement  is  therefore  very 
much  like  that  of  the  ideal  carpus.  Figures  of  the  carpus  of  turtles  are  given 
in  Wd,  pages  163,  164.  Draw,  showing  the  bones  of  the  carpus. 

7.  The  pectoral  girdle,  sternum,  and  fore  limb  of  birds. — The  pectoral 
girdle  of  birds  is  a  very  complete  and  generalized  girdle.  It  is  composed  of  the 
scapula,  a  long  swordlike  bone  lying  above  the  ribs;  the  coracoid,  a  stout  bone 
reaching  the  sternum;  and  the  wishbone,  or  furcula,  in  front  of  the  coracoids  and 
attached  at  the  lower  end  by  ligaments  only.  The  furcula  is  membrane  bone 
and  really  consists  of  two  clavicles,  the  two  forks  of  the  wishbone,  united  to  a 
rounded  piece,  the  interclavicle.  Coracoid  and  scapula  take  part  in  the  forma- 
tion of  the  glenoid  fossa.  The  sternum  of  birds  is  highly  specialized.  It  is  an 
elongated  bone  bearing  a  strong  ventral  projection,  the  keel  or  carina.  The  carina 
serves  for  the  attachment  of  the  powerful  wing  muscles.  As  in  reptiles,  the  ribs 
are  joined  to  the  sternum  by  their  costal  cartilages.  The  front  end  of  the  sternum 
has  short  costal  processes,  each  side  two  long  xiphisternal  processes.  Draw  from 
the  side,  showing  girdle  and  sternum. 


92       LABORATORY  MANUAL  FOR  VERTEBRATE  ANATOMY 

On  a  demonstration  specimen  of  the  sternum  of  an  ostrich  or  other  flightless 
bird  note  the  absence  of  the  keel  or  carina,  the  sternum  presenting  a  convex  ven- 
tral face. 

The  fore  limb  of  birds  is  greatly  modified  as  a  wing,  particularly  in  its  distal 
portions.  The  humerus  is  stout,  fitting  into  the  glenoid  fossa  by  its  convex 
head.  On  either  side  and  slightly  distal  to  the  head  are  prominent  projections, 
the  greater  and  lesser  tuberosities,  corresponding  to  the  trochanters  of  the  femur. 
The  lesser  tuberosity  is  continued  distally  into  a  sharp  ridge,  the  deltoid  ridge. 
This  side  of  the  humerus  bearing  the  deltoid  ridge  is  the  preaxial  or  radial  side, 
and  it  is  readily  observed  that  a  rotation  has  occurred  so  that  the  preaxial  surface 
faces  dorsally  in  the  folded  position  of  the  wing.  The  greater  tuberosity  is  post- 
axial  and  bears  on  its  under  surface  a  large  hole,  the  pneumatic  foramen,  leading 
into  the  air  space  of  the  humerus.  The  radius  and  ulna  are  typical,  the  former 
the  more  slender  of  the  two,  the  latter  exhibiting  at  its  proximal  end  a  projection, 
the  olecranon  process  or  elbow,  here  met  for  the  first  time.  The  wrist  is  greatly 
altered,  consisting  of  but  two  separate  bones — the  radiale  at  the  base  of  the  radius 
and  the  ulnare  at  the  base  of  the  ulna.  The  remaining  wrist  bones  are  fused  to  the 
metacarpals  to  form  the  carpometacarpus,  consisting  of  two  elongated  bones. 
The  metacarpals  contributing  to  these  elements  are  the  third  and  fourth.1  The 
second  metacarpal  is  fused  to  the  preaxial  side  of  the  proximal  end  of  the  third 
metacarpal  where  it  forms  a  pronounced  hump.  From  this  hump  projects  the 
second  digit.  The  third  digit  is  the  longest  and  consists  of  two  phalanges, 
of  which  the  proximal  one  is  much  flattened.  The  fourth  digit  is  a  small  projec- 
tion fused  to  the  postaxial  side  of  the  proximal  phalanx  of  the  third  digit.  First 
and  fifth  metacarpals  and  digits  are  wanting. 

8.  The  pectoral  girdle,  sternum,  and  fore  limb  of  mammals. — The  pectoral 
girdle  of  most  mammals  is  reduced  and  somewhat  modified.  Only  the  mono- 
tremes  have  complete  pectoral  girdles,  consisting  of  scapulae,  coracoids,  clavicles, 
and  interclavicle  (see  Fig.  2qD,  p.  88).  In  all  of  the  placental  mammals,  the  cora- 
coid  is  reduced  to  a  small  process,  the  coracoid  process,  the  interclavicle  is 
missing,  and  the  clavicles  are  often  reduced  or  wanting  (Fig.  2$E).  Conse- 
quently the  pectoral  girdle  of  most  mammals  consists  of  the  scapulae  only, 
which  are  correspondingly  enlarged  and  important  as  places  of  muscle  attach- 
ment. 

Study  the  pectoral  girdle  of  the  rabbit  or  cat.  It  consists  of  two  pairs  of 
bones,  the  clavicles  and  the  scapulae.  The  clavicles  are  small  slender  bones 
imbedded  in  the  muscles  of  the  front  of  the  shoulder,  and  as  they  are  not  artic- 
ulated to  the  rest  of  the  girdle  they  generally  fall  off  in  prepared  skeletons.  Iso- 
lated clavicles  will  be  demonstrated  and  they  will  be  seen  in  place  later  when  the 

1  Regarded  in  some  texts,  however,  as  the  second  and  third,  and  the  metacarpal  here  called  second 
as  the  first.  According  to  Lillie,  Development  of  the  Chick,  the  embryological  evidence  strongly  indicates 
that  the  persistent  metacarpals  are  the  second,  third,  and  fourth. 


THE  ENDOSKELETON:  GIRDLES,  THE  STERNUM,  AND  APPENDAGES     93 

muscles  are  dissected.  The  scapulae  or  shoulder  blades  are  the  large,  flat  triangu- 
lar bones  situated  dorsal  to  the  anterior  ribs.  The  mammalian  scapula  has  certain 
characteristics  which  differentiate  it  readily  from  the  scapulae  of  other  verte- 
brates. It  is  triangular  in  form,  the  apex  of  the  triangle  articulating  with  the 
humerus.  Its  outer  surface  bears  a  prominent  ridge,  the  spine  of  the  scapula. 
The  ventral  end  of  the  spine  terminates  ventrally  in  a  pointed  projection,  the  aero- 
mion  process;  just  above  this  and  projecting  laterally  and  posteriorly  is  the  meta- 
cromion  process,  very  long  in  the  rabbit.  The  apex  of  the  scapula  is  concavely 
curved,  forming  the  glenoid  fossa.  From  the  anterior  side  of  the  rim  of  the  fossa 
a  small  beaklike  process  projects  medially;  this  is  the  coracoid  process,  the  vestige 
of  the  coracoid  bone.  In  mammalian  embryos,  this  process  is  a  separate  bone. 
For  facilitating  the  description  of  muscle  attachments  the  various  surfaces  and 
borders  of  the  scapula  are  named  as  follows :  the  part  of  the  external  surface  ante- 
rior to  the  spine  is  the  supraspinous  fossa;  the  part  posterior  to  the  spine,  the 
infraspinous  fossa;  the  whole  of  the  internal  surface  is  the  subscapular  fossa; 
the  dorsal  border  is  the  vertebral  border;  the  anterior  margin,  the  anterior  border; 
the  posterior  margin,  the  axillary  border.  Draw  the  scapula  from  the  outer  surface. 

The  sternum  consists  of  a  longitudinal  series  of  pieces,  the  sternebrae,  eight 
in  the  cat,  six  in  the  rabbit.  The  first  sternebra  is  called  the  manubrium 
and  articulates  with  the  first  thoracic  rib  at  its  center.  The  next  six  (cat)  or 
four  (rabbit)  sternebrae  constitute  the  body  of  the  sternum.  The  last  piece  is 
called  the  xiphisternum  and  terminates  in  a  xiphoid  or  ensiform  cartilage.  Note 
points  of  articulation  of  the  sternebrae  with  the  ribs. 

The  fore  limb  is  fairly  typical.  The  humerus  has  a  large  rounded  head  fitting 
into  the  glenoid  fossa  and  greater  and  lesser  tuberosities  at  the  sides  of  the  head. 
The  anterior  surface  of  the  humerus  below  the  tuberosities  is  slightly  elevated 
into  ridges  or  crests  (two  in  the  cat,  one  in  the  rabbit)  which  serve  as  points  of 
muscle  attachment.  The  lower  end  of  the  humerus  is  rounded  for  articulation 
with  the  bones  of  the  forearm  and  is  divided  into  two  portions,  an  outer  mass, 
the  capitulum,  and  a  medial  mass,  the  trochlea.  Above  the  capitulum  is  a  pro- 
jecting ridge,  the  lateral  epicondyle;  and  a  similar  medial  epicondyle  is  situated 
above  the  trochlea.  Near  the  medial  epicondyle  the  bone  is  pierced  by  an 
opening,  the  supracondyloid  foramen,  absent  in  the  rabbit. 

The  forearm  consists  of  radius  and  ulna,  the  latter  the  larger.  The  proximal 
end  of  the  ulna  forms  a  prominent  projection,  the  elbow  or  olecranon.  Distal  to 
this  is  a  deep  semicircular  concavity,  the  semilunar  notch,  which  articulates  with 
the  trochlea  of  the  humerus.  The  distal  border  of  the  notch  forms  another 
projection,  the  coronoid  process.  Observe  that  the  proximal  end  of  the  radius  is 
situated  lateral  to  or  in  front  of  the  proximal  end  of  the  ulna,  while  its  distal 
end  is  medial  to  the  distal  end  of  the  ulna.  In  other  words,  the  radius  crosses 
obliquely  in  front  of  the  ulna.  The  cause  of  this  crossing  was  explained  in 
Section  III. 


94  LABORATORY  MANUAL  FOR  VERTEBRATE  ANATOMY 

The  wrist  is  composed  of  a  number  of  small  bones,  arranged  in  two  transverse 
rows.  The  proximal  row  consists  of  four  pieces  in  the  rabbit,  three  in  the  cat; 
the  distal  row  of  five  in  the  rabbit,  four  in  the  cat.  Articulating  with  the  distal 
end  of  the  radius  is  the  large  scapholunar  in  the  cat,  separated  in  the  rabbit  into 
a  medial  navicular  (radiate)  and  a  lateral  lunate  bone  (intermedium).  Lateral 
to  the  lunate  portion  or  bone  and  articulating  with  the  ulna  is  the  triquetral  bone 
(ulnare).  The  pisiform  is  the  element  projecting  prominently  lateral  to  the 
triquetral  bone  in  the  cat  or  behind  it  in  the  rabbit.  The  distal  row  of  pieces 
beginning  at  the  medial  side  and  proceeding  laterally  are :  the  greater  multangular 
(first  car  pale),  the  lesser  multangular  (second  car  pale),  the  central  (centrale,  missing 
in  the  cat),  the  capitate  (third  car  pale),  and  the  hamate  (fourth  and  fifth  car  pales 
fused).  These  carpales  are  situated  at  or  near  the  proximal  ends  of  their  respec- 
tive metacarpals.  There  are  five  metacarpals  of  which  the  first  is  very  much 
reduced,  and  five  digits  whose  terminal  phalanges  support  the  horny  claws. 

D.      GENERAL  SUMMARY    OF    THE    GIRDLES,    THE    STERNUM,    AND   THE 
PAIRED   APPENDAGES 

1.  The  paired  appendages  probably  represent  remnants  of  a  pair  of  continuous  lateral  fin 
folds,  supported  by  cartilaginous  fin  rays. 

2.  The  two  girdles  probably  arose  through  the  fusion  in  the  median  line  of  some  of  these 
fin  rays. 

3.  The  primitive  girdles  are  bars  or  plates  of  cartilage  in  which  subsequently  ossification 
occurred  with  the  formation  of  cartilage  bones. 

4.  In  each  girdle  three  pairs  of  cartilage  bones  arise.    These  are  ilium,  pubis,  and  ischium 
in  the  pelvic  girdle;  scapula,  procoracoid,  and  coracoid  in  the  same  respective  positions  in  the 
pectoral  girdle. 

5.  The  pelvic  girdle  after  the  three  pairs  of  bones  have  arisen  undergoes  little  change.    In 
birds  and  mammals  the  boundaries  between  the  bones  are  lost  by  fusion,  producing  on  each 
side  an  innominate  bone.    There  are  never  any  membrane  bones  associated  with  the  pelvic 
girdle. 

6.  The  pelvic  girdle  is  free  in  fishes  but  in  all  other  vertebrates  is  firmly  articulated  or 
ankylosed  (i.e.,  immovably  fused)  to  the  sacrum  by  means  of  the  sacral  ribs. 

7.  The  pectoral  girdle,  on   the  other  hand,   exhibits  many  modifications  among  the 
vertebrates  and  is  further  complicated  by  the  addition  of  membrane  bones. 

8.  In  the  majority  of  the  land  vertebrates  only  two  of  the  three  cartilage  bones  of  the 
primitive  girdle  persist.    The  scapula  is  always  present.     One  ventral  element  is  generally 
present,  called  the  coracoid,  and  believed  by  some  to  be  homologous  with  the  coracoid  in  the 
primitive  girdle,  by  others  to  be  homologous  with  the  procoracoid. 

9.  In  all  of  the  placental  mammals  the  coracoid  element  is  reduced  to  a  vestige,  the 
coracoid  process,  borne  on  the  scapula. 

10.  The  mammalian  scapula  is  distinguished  by  the  possession  of  a  spine  and  of  the  cora- 
coid process  mentioned  in  paragraph  9. 

11.  The  membrane  bones  added  to  the  pectoral  girdle  vary  in  different  forms,  but  in  living 
land  vertebrates  usually  consist  of  paired  clavicles  and  a  median  unpaired  interclavicle. 

12.  The  pectoral  girdle  is  very  rarely  connnected  with  the  vertebral  column  by  an  articula- 
tion or  ankylosis. 


THE  ENDOSKELETON:    GIRDLES,  THE  STERNUM,  AND  APPENDAGES     95 

13.  The  sternum  occurs  first  in  the  Amphibia  and  is  found  in  most  vertebrates  above 
A  mphibia.    It  is  probably  the  median  ventral  portion  of  the  pectoral  girdle.     In  many  reptiles, 
and  in  birds  and  mammals,  the  ribs  articulate  with  the  sternum. 

14.  The  bones  of  the  limbs  are  probably  derived  from  the  cartilaginous  fin  rays  of  the  lower 
fishes.    They  are  very  similar  in  all  land  vertebrates,  the  distal  portions  being  subject  to  the 
most  modifications. 

1 5.  The  limbs  are  divided  into  three  segments.    The  bones  of  these  segments  are  as  follows, 
those  of  the  hind  limb  being  named  first:   proximal  segment,  one  bone,  femur  or  humerus; 
middle  segment,  two  parallel  bones,  tibia  and  fibula,  radius  and  ulna;  distal  segment,  com- 
posed of  three  parts,  ankle  or  wrist,  sole  or  palm,  and  digits.    Ankle  or  wrist  consists  primitively 
of  nine  or  ten  bones  in  three  rows:  a  proximal  row  of  three,  named  tibiale  or  radiale,  inter- 
medium, and  fibulare  or  ulnare;  a  middle  row  of  one  or  two  centrales;  and  a  distal  row  of 
five  tarsales  or  carpales.    Loss  or  fusion  of  any  of  these  elements  is  quite  common.    The 
palm  or  sole  is  composed  of  five  parallel  bones,  the  metatarsals  or  metacarpals.    The  digits 
consist  of  phalanges  of  which  the  primitive  numbers  are,  proceeding  from  the  first  to  the  fifth 
digit,  2,  3,  3,  3,  3.     Loss  of  digits  is  also  common;  nearly  always  the  inner  and  outer  digits 
disappear  with  retention  of  middle  ones;  very  rarely  are  middle  digits  lost.     Extra  digits 
may  also  occur,  usually  on  aquatic  forms. 


THE   ENDOSKELETON:     THE    COMPARATIVE   ANATOMY   OF 
THE  SKULL  AND  THE  VISCERAL  SKELETON 

The  skull  or  cranium  is  that  part  of  the  endoskeleton  which  is  found  within  the  head 
where  it  covers  and  protects  the  brain.  The  skull  is  the  most  complex  of  all  the  parts  of  the 
endoskeleton,  owing  in  part  to  the  fact  that  it  is  derived  from  several  different  sources.  To 
determine  the  homology  of  the  bones  of  the  skull  of  different  vertebrates  is  a  tremendous 
task  which  is  not  yet  completed.  The  earlier  investigators  named  bones  in  different  skulls 
by  the  same  names  without  any  adequate  evidence  that  they  really  were  homologous,  with 


olfactory  sac . 
nasal  capsule 

prechordal 


olfactory  sac 

nasal  capsule 
ethmoid  plate 


vertebral 
sclerotome 


notochord 


vertebral 
sclerotome 


A  B 

FIG.  31. — Diagrams  to  illustrate  the  development  of  the  chondrocranium  from  the  prechordals 
parachordals,  sense  capsules,  and  sclerotomes  of  the  vertebrae  of  the  occipital  region.  A,  early  stage 
with  the  various  cartilages  separate.  B,  later  stage,  showing  union  of  the  prechordals  with  each  other 
and  with  the  olfactory  capsules  to  form  the  ethmoid  plate,  and  union  of  the  parachordals  with  each 
other  and  with  the  otic  capsules  and  occipital  vertebrae  to  form  the  basal  plate.  (Combined  from 
figures  by  Goodrich  and  Wilder.) 

resulting  confusion.  In  the  following  pages  an  attempt  is  made  to  facilitate  the  study  of 
the  skull  by  analyzing  the  skull  into  its  various  components  and  tracing  in  the  phylogenetic 
scale  the  union  of  these  components  into  the  complete  skull. 


A.      THE   CARTILAGE   STAGE   OF  THE   SKULL 

i.  Origin  of  the  chondrocranium. — The  skull  arises  in  the  mesenchyme  of  the  head.  The 
first  step  in  the  formation  of  the  skull  is  the  production  of  cartilage  by  the  mesenchyme.  Five 
pairs  of  cartilages  appear.  One  pair  forms  along  the  sides  of  the  anterior  end  of  the  notochord; 

96 


THE  ENDOSKELETON:    SKULL  AND  VISCERAL  SKELETON 


97 


cornua 
trabeculae 

ethmoid  plate 


these  are  designated  as  the  parachordal  cartilages.  Another  pair  is  laid  down  immediately 
anterior  to  the  parachordals;  these  are  the  prechordal  cartilages  or  trabeculae.  Cartilaginous 
capsules  also  develop  around  each  of  the  paired  sense  organs  of  the  head — olfactory  sacs, 
eyes,  and  ears.  These  are  known  respectively  as  the  nasal  capsules  around  the  olfactory- 
sacs,  the  optic  capsules  around  the  eyes,  and  the  otic  capsules  around  the  internal  ears 
(Fig.  31-4).  Fusion  of  these  five  paired  cartilages  then  occurs.  The  anterior  ends  of 
the  prechordals  fuse  together  to  form  the  ethmoid  plate  which  in  turn  fuses  laterally  with  the 
nasal  capsules.  The  prechordals  continue  in  front  of  the  ethmoid  plate  as  two  projections,  the 
horns  or  cornua  trabeculae.  The  parachordals  unite  to  form  the  basal  plate  which  then  fuses 
on  either  side  with  the  otic  capsules.  The  posterior  ends  of  the  prechordals  unite  with  the 
basal  plate  (Fig.  31 B).  The  optic  capsules 
never  become  fused  to  the  skull  since  it  is 
necessary  that  the  eyes  remain  independ- 
ently movable;  the  optic  capsules  persist 
around  the  eye  as  a  tough  protective  coat, 
the  sclera.  As  a  result  of  the  fusion  of 
prechordals,  nasal  capsules,  parachordals, 
and  otic  capsules,  a  nearly  continuous  plate 
of  cartilage  is  formed  below  the  ventral 
side  of  the  brain  and  inclosing  the  sense 
organs.  In  addition,  it  is  highly  probable 
that  certain  vertebrae  are  fused  to  the 
posterior  end  of  the  basal  plate.  The 
cartilaginous  structure  arising  in  this  way 
is  named  the  chondrocranium.  It  occurs  as 
a  stage  in  the  development  of  the  skull  in 
the  embryos  of  all  vertebrates.  It  must 
be  emphasized  that  in  the  majority  of 
vertebrate  embryos  the  chondrocranium 


occipital 
condyle 


notochord 


FIG.  32. — Chondrocranium  of  a  urodele  larva. 
(After  Gaupp  in  Hertwig's  Handbuch  der  ver- 
gleichenden  und  experimentellen  Entwickelungskhre 
der  Wirbeltiere.) 


has  the  form  of  an  elongated,  somewhat 
curved  plate  ventral  to  the  brain  and  sup- 
porting the  brain  on  its  concave  dorsal 
surface.  Generally  it  has  neither  lateral 
nor  dorsal  walls  except  at  the  extreme  pos- 
terior end  where  it  gives  rise  to  a  narrow 

dorsal  arch,  the  synotic  tectum.  Usually  there  are  gaps  in  this  plate,  known  zsfenestrac 
(Fig.  32).  In  the  elasmobranch  and  other  fishes  the  chondrocranium  gradually  extends  dorsally 
and  finally  incloses  the  brain  in  a  cartilaginous  box.  The  roof  of  this  is  generally  incomplete, 
presenting  one  or  more  gaps,  the  fontanelles,  closed  by  membranes.  Diagrams  of  the 

formation  of  the  chondrocranium  are  given  in  Figure  31;  similar  figures  will  also  be  found  in 

W,  Wd,  P  and  H,  and  K. 

For  more  complete  accounts  of  the  development  and  comparative  anatomy  of  the  skull, 

K,  W,  and  Wd  should  be  consulted. 

2.  The  chondrocranium  of  the  dogfish.— Study  the  chondrocranium  of  the 
dogfish  preserved  in  jars.  It  is  a  cartilaginous  mass  inclosing  the  brain  and 
without  sutures  or  divisions.  The  dorsal  surface  is  broad  and  flat  while  the 
ventral  surface  is  narrower  and  more  irregular.  The  anterior  end  of  the  chondro- 
cranium  is  drawn  out  into  a  troughlike  structure,  the  rostrum.  Each  side  of 


98        LABORATORY  MANUAL  FOR  VERTEBRATE  ANATOMY 

the  chondrocranium  presents  a  large  depression,  the  orbit,  which  in  life  holds 
the  eye. 

a)  Dorsal  surface  of  the  chondrocranium:   Study  the  dorsal  surface.     The 
rostrum  opens  dorsally  by  a  large,  egg-shaped  cavity,  the  anterior  fontanelle. 
At  either  side  of  the  base  of  the  rostrum  is  an  olfactory  capsule,  a  projecting 
structure  with  thin  walls.     Posterior  to  the  capsule  and  continuous  with  its 
dorsal  wall  is  a  thick  projecting  shelf,  the  supraorbital  crest,  which  forms  the 
dorsal  wall  of  the  orbit.     The  posterior  end  of  the  crest  continues  into  a  pro- 
jection, the  postorbital  process.    Along  the  medial  borders  of  the  supraorbital 
crests  runs  a  row  of  openings,  which  are  nerve  foramina,  i.e.,  openings  through 
which  nerves  pass  to  and  from  the  brain.     In  the  median  line  just  back  of  the 
anterior  fontanelle  is  an  opening,  the  epiphyseal  foramen,  through  which  a  por- 
tion of  the  brain  (epiphysis  or  pineal  body)  extends.     In  the  median  line  of  the 
posterior  part  of  the  roof  is  a  rounded  depression,  the  endolymphatic  fossa,  in  which 
are  situated  two  pairs  of  openings,  the  terminations  of  the  endolymph  (smaller 
outer  holes)  and  perilymph  (larger  medial  holes)  ducts  of  the  internal  ear.     These 
ducts  are  canals  which  connect  the  fluid-filled  channels  of  the  ear  with  the  surface 
of  the  skull.     On  each  side  of  the  endolymphatic  fossa  is  a  massive  region  which 
is  the  auditory  or  otic  capsule,  fused  to  the  cranium.     The  posterior  end  of  the 
cranium  bears  an  opening,  the  foramen  magnum,  visible  dorsally  just  back  of 
the  endolymphatic  fossa.     Through  this  opening  the  brain  is  continuous  with 
the  spinal  cord.     Draw  the  chondrocranium  from  the  dorsal  side. 

b)  Ventral  and  lateral  surfaces  of  the  chondrocranium:  The  ventral  surface  of 
the  rostrum  bears  a  projecting  carina  or  keel.     On  either  side  of  this  medial  keel 
is  an  oval  opening,  which  may  be  designated  the  rostral  fenestra.     Through  the 
rostral  fenestrae  one  can  look  into  the  cavity  of  the  cranium,  which  in  life  is 
occupied  by  the  brain.     Lateral  to  each  rostral  fenestra  is  the  thin-walled, 
olfactory  capsule.     In  most  specimens  part  of  the  walls  of  this  will  probably 
be  broken  away.     When  complete,  the  nasal  capsule  is  a  nearly  spherical  struc- 
ture with  a  relatively  small  opening  to  the  exterior.     At  the  bottom  of  the  nasal 
capsule  is  a  large  opening  which  leads  into  the  cavity  of  the  skull.    Posterior 
to  each  nasal  capsule  and  continuous  with  its  posterior  wall  is  the  preorbital 
process,  which  forms  the  anterior  wall  of  the  orbit.     The  walls  of  the  orbit  are 
pierced  by  nerve  foramina  of  which  the  largest,  located  in  the  anteroventral 
region  of  the  orbit,  is  the  optic  foramen,  through  which  the  optic  nerves  pass 
from  the  eye.1    The  ventral  wall  of  the  skull  between  the  two  orbits  is  rather 
narrow,  its  narrowest  portion  being  the  place  where  a  process  of  the  upper  jaw 
articulates.     The  posterior  part  of  the  ventral  surface  forms  a  broad  basal  plate, 
whose  sides  are  composed  of  the  otic  capsules.     In  the  median  line  of  the  basal 
plate  will  be  seen  a  streak  of  slightly  different  color  from  the  chondrocranium. 

1  Projecting  into  the  orbit  may  be  present  a  mushroom-shaped  structure,  the  optic  pedicel,  which 
in  life  supports  the  eyeball. 


THE  ENDOSKELETON:    SKULL  AND  VISCERAL  SKELETON  99 

This  streak  is  the  notochord.  Its  anterior  end  turns  dorsally  into  the  cartilage 
and  terminates  at  about  the  level  of  the  small  median  foramen  lying  near  the 
anterior  end  of  the  basal  plate  through  which  the  internal  carotid  artery  passes. 
It  will  be  understood  that  those  portions  of  the  chondrocranium  lying  to  either 
side  of  the  notochord  are  the  products  of  parachordals  and  that  the  parts  anterior 
to  this  up  to  the  olfactory  capsules  are  the  products  of  the  prechordals.  The 
parachordals  are  fused  laterally  with  the  otic  capsules  and  the  prechordals  with 
the  olfactory  capsules.  The  lateral  and  dorsal  walls  of  the  brain  are  produced 
by  extension  of  these  original  parts.  The  posterior  end  of  the  ventral  surface 
of  the  skull  is  produced  at  either  side  of  the  notochord  into  a  slight  process,  the 
occipital  condyle,  which  articulates  with  the  first  vertebra.  Draw  a  ventral 
view  of  the  chondrocranium. 

B.      THE  VISCERAL  SKELETON 

The  visceral  skeleton  or  splanchnocranium  is  that  part  of  the  endoskeleton 
which  supports  the  gills.  The  gills  are  located  in  the  ventral  and  posterior  part 
of  the  head.  To  support  the  gills  and  enable  them  to  be  moved  for  respiratory 
purposes  a  special  skeleton  is  present  which  is  called  the  visceral  skeleton, 
because  gills  are  part  of  the  walls  of  the  digestive  tract,  as  will  be  demonstrated 
later.  The  visceral  skeleton  consists  of  a  longitudinal  series  of  crescent-shaped 
cartilages  (or  bones)  situated  between  the  gill  slits  in  the  pharyngeal  wall.  Each 
such  element  is  designated  a  gill  arch.  There  are  typically  seven  gill  arches  in 
vertebrates,  although  some  elasmobranchs  have  nine  (Fig.  33).  The  gill  arches 
in  the  lower  vertebrates  are  closely  associated  with  the  chondrocranium,  and  in 
the  course  of  evolution  some  of  them  take  part  in  the  production  of  the  skull. 

i.  The  visceral  skeleton  of  the  dogfish. — Obtain  a  specimen  in  which  the 
visceral  skeleton  has  been  left  attached  to  the  chondrocranium,  and  study  it 
carefully.  The  seven  gill  arches  form  a  series  of  curved  cartilages  ventral  to 
the  posterior  part  of  the  chondrocranium  and  extending  posteriorly  to  the  pectoral 
girdle.  The  first  gill  arch,  the  mandibular  arch,  is  the  largest  and  most  modified 
of  the  series.  It  is  seen  when  viewed  from  below  to  consist  of  dorsal  and  ventral 
halves.  Each  side  of  the  dorsal  half  is  called  the  palatoquadrate  or  pterygoquadrate 
cartilage;  in  profile  view  it  will  be  seen  that  this  cartilage  is  closely  applied  to  the 
ventral  surface  of  the  chondrocranium,  to  which  in  life  it  is  attached  by  a  liga- 
ment. It  sends  up  a  well-developed  palatal  process  into  the  orbit.  The  pterygo- 
quadrate cartilages  bear  teeth  and,  in  fact,  constitute  the  upper  jaw  of  the  animal. 
The  ventral  half  of  the  mandibular  arch  consists  of  two  halves,  each  of  which  is 
known  as  MeckeVs  cartilage.  These  bear  teeth  and  together  constitute  the  lower 
jaw  of  the  dogfish.  The  wide  gap  between  the  two  jaws  is  the  mouth  opening. 
At  their  posterior  ends  the  pterygoquadrate  and  Meckel's  cartilages  join  at  an 
acute  angle  called  the  angle  of  the  jaw,  forming  a  hinge  joint,  permitting  opening 
and  closing  of  the  mouth. 


100 


LABORATORY  MANUAL  FOR  VERTEBRATE  ANATOMY 


The  second  or  hyoid  arch  is  more  slender  than  the  mandibular  arch  to  the 
posterior  face  of  which  it  is  closely  applied.  It  consists  of  a  ventral  median 
piece,  the  basihyal;  a  slender  bar,  the  ceratohyal,  on  each  side  of  the  basihyal; 
and  a  stout  piece,  the  hyomandibular,  dorsal  to  each  ceratohyal.  The  hyoman- 
dibular  articulates  to  the  otic  region  of  the  skull  and  thus  acts  as  a  suspensor  of 
the  lower  jaw.  The  hyomandibular  bears  on  its  posterior  margin  some  slender 
projections,  the  gill  rays,  which  in  life  support  the  gills. 

The  remaining  arches,  known  simply  as  gill  or  branchial  arches,  are  similar 
to  each  other.  Each  consists  typically  of  five  pieces,  named  from  the  dorsal 
side  ventrally:  pharyngobranchial,  the  most  dorsal  piece,  elongated  and  directed 
posteriorly;  epibranchial,  the  succeeding  much  shorter  piece;  ceratobranchial, 


orbit 


chondrocranium 
olfactory  capsule 


otic  capsule 

hyomandibular 


pterygoquadrate 

cartilage 
mandibular  (first) 

gill  arch 

Meckel's 
cartilage 


hyoid  (second) 
gill  arch 


ceratohyal 


third  to  seventh 
gill  arches 


neural  arch     intercalary  arch 


pharyngobranchial 
epibranchial 

ceratobranchial 
hypobnnchiai 

basibranchial 

or  copula 


FIG.  33. — Diagram  of  the  chondrocranium,  vertebral  column,  and  gill  arches  of  an  elasmobranch 
to  show  particularly  the  parts  and  relations  of  the  seven  gill  arches.  (Slightly  modified  from  Vialleton's 
Elements  de  Morphologie  des  Vertebres.) 

another  elongated  piece;  hypobranchial,  curved  ventral  pieces,  of  which  there 
are  but  three  pairs  to  the  five  branchial  arches;  and  the  basibranchials,  two  in 
number — an  anterior  small  one  situated  between  the  medial  ends  of  the  first 
and  second  pairs  of  hypobranchials  and  a  large  posterior  piece  between  the  bases 
of  the  fifth  ceratobranchials  and  terminating  in  a  caudally  directed  point. 
Epi-  and  ceratobranchials  bear  gill  rays.  Note  that  the  gill  arches  are  not 
attached  to  the  vertebral  column.  A  diagram  of  the  gill  arches  is  given  in 
Figure  33. 

Draw  from  the  side,  showing  chondrocranium  and  visceral  skeleton. 


C.   THE  FORMATION  OF  THE  MEMBRANE  BONES  OF  THE  SKULL 

i.  Origin  of  the  membrane  bones  of  the  skull. — In  addition  to  the  chondrocranium  and 
gill  arches,  still  another  component  enters  into  the  formation  of  the  skull.  This  component 
consists  of  the  dermal  or  membrane  bones  of  the  skull.  These  elements  first  appear  in  the 
ganoid  fishes.  In  these  forms  it  can  be  observed  that  the  ganoid  scales  have  become  fused 
on  the  head  to  form  large  bony  plates  incasing  the  head.  These  scales,  which  as  we  have 
already  learned,  are  dermal  in  origin,  sink  into  the  head  and  apply  themselves  closely  to  the 
chondrocranium  and  mandibular  arch.  It  has  already  been  pointed  out  that  in  the  majority 


THE  ENDOSKELETON:    SKULL  AND  VISCERAL  SKELETON  101 

of  vertebrates  the  chondrocranium  consists  chiefly  of  a  ventrally  located  plate  and  lacks  side 
walls  and  roof.  Walls  and  roof  are  completed  by  these  dermal  scales,  which  thereupon 
become  the  superficial  bones  of  the  skull.  Similarly  dermal  scales  incase  the  pterygoquadrate 
and  Meckelian  cartilages,  which  are  the  primitive  upper  and  lower  jaws,  and  become  the  super- 
ficial bones  of  the  jaws.  In  this  fashion  the  entire  skull  becomes  sheathed  in  dermal  scales, 
originating  in  the  skin  and  in  reality  belonging  to  the  exoskeleton.  These  incasing  bones  of 
the  skull  and  jaws  are  known  as  dermal,  membrane,  or  investing  bones,  because  of  their 
manner  of  origin.  The  membrane  bones  of  the  skull  are  present  in  their  most  complete  and 
generalized  condition  in  the  earliest  Amphibia  (Stegocephald)  and  the  earliest  reptiles  (Coty- 
losauria) ;  but  in  the  course  of  evolution  gradually  decrease  in  number  owing  to  processes  of 
loss  and  fusion  (see  Fig.  36,  p.  106). 

2.  Membrane  bones  of  some  typical  ganoids. — Study  the  ganoid  scales  on 
the  head  of  any  one  of  the  following  ganoids:  Polypterus,  sturgeon  (Acipenser), 
the  gar  pike  (Lepidosteus) ,  or  the  bowfin  (Amid).  Some  of  these  enlarged 
scales  on  the  heads  of  ganoid  fishes  correspond  to  the  superficial  bones  of  the 
skull  of  higher  vertebrates,  and  have  received  the  same  names;  others,  especially 
those  supporting  the  operculum,  have  been  lost  subsequently.  The  student 
should  understand  that  these  dermal  scales  are  on  the  outside  of  the  animal's 
head,  imbedded  in  the  skin,  and  that  a  typical  cartilaginous  or  partially  ossified 
chondrocranium,  like  that  of  the  dogfish,  is  present  inside  of  the  covering  of 
scales. 

a)  Membrane  bones  of  Polypterus:  (see  Fig.  34^)   The  dermal  scales  on  the 
head  of  Polypterus  are  very  much  like  the  bones  on  the  dorsal  side  of  the 
skull  of  higher  forms.     Identify  on  the  dorsal  side  of  the  head  the  two  small 
nasal  openings  near  the  tip  and  the  larger  oval  orbits  posterior  to  them.     In 
front  of  the  nasal  openings  is  a  pair  of  bones  bearing  teeth,  the  premaxillae; 
behind  the  nasal  openings  is  a  pair  of  nasals;  between  the  two  nasals  is  situated 
a  small  triangular  membrane  bone,  the  dermal  mesethmoid.    Posterior  to  the 
nasals  are  two  large  frontal  bones,  and  posterior  to  them,  two  smaller  parietal 
bones.     The  several  small  bones  posterior  to  the  parietals  are  called  temporals. 
A  row  of  small  bones  extends  from  each  orbit  posteriorly;  these  are  poster Utah. 
Below  the  orbit  is  an  elongated  bone  bearing  teeth,  the  maxilla.    The  sides 
of  the  head  behind  the  orbit  are  covered  by  large  flat  bones,  the  operculars. 
The  lower  jaw  is  similarly  clothed  in  dermal  bones,  consisting  of  a  tooth-bearing 
dentary  hi  front  and  a  toothless  angular  behind. 

b)  Membrane  bones  of  the  sturgeon:   (Fig.  34*?)   The  anterior  end  of   the 
sturgeon's  head  is  extended  into  a  snout  or  rostrum,  covered  by  many  small 
rostral  scales.     At  the  sides  of  the  base  of  the  snout  are  the  two  pairs  of  nasal 
openings,  and  just  posterior  to  them,  the  orbit.     On  the  dorsal  side  between 
the  two  orbits  are  two  large  scales,  the  frontals,  and  posterior  to  them,  two 
parietals.    Numerous  other  small  bones  need  not  be  considered.     The  jaws  and 
visceral  skeleton  of  the  sturgeon  are  degenerate  on  account  of  its  method  of 
feeding. 


IO2 


LABORATORY  MANUAL  FOR  VERTEBRATE  ANATOMY 


nans 


jugal 

maxilla 

suborbitals 

frontal 
postfrontat 
postorbitals 
squamosal 
parietal 
preopercular 
supratemporal 


posttemporal 
opercular 


nans 


rostal  plates 


postfrontal 

spiracle 

parietal 

squamosal 


prespiraculars 
preopercular 

-spiracular 

•  spiracle 

•  parietal 

subopercular 
opercular 

postspiraculars 
supratemporals 
posttemporal 


nasal 
naris 

adnasal 


ethmonasal 


maxilla 


posttemporal 


parietal 
squamosal 
.  supratemporals 


posttemporal 


FIG.  34. — Dermal  scales  (superficial  bones  of  the  skull)  of  the  head  of  some  ganoid  fishes.  A , 
Amia.  B,  Polypterus.  C,  sturgeon  (Acipenser).  D,  gar  pike  (Lepidosteus) .  The  arrangement  of 
some  of  these  scales,  which  in  teleosts  become  the  bones  of  the  dorsal  surface  of  the  skull,  greatly  resembles 
that  of  the  dorsal  skull  bones  of  the  land  vertebrates,  and  these  scales  have  received  the  same  names 
as  the  skull  bones,  although  the  strict  homology  is  doubtful.  (After  Goodrich  in  Part  IX  of  Lankester's 
Treatise  on  Zoology,  courtesy  of  the  Macmillan  Company.) 


THE  ENDOSKELETON:    SKULL  AND  VISCERAL  SKELETON  103 

c)  Membrane  bones  of  the  gar  pike:   (Fig.  34!))   The  head  is  prolonged  into 
a  long  snout  having  the  nasal  openings  at  its  extremity.     The  small  nasal  bones 
surround  the  nasal  openings.     Posterior  to  these  occupying  the  center  of  the 
dorsal  surface  of  the  snout  are  the  elongated  ethmonasal  bones.    Posterior  to 
them  are  the  frontals,  whose  anterior  ends  inclose  the  posterior  ends  of  the 
ethmonasals.     Posterior  to  the  frontals  are  the  large  parietals  and  behind  them 
a  number  of  temporals.     The  edges  of  the  upper  jaw  are  composed  of  the  maxillae, 
bearing  teeth  and  each  consisting  of  a  series  of  squarish  bones.     The  lower  jaws 
consist  chiefly  of  the  long  dentary  bones,  bearing  teeth.     The  sides  of  the  head 
behind  the  orbit  are  covered  by  a  large  number  of  small  cheek  plates  and,  posterior 
to  them,  by  the  larger  operculars. 

d)  Membrane  bones  of  the  bowfin:  The  skull  of  Amia  is  "perhaps  less  special- 
ized than  in  any  other  living  teleostome, "  according  to  L  (Fig.  34^4).    Identify 
on  the  skull  anterior  stalked  and  posterior  nostrils  and  the  orbits.     Between  the 
anterior  nostrils  projects  the  small  triangular  dermal  mesethmoid,  a  membrane 
bone.     Anterior  to  this  are  the  premaxillae,  bearing  teeth.     Covering  the  space 
between  the  four  nostrils  are  the  nasals,  posterior  to  them  in  the  median  line  the 
large  frontals,  posterior  to  them  the  parietals,  and  at  the  posterior  end  of  the 
skull  the  four  temporals.     Below  and  anterior  to  the  orbit  is  the  large  lacrimal, 
below  the  orbits  the  two  suborbitals,  and  posterior  to  the  orbit  two  large  post- 
orbitals.     Forming  the  sides  of  the  upper  jaw  are  the  tooth-bearing  maxillae. 
Between    the   dorsal,    postorbital,    and    the   parietal   is    the   squamosal.     The 
operculum  is  supported  by  several  opercular  bones. 

e)  Demonstration  of  the  separate  origin  of  chondrocranium  and  membrane 
bones:  Examine  the  demonstration  specimen  of  the  head  of  a  ganoid  fish  in  which 
the  incasing  membrane  bones  have  been  loosened  from  the  underlying  chondro- 
cranium.    Remove  the  sheath  of  membrane  bones,  noting  that  they  are  situated 
in  the  skin,  and  note  the  chondrocranium,  similar  to  that  of  the  dogfish,  lying 
within  the  sheath.     Such  a  specimen  shows  clearly  the  origin  of  the  skull  from 
two  separate  sources,  the  cartilaginous  chondrocranium  and  the  dermal  scales. 


D.   THE  FORMATION  OF  THE  CARTILAGE  BONES  OF  THE  SKULL  AND  THE 
COMPOSITION  OF  THE  COMPLETE  SKULL 

The  next  and  last  step  in  the  formation  of  the  skull  is  the  production  of  cartilage  bones 
in  the  chondrocranium,  including  the  sense  capsules,  and  in  the  mandibular  and  hyoid  arches. 
In  definite  regions  of  these  structures  centers  of  ossification — that  is,  centers  of  bone  forma- 
tion— arise.  Each  of  these  centers  transforms  a  certain  area  of  cartilage  into  bone,  and 
each  such  bone  is  primitively  a  cartilage  bone  of  the  skull.  It  has  already  been  stated  that 
in  the  majority  of  vertebrates  the  chondrocranium,  present  in  the  embryonic  stage  only, 
consists  of  a  ventrally  situated  plate  with  neither  lateral  nor  dorsal  walls,  except  at  the  extreme 
posterior  end  (Fig.  3  2) .  Consequently,  the  cartilage  bones  of  the  skull,  formed  in  the  chondro- 
cranium, are  likewise  limited  to  the  ventral  and  extreme  posterior  parts  of  the  skull,  where 
thev  form  a  floor  for  the  brain.  The  roof  and  sides  of  the  skull  are  completed  by  membrane 


104 


LABORATORY  MANUAL  FOR  VERTEBRATE  ANATOMY 


bones,  derived,  as  we  have  already  seen,  from  the  dermal  scales  of  the  ganoid  fishes.  In  a 
similar  way  certain  cartilage  bones  arise  in  the  mandibular  and  hyoid  arches  and  become 
sheathed  in  membrane  bones.  The  complete  vertebrate  skull,  therefore,  consists  of  cartilage 
bones  derived  from  the  chondrocranium  and  its  sense  capsules,  and  the  first  and  second  gill 
arches,  and  of  membrane  bones  covering  the  cartilage  bones  everywhere  except  on  the  ventral 
surface  and  posterior  end  of  the  skull.  We  may  now  state  in  detail  the  bones  derived  in  these 
ways  to  form  the  skull. 

1.  Cartilage  bones  derived  from  the  pre-  and  parachordals. — There  are  four  groups  of 
such  cartilage  bones:  one  occipital,  two  sphenoid,  and  one  ethmoid  group  (see  Fig.  35). 

a)  Occipital  group:  This  consists  of  four  occipital 
bones  situated  at  the  posterior  end  of  the  skull, 
forming  a  ring  around  the  foramen  magnum.  They 
are  one  supraoccipital,  two  exoccipitals,  and  one 
basioccipital.  They  arise  from  the  parachordal  car- 
tilages, together  with  their  dorsal  extension — the 
synotic  tectum — and  the  vertebral  elements  which 
are  fused  to  the  posterior  end  of  the  parachordals 
(Fig.  35). 

b)  Posterior   sphenoid   group:    This   comprises 
three  sphenoid  bones,  on  the  ventral  side  of  the 
skull,  in  front  of  the  basioccipital.     They  are  the 
median  unpaired  basisphenoid  and  the  paired  ali- 
sphenoids,  one  on  each  side  extending  up  into  the 
back  wall  of  the  orbit.    These  bones  come  from  the 
posterior  part  of  the  prechordals. 

c)  Anterior  sphenoid  group:  This  includes  three 
bones  on  the  ventral  side  anterior  to  the  preceding 
and  also  derived  from  the  prechordal  cartilages. 
They  are  a  median  unpaired  presphenoid  and  paired 
orbitosphenoids,  one  on  each  side  forming  part  of 
the  walls  of  the  orbit. 

d)  Ethmoid  group:  This  group  consists  of  three 
bones  at  the  anterior  end  of  the  ventral  side  of  the 
skull,  just  behind  and  fused  with  the  olfactory  cap- 
sules.   The  bones  are  a  median  mesethmoid  and 
paired  ectethmoids,  one  on  each  side  of  the  preceding. 
The  mesethmoid  forms  a  median  partition  or  sep- 
tum between  the  two  olfactory  capsules,  while  the 

ectethmoids  contribute  to  the  posterior  walls  of  these  capsules.  The  ethmoids  arise  in  the 
ethmoid  plate,  which  represents  the  fused  anterior  ends  of  the  prechordals. 

2.  Cartilage  bones  derived  from  the  sense  capsules. — 

a)  From  the  otic  or  auditory  capsules:  A  number  of  otic  bones  arise  in  the  walls  of  the 
otic  capsules.  Since  the  latter  are  fused  with  the  parachordal  region  of  the  skull,  the  otic 
bones  are  naturally  closely  associated  with  the  occipital  group  of  bones  and  often  fused  with 
them.  There  may  be  as  many  as  five  otic  bones  (in  teleosts),  the  prootic,  epiotic,  opisthotic, 
pterotic,  and  sphenotic,  but  they  are  commonly  fused  together  or  fused  with  nearby  bones. 
The  three  first  named  are  the  ones  most  constant  in  the  higher  vertebrates.  When  fused 
into  one  bone  they  are  designated  as  the  periotic  or  petromastoid  bone. 

b}  From  the  optic  capsules:  As  already  explained,  the  optic  capsules  do  not  fuse  with  the 
skull,  owing  to  the  necessity  for  retaining  free  movement  in  the  eyes.  The  optic  capsule 


exoccipital 
supraoccipital 

FIG.  35. — Diagram  to  show  the  ar- 
rangement of  the  cartilage  bones  of  the 
skull,  seen  from  above.  The  separate 
otic  bones,  which  may  be  as  many  as  five 
in  number,  are  not  represented.  The 
supraoccipital  forms  an  arch  above  the 
basioccipital.  (Modified  from  Kingsley's 
Comparative  Anatomy  of  Vertebrates.) 


THE  ENDOSKELETON:    SKULL  AND  VISCERAL  SKELETON  105 

generally  forms  in  vertebrates  a  tough  membrane,  the  sclera  or  sclerotic  coat,  which  is  the 
outer  coat  of  the  eyeball.  In  some  vertebrates,  however,  particularly  birds  and  reptiles,  a 
ring  of  sclerotic  bones  arises  in  the  sclera,  but  these  always  remain  free  from  the  skull. 

c)  From  the  olfactory  capsules:   Additional  cartilage  bones,  closely  fused  to  and  almost 
indistinguishable  from  the  ethmoid  bones  already  mentioned  may  arise  in  the  walls  of  the 
olfactory  capsules.    These  are  the  lateral  ethmoids  and  the  turbinals  or  conchae  in  part. 
The  latter  are  curiously  scrolled  or  grooved  bones  situated  on   the  walls  of  the  nasal 
cavities. 

3.  Cartilage  bones  derived  from  the  gill  arches. — 

a)  From  the  pterygoquadrate  cartilage:  These  cartilages  form,  as  we  have  seen,  the  primi- 
tive upper  jaw  and  the  dorsal  half  of  the  first  or  mandibular  gill  arch.    The  anterior  part  of 
these  cartilages  may  form  the  palatines,  the  middle  part  one  or  more  paired  pterygoids,  while 
the  posterior  part  invariably  gives  rise  to  the  paired  quadrate  bones  to  which  the  lower  jaw 
is  generally  articulated.     It  should  be  noted  that  the  bones  designated  in  various  vertebrates 
as  palatines  and  pterygoids  are  in  some  cases  cartilage  bones  derived  from  the  pterygoquadrate 
cartilage  and  in  many  cases  are  membrane  bones.     The  quadrates  are  therefore  the  only 
constant  cartilage  bones  originating  from  the  upper  jaw  (Fig.  37,  p.  107). 

b)  From  Meckel's  cartilage:  These  two  cartilages  constitute  the  two  halves  of  the  lower 
jaw  and  the  ventral  half  of  the  first  gill  arch.    The  greater  part  of  Meckel's  cartilages  fails 
to  ossify,  and  either  remains  as  a  cartilaginous  core  of  the  lower  jaw  or  disappears.    The 
posterior  end  of  each  cartilage  commonly  ossifies  to  form  the  articular  bone  which  articulates 
with  the  quadrate,  forming  a  hinge  joint  for  the  lower  jaw.    The  anterior  tip  of  the  cartilage 
may  rarely  produce  a  cartilage  bone  (mentomeckelian  bone).    The  articular  bones  are  thus  the 
chief  contribution  of  the  lower  jaw  to  the  skull. 

c}  From  the  second  or  hyoid  gill  arch:  The  dorsal  portion  of  the  hyoid  arch  forms  the 
hyomandibular  cartilage  as  we  saw  in  elasmobranchs.  This  ossifies  into  a  hyomandibular 
bone,  which  in  many  fishes  suspends  the  lower  jaw.  The  remainder  of  the  hyoid  arch  together 
with  parts  of  the  succeeding  gill  arches  forms  the  hyoid  apparatus.  This  is  a  plate  or  bar  of 
cartilage  or  bone,  situated  in  the  floor  of  the  mouth  cavity  and  throat,  from  which  processes 
extend  posteriorly  on  each  side  to  the  otic  region.  It  is  variable  in  form  and  composition 
in  different  vertebrates. 

d )  From  the  remaining  gill  arches:  These  arches  ossify  in  fishes,  but  above  fishes  gradually 
retrogress  and  become  much  modified,  taking  part  in  the  formation  of  the  hyoid  apparatus 
and  transforming  into  the  cartilages  of  the  larynx.    The  larynx  is  the  Adam's  apple,  so-called, 
of  man,  and  is  the  chamber  at  the  top  of  the  windpipe  from  which  the  voice  comes.    The 
walls  of  this  chamber  are  supported  by  cartilages  which  are  the  remains  of  the  cartilages  of 
the  gill  arches.     Wd,  page  374,  gives  a  figure  to  show  the  origin  of  the  larynx  of  man  from  the 
gill  arches. 

4.  Membrane  bones  added  to  the  dorsal  surface  of  the  skull. — As  has  been  said,  the 
chondrocranium  is  open  dorsally  (except  in  fishes)  and  no  cartilage  bones  are  ever  formed 
there  in  any  vertebrates,  except  at  the  extreme  posterior  end.    Instead,  the  roof  of  the  skull 
consists  of  membrane  bones,  evolved  from  the  dermal  scales  of  the  ganoid  fishes.    The  chief 
membrane  bones  covering  the  olfactory  capsules  and  roof  of  the  skull  are:  nasals,  lacrimals, 
prefrontals,  frontals,  post/rentals,  and  parietals.    A  number  of  others  are  present  in  fishes, 
such  as  orbitals,  temporals,  operculars,  etc.,  but  as  these  do  not  persist,  we  shall  not  consider 
them  further.     The  earliest  Amphibia  and  reptiles  also  had  a  considerably  larger  number  of 
membrane  bones  in  the  roof  of  the  skull  than  do  any  living  land  vertebrates.    These  are 
represented  in  Figure  3  64  and  B.     From  such  a  generalized  condition  the  dorsal  aspect  of  the 
skull  of  present-day  land  vertebrates  has  resulted  through  the  loss  of  certain  bones  (see 
Fig    joCandP). 


io6 


LABORATORY  MANUAL  FOR  VERTEBRATE  ANATOMY 


lacrimal 

prefrontal 

frontal 

jugal 

orbit 

postfrontal 
postorbital 
parietal 
supratemporal 

squamosal 

r'^adratojugal 

tabulare 

quadrate 

dermosupraoccipital 
exoccipital 


orbit 
jugal 

postfrontal 
postorbital 
intertemporal 
squamosal 
supratemporal 
'parietal 

quadratojugal 

tabulare 
dermosupraoccipital 


B 


nans 
premaxilla 


premaxilla 


supratemporal 
arcade 


interparietal 
occipital 


quadratojugal 


FIG.  36. — Dorsal  view  of  the  skulls  of  four  representative  vertebrates  to  show  the  reduction  of  the 
membrane  bones  of  the  roof  in  the  course  of  evolution.  A,  skull  of  an  extinct  amphibian,  Capilosaurus, 
belonging  to  the  Stegocephala;  note  the  large  number  of  membrane  bones  completely  roofing  the  skull. 
B,  skull  of  one  of  the  most  ancient  reptiles,  Seymour ia,  belonging  to  the  Cotylosauria;  the  membrane 
bones  are  nearly  as  numerous  as  in  the  amphibian,  are  similarly  arranged,  and  completely  roof  the 
skull.  C,  skull  of  a  modern  reptile,  the  alligator;  several  of  the  membrane  bones  present  in  the  extinct 
forms  have  been  lost,  and  the  roof  bears  several  openings.  D,  skull  of  a  modern  mammal,  the  dog, 
showing  still  greater  loss  of  membrane  bones.  Membrane  bones  blank;  cartilage  bones  stippled. 
(A  from  Reynolds'  The  Vertebrate  Skeleton,  courtesy  of  the  Macmillan  Company;  B  from  Willis  ton's 
Water  Reptiles  of  the  Past  and  Present,  University  of  Chicago  Press.) 


THE  ENDOSKELETON:   SKULL  AND  VISCERAL  SKELETON 


107 


premaxflla 


5.  Membrane  bones  added  to  the  ventral  surface  of  the  skull. — These  are  from  the 
anterior  end,  posteriorly,  chiefly:  vomers,  palatines,  ptery  golds  (Fig.  37).     It  should  be  stated 
again  that  palatines  and  pterygoids  are  membrane  bones  in  some  animals  and  cartilage  bones 
in  others.    The  parasphenoid  is  a  membrane  bone  which  in  Amphibia  typically  forms  the 
chief  bone  on  the  ventral  surface  of  the  skull,  but  it  does  not  persist  in  higher  forms.    These 
membrane  bones  are  situated  ventral  to  the  ethmoid  and  part  of  the  sphenoid  bones,  which 
they  conceal  from  surface  view,  but  the  posterior  part  of  the  ventral  side  of  the  skull  is  com- 
posed of  cartilage  bones,  sphenoids  and  occipitals. 

6.  Membrane  bones  added  to  the  jaws,  and  other  gill  arches. — 

a)  Upper  jaw:  The  upper  jaw  (pterygoquadrate  cartilages)  becomes  incased  in  membrane 
bones.    The  chief  ones  from  the  tip  of  the  jaw  posteriorly  are,  on  each  side:   premaxilla, 
maxilla,  jugal  (malar],  quadratojugal,  and 

squamosal  (see  Fig.  37).  The  upper  jaw  in 
elasmobranchs  is  separate  from  the  chon- 
drocranium,  to  which  it  is  generally  at- 
tached by  ligaments,  but  in  all  of  the  land 
vertebrates  the  upper  jaw  is  inseparably 
fused  to  the  ventral  side  of  the  chondrocra- 
nium  and  becomes  an  integral  part  of  the 
skull. 

b)  Lower  jaw:  The  lower  jaw  (Meckel's 
cartilages)  is  similarly  sheathed  in  mem- 
brane bones,  Meckel's  cartilage  often  per- 
sisting within  them.    The  chief  membrane 
bones  of  the  lower  jaw  are  the  dentary,  the 
splenial,  the  angular,  the  sur angular  (also 
given  as  supra-angular),  the  coronoid,  the 
gonial,  and  two  or  three  others  occurring 
only  in  extinct  forms.    In  the  evolution  of 
the  lower  jaw  there  has  been  a  continual 
decrease  in  the  number  of  membrane  bones 


maxilla 


external  nares 


palatine 


pterygoid 


quadrate 


sQuaniosal 


FIG.  37. — Diagram  of  the  cartilage  and  mem- 
brane bones  of  the  upper  jaw  and  floor  of  the 
skull.  Membrane  bones  blank;  cartilage  bones 
stippled.  The  cartilage  bones  come  from  the  ptery- 
goquadrate cartilage.  (Modified  from  Kingsley's 
Comparative  Anatomy  of  Vertebrates.) 


(see  Fig.  38). 

c)  Other  gill  arches:  No  membrane 
bones  occur  in  connection  with  the  hyoid 
or  other  gill  arches. 

With  this  account  of  the  composition  of  the  skull  in  mind  we  may  now  turn  to  the  study 
of  some  vertebrate  skulls. 


E.      THE   SKULL  OF  NECTURUS,   A  PARTIALLY  OSSIFIED   SKULL 

In  the  skull  of  Amphibia  the  ossification  of  the  chondrocranium  has  pro- 
ceeded to  but  a  limited  extent  so  that  a  partially  cartilaginous  chondrocranium 
is  present  from  which  the  incasing  membrane  bones  can  be  readily  separated  as 
in  fishes.  The  pterygoquadrate  cartilages  are  inseparably  fused  to  the  ventral 
and  lateral  sides  of  the  skull  proper  and  are  partially  ossified.  In  the  lower 
jaws  Meckel's  cartilages  are  persistent  as  cores  inclosed  by  membrane  bones. 
The  number  of  membrane  bones  in  the  skulls  of  present-day  Amphibia  is  con- 
siderably less  than  that  of  extinct  forms,  such  as  shown  in  Figure  36  A. 


io8 


LABORATORY  MANUAL  FOR  VERTEBRATE  ANATOMY 


For  the  study  of  the  skull  of  Necturus,  complete  skulls,  preferably  preserved 
in  fluid,  and  chondrocrania,  from  which  the  membrane  bones  have  been  removed, 
should  be  at  hand. 

i.  General  regions  of  the  skull. — The  skull  is  partly  bony,  partly  cartilagi- 
nous. The  bony  part  exists  in  the  form  of  distinct  areas  or  bones,  separated  from 


FIG.  38. — Lower  jaws  of  four  vertebrates  to  show  the  reduction  in  the  number  of  bones  in  the 
course  of  evolution.  A,  lower  jaw  of  an  extinct  amphibian,  Trimerorhachis,  belonging  to  the  Stego- 
cephala;  inner  surface  above,  outer  surface  below;  note  large  number  of  membrane  bones.  B,  lower 
jaw  of  an  extinct  reptile,  Labidosaurus ,  belonging  to  the  Cotylosauria;  inner  surface  above,  outer  surface 
below;  note  reduction  in  the  number  of  membrane  bones  and  increased  size  of  the  dentary  h  and  the 
splenial  i.  C,  lower  jaw  of  a  modem  reptile,  a  lizard,  Varanus,  showing  still  farther  reduction  in  the 
number  of  bones;  outer  surface  above,  inner  surface  below.  D,  half  of  the  lower  jaw  of  man,  seen 
from  the  outer  surface;  it  consists  of  but  one  bone,  the  dentary,  all  other  bones  having  vanished.  Mem- 
brane bones  blank;  cartilage  bones  stippled,  a,  precoronoid;  b,  intercoronoid;  c,  postsplenial;  d,  coro- 
noid;  e,  articular;  /,  angular;  g,  prearticular;  h,  dentary;  i,  splenial;  j,  supra-angular  or  surangular; 
/,  coronoid  process;  m,  condyloid  process;  n,  ramus;  o,  body;  p,  mental  foramen.  (A  and  B  from 
Williston's  Water  Reptiles  of  the  Past  and  Present,  University  of  Chicago  Press;  C  from  Reynolds  The 
Vertebrate  Skeleton,  courtesy  of  the  Macmillan  Company;  D  from  a  specimen  loaned  by  the  anatomy 
department.) 

each  other  along  wavy  or  jagged  lines,  the  sutures.  The  membrane  bones  are 
somewhat  distinguishable  from  the  cartilage  bones  by  their  more  superficial 
positions.  The  skull  is  divisible  into  a  median  portion,  the  skull  proper,  and 


THE  ENDOSKELETON:    SKULL  AND  VISCERAL  SKELETON  109 

lateral  regions,  which  constitute  the  two  halves  of  the  upper  jaw.  The  upper 
jaw  forms  the  margins  of  the  skull  and  consists  in  part  of  tooth-bearing  bones. 
The  teeth  are  arranged  in  two  rows.  About  the  middle  of  the  margin  is  a  project- 
ing process;  on  examining  the  ventral  surface  of  this  it  will  be  seen  to  bear  a 
fossa  for  articulation  with  the  lower  jaw.  At  the  posterior  end  of  the  skull  is 
an  opening,  the  foramen  magnum,  through  which  the  brain  is  continuous  with 
the  spinal  cord.  On  either  side  ventral  to  the  foramen  magnum  is  a  projection, 
the  occipital  condyle,  bearing  a  smooth  face  for  articulation  with  the  atlas.  On 
each  side  of  the  foramen  magnum  and  extending  anteriorly  is  an  expanded 
region  with  partially  cartilaginous  walls.  This  is  the  otic  capsule  fused  on  each 
side  to  the  occipital  region  of  the  skull.  At  the  anterior  end  of  each  otic  capsule 
is  a  depression  for  the  passage  of  nerves.  Anterior  to  this  depression  is  another, 
the  orbit,  in  which  the  optic  nerve  runs  to  the  eyeball.  Anterior  to  the  orbit  is 
a  slitlike  cavity,  the  base  of  the  cavity  of  the  olfactory  capsules. 
2.  Bones  of  the  skull  proper. — 

a)  Membrane  bones  of  the  roof  of  the  skull:  The  roof  of  the  skull  is  formed  by 
two  pairs  of  membrane  bones,  in  the  median  region — a  posterior  pair  of  parietals 
and  an  anterior  pair  of  frontals.    The  parietals  extend  from  the  dorsal  rim  of 
the  foramen  magnum  to  a  little  beyond  the  middle  of  the  roof  where  they  termi- 
nate in  a  median  point.     The  frontals  lie  on  each  side  of  and  partially  cover  the 
anterior  end  of  the  parietals  and  extend  forward  nearly  to  the  tip  of  the  skull. 

b)  Membrane  bones  of  the  floor  of  the  skull:  Examine  the  ventral  surface.    It 
is  composed  almost  entirely  of  the  very  large  parasphenoid  or  parabasal  bone 
extending  from  the  occipital  condyles  forward  nearly  to  the  tip  of  the  skull. 
It  is  shaped  somewhat  like  a  short-necked  bottle,  the  neck  of  the  bottle  lying 
between  a  pair  of  bones,  the  vomers,  which  complete  the  sides  of  the  anterior 
part  of  the  ventral  surface.     Each  vomer  bears  teeth  on  its  margin  and  extends 
to  the  dorsal  side,  forming  the  floor  of  the  nasal  capsule.     On  the  ventral  side 
between  the  anterior  parts  of  the  vomers  and  in  front  of  the  termination  of  the 
parasphenoid  is  a  cartilage,  the  ethmoid  plate,  which  is  the  anterior  part  of  the 
chondrocranium. 

c)  Cartilage  bones  of  the  chondrocranium  and  otic  capsules:    The  chondro- 
cranium is  ossified  only  at  its  posterior  end,  all  the  rest  of  it  remaining  in  the 
cartilage  stage.     The  ossification  consists  of  a  pair  of  exoccipital  bones  which 
bear  the  occipital  condyles.     The  exoccipitals,  besides  bearing  the  occipital 
condyles,  form  the  lateral  walls  of  the  foramen  magnum;  the  dorsal  rim  of  the 
foramen  magnum  is  formed  by  a  strip  of  cartilage,  the  synotic  tectum,  which  is 
partially  covered  by  the  posterior  ends  of  the  parietals. 

The  otic  capsules  are  partially  ossified  each  containing  two  cartilage  bones, 
the  opisthotic  and  the  prootic.  The  opisthotic  is  lateral  to  the  exoccipital  and 
is  a  cone-shaped  bone  composing  the  projecting  angle  of  the  skull  noticeable  on 
each  side  of  the  occipital  region.  The  dorsal  portion  of  the  opisthotic  articulates 


no       LABORATORY  MANUAL  FOR  VERTEBRATE  ANATOMY 

with  the  parietal.  In  front  of  the  dorsal  portion  of  the  opisthotic  is  an  area 
of  cartilage,  and  in  front  of  this  is  the  prootic  bone.  Extending  from  the  opisth- 
otic obliquely  forward  and  laterally  is  a  slender  membrane  bone,  the  squamosal. 
The  posterior  part  of  the  squamosal  covers  a  part  of  the  opisthotic  and  lies 
above  the  lateral  border  of  the  otic  capsule.  The  prootic  bone  is  wedged  in 
between  the  squamosal  and  the  parietal.  On  turning  the  skull  laterally  so  as 
to  obtain  a  lateral  view  of  the  otic  capsule  a  small  rounded  bone  will  be  found 
situated  in  the  lateral  wall  of  the  otic  capsule.  It  articulates  behind  with  the 
opisthotic  and  bears  a  dorsally  projecting  process  which  meets  a  ventrally  directed 
process  from  the  middle  of  the  squamosal.  This  rounded  bone  is  called  the 
columella  and  is  believed  to  be  equivalent  to  the  hyomandibular  bone  of  fishes, 
that  is,  the  dorsal  portion  of  the  hyoid  gill  arch.  The  internal  ear  is  located 
inside  of  the  prootic  bone,  and  the  columella  fits  into  an  opening  in  the  prootic 
bone,  called  the  oval  window  orfenestra  ovalis.  The  columella,  when  the  middle 
ear  appears  (first  in  Anura),  becomes  a  bone  of  the  middle  ear,  and  it,  or  at 
least  that  portion  of  it  which  fits  into  the  fenestra  ovalis,  is  probably  homologous 
with  the  stapes  of  the  mammalian  middle  ear.  The  homologies  of  the  middle 
ear  bones  are,  however,  in  some  doubt  (see  K,  pp.  80-82). 

3.  Bones  of  the  upper  jaw. — The  upper  jaw,  composed  originally  of  the  two 
pterygoquadrate  cartilages,  is  fused  to  the  skull  proper  and  forms  its  lateral 
portions.  It  consists,  in  Necturus,  in  part  of  the  persistent  portions  of  the 
pterygoquadrate  cartilages,  in  part  of  the  cartilage  bones  formed  in  these 
cartilages,  and  in  part  of  membrane  bones  ensheathing  and  replacing  the 
cartilage. 

a)  Membrane  bones  of  the  upper  jaw:  The  anterior  tip  of  the  jaw  is  formed 
of  the  premaxillae,  V-shaped  bones,  the  angle  of  the  V  directed  forward  and 
constituting  the  tip  of  the  snout.     One  limb  of  the  V  is  a  slender  process  situated 
dorsally  on  top  of  the  anterior  end  of  the  frontal  bone.     The  other  limb  of  the 
V  is  a  tooth-bearing  process  forming  the  margin  of  the  anterior  end  of  the  skull. 
Posterior  to  the  premaxilla  is  the  tooth-bearing  vomer  already  mentioned  and 
not  regarded  as  a  bone  of  the  jaw.     Posterior  to  the  vomer  is  the  palatopterygoid 
bone,  the  anterior  portion  of  which  bears  teeth.     From  the  projecting  angle  to 
which  the  lower  jaw  articulates  to  the  posterior  angle  of  the  otic  capsule  extends 
the  slender  squamosal  bone. 

b)  Cartilage  bones  of  the  upper  jaw:  The  quadrate  bone  forms  the  fossa  into 
which  the  end  of  the  lower  jaw  articulates  and  extends  posteriorly  from  the  fossa 
for  a  short,  distance  in  contact  with  the  inner  side  of  the  squamosal.     Medial  to 
the  quadrate  bone  is  a  considerable  area  of  cartilage,  the  quadrate  cartilage, 
representing  unossified  portions  of  the  pterygoquadrate  cartilage.     Besides  the 
quadrate  bone,  a  portion  of  the  palatopterygoid  bone  is  likewise  a  cartilage  bone 
ossified  in  the  pterygoquadrate  cartilage. 

Draw  dorsal  and  ventral  views  of  the  skull,  outlining  the  bones  accurately. 


THE  ENDOSKELETON:    SKULL  AND  VISCERAL  SKELETON  in 

4.  The  chondrocranium.— When  the  skull  of  Necturus  is  soaked  in  warm 
soap  solution,  the  membrane  bones  can  easily  be  lifted  off,  revealing  the  chondro- 
cranium beneath.     The  chondrocranium  (including  the  olfactory  and  otic  cap- 
sules) consists  in  large  part  of  cartilage  with  a  few  cartilage  bones,  which  have 
already  been  identified. 

Study  prepared  chondrocrania.  The  form  of  the  chondrocranium  is  similar 
to  that  illustrated  in  Figure  32,  p.  97.  The  posterior  part  of  the  chondrocranium 
consists  chiefly  of  the  rounded,  hollow  otic  capsules.  These  are  connected  dorsally 
by  a  narrow  arch  of  cartilage,  the  synotic  tectum,  and  ventrally  by  the  broader 
basal  plate,  formed  by  the  fusion  of  the  two  parachordals.  The  two  exoccipital 
bones  are  ossified  in  the  basal  plate.  From  the  anterior  end  of  each  otic  cap- 
sule a  slender  curved  prechordal  cartilage  extends  forward.  Between  these  is 
a  large  space,  the  basicranial  fenestra,  which  in  the  complete  skull  is  covered 
above  by  frontals  and  parietals  and  below  by  the  parasphenoid.  Near  the 
anterior  end  in  front  of  the  point  where  in  the  complete  skull  the  parasphenoid 
ends,  the  prechordals  are  fused  across  to  form  the  ethmoid  plate,  already  noted. 
From  the  ethmoid  plate  a  slender  process  continues  forward  on  each  side,  the 
cornua  or  horns  of  the  prechordals.  In  front  of  the  otic  capsule  is  the  quadrate 
bone  and  quadrate  cartilage.  The  latter  sends  processes  to  the  otic  capsule  and 
the  prechordal.  In  the  walls  of  the  otic  capsule,  opisthotic,  prootic,  and  colu- 
mella  may  be  identified  and  their  boundaries  more  clearly  distinguished  than 
in  the  study  of  the  entire  skull.  The  olfactory  capsules  are  so  delicate  as  to 
be  lost  in  preparing  the  chondrocranium. 

5.  The  lower  jaw. — The  lower  jaw  consists  of  a  pair  of  MeckePs  cartilages 
united  in  front  and  sheathed  for  the  greater  part  of  their  course  in  membrane 
bones.     The  outer  surface  of  each  half  of  the  lower  jaw  consists  of  the  dentary, 
a  membrane  bone  bearing  teeth.     The  inner  surface  is  formed  of  two  membrane 
bones,  the  splenial  and  the  angular.    The  former  is  a  small  bone  situated  at 
about  the  middle  of  the  inner  surface,  bearing  the  last  group  of  teeth,  consisting 
of  five  or  six  teeth.     The  angular  covers  the  remainder  of  the  inner  surface  and 
passes  onto  the  outer  surface  at  the  extreme  posterior  end  of  the  jaw,  below  the 
posterior  end  of  the  dentary.     The  articulating  surfaces  of  the  lower  jaw  are 
composed  of  cartilage,  which  is  the  posterior  end  of  Meckel's  cartilage.    This 
articulates  with  the  quadrate  of  the  upper  jaw.     Meckel's  cartilage  runs  almost 
the  entire  length  of  the  jaw  concealed  between  the  dentary  and  the  angular.    It 
can  be  revealed  by  removing  these  membrane  bones. 

6.  The  remaining  gill  arches. — The  hyoid  and  three  succeeding  gill  arches 
are  present  in  Necturus  and  are  almost  completely  cartilaginous.     Preserved 
material  is  necessary  for  their  study.     The  hyoid  arch  is  a  broad  somewhat 
V-shaped  cartilage  situated  just  posterior  to  the  lower  jaw  in  the  floor  of  the 
mouth  cavity.     On  each  side  it  is  divisible  into  two  cartilages — a  small  anterior 
hypohyal  and  a  much  larger  posterior  ceratohyal.     The  third  gill  arch  (first  true 


ii2       LABORATORY  MANUAL  FOR  VERTEBRATE  ANATOMY 

gill-bearing  arch)  is  more  elongated  and  slender  than  the  hyoid  arch  and  is  like- 
wise divisible  into  two  pieces,  an  anterior  ceratobranchial  and  a  posterior, 
slightly  longer,  epibranchial.  Between  the  median  ventral  portions  of  the 
hyoid  and  third  arch  is  a  triangular  copula  representing  a  basibranchial  piece. 
The  fourth  and  fifth  gill  arches  are  short  curved  rods  of  cartilage  on  each  side, 
composed  of  epibranchials.  At  the  anterior  end  of  the  epibranchial  of  the  fourth 
arch  is  a  small  ceratobranchial.  In  the  median  ventral  line  is  a  slender  bone, 
the  second  basibranchial.  It  will  be  seen  that  the  gill  arches  of  Necturus  are 
reduced  both  in  number  and  as  regards  the  pieces  of  which  they  are  composed 
as  compared  with  the  gill  arches  of  the  elasmobranchs.  For  the  typical  condi- 
tion of  the  gill  arches  in  elasmobranchs  see  Figure  33,  page  100,  and  compare 
with  the  condition  in  Necturus. 

F.   THE  SKULL  OF  THE  ALLIGATOR,  A  TYPICAL  MODERN  SKULL 

The  skull  of  the  alligator  is  a  nearly  completely  ossified  skull  in  which  the 
various  components  of  which  the  skull  is  composed  are  so  closely  knit  into  a 
single  structure  as  to  be  inseparable.  The  skull  of  the  alligator  is  probably  as 
typical  and  generalized  a  skull  as  is  to  be  found  among  living  land  vertebrates 
and  has  further  the  advantage  of  large  size.  The  student  must  learn  the  bones 
of  the  skull  and  jaw  of  the  alligator  and  be  able  to  distinguish  the  cartilage  from 
the  membrane  bones.  Figures  of  the  alligator  skull  will  be  found  in  R,  pages 
252,  254,  256,  261 ;  P  and  H,  page  342 ;  CNA,  Vol.  VIII,  page  458;  K,  page  104. 

i.  General  regions  and  cavities  of  the  skull. — Obtain  a  skull  and  study  its 
structure.  It  is  composed  of  a  number  of  separate  bones  closely  jointed  to  each 
other  along  somewhat  jagged  lines  known  as  sutures.  The  various  components 
of  which  we  learned  the  skull  is  constructed  are  here  morphologically  indistin- 
guishable from  each  other  but  will  be  pointed  out  later.  The  bones  of  the  roof 
of  the  skull  are  pitted  and  roughened.  At  the  anterior  end  of  the  dorsal  surface 
are  the  two  nasal  openings  or  anterior  nares.  Posterior  to  the  middle  of  the 
roof  are  the  two  large  orbits,  the  largest  openings  in  the  roof  of  the  skull.  Pos- 
terior to  each  orbit  is  the  temporal  region  in  which  are  several  holes,  known  as 
fossae  or  vacuities.  The  development  of  these  temporal  fossae  is  characteristic 
of  reptiles  and  has  occurred  during  their  evolution,  since  the  early  reptiles,  as 
in  Figure  3 6B,  possessed  completely  roofed  skulls.  (See  further  on  this  point, 
R,  pp.  285-87,  and  Fig.  44,  p.  176.)  The  alligator  skull  has  two  pairs  of  these 
openings,  a  dorsal  pair — the  supratemporal  fossae,  on  the  dorsal  side  of  the  pos- 
terior end  of  the  skull — and  a  lateral  pair  —the  infratemporal  fossae,  just  posterior 
to  the  orbits  from  which  each  is  separated  by  a  rod  of  bone,  the  postorbital  bar. 
Lateral  to  each  supratemporal  fossa  is  a  projecting  ledge  of  bone,  the  supra- 
temporal  arcade.  Underneath  this  overhanging  ledge  is  an  opening,  the  external 
auditory  meatus,  which  leads  into  the  cavity  of  the  middle  ear  or  tympanic  cavity. 
Numerous  canals  and  passages  will  be  found  entering  the  tympanic  cavity,  and 


THE  ENDOSKELETON:    SKULL  AND  VISCERAL  SKELETON  113 

the  cavities  of  the  two  sides  are  connected  by  a  passage  in  the  roof  of  the  skull. 
The  bones  surrounding  the  tympanic  cavity  are  bones  ossified  in  the  otic  capsules, 
which  are  thus  seen  to  be  closely  knit  into  the  structure  of  the  skull.  At  the 
posterior  end  of  the  skull  in  the  median  line  is  a  rounded  opening,  the  foramen 
magnum,  surrounded  by  the  occipital  group  of  bones.  Below  the  foramen  is  a 
rounded  projection,  the  occipitaj  condyle.  At  either  side  of  the  posterior  end  of 
the  skull  is  a  very  large  cavity,  the  pterygoid  fossa,  continuous  dorsally  with  the 
orbits  and  infratemporal  fossae.  The  lateral  margins  of  the  posterior  part  of 
the  skull  are  formed  by  a  bar  of  bone,  which  constitutes  the  lateral  boundaries 
of  the  orbits  and  infra  temporal  fossae;  this  is  called  the  infratemporal  arcade. 

On  the  ventral  surface  of  the  skull  identify:  a  small  opening  near  the  tip, 
the  anterior  palatine  vacuity;  a  pair  of  larger  oval  openings  ventral  to  the  orbits, 
the  posterior  palatine  vacuities;  and  the  posterior  nares,  a  pair  of  small  openings 
in  the  median  ventral  line  posterior  to  the  preceding. 

The  smaller  holes  in  the  skull  are  nerve  foramina  through  which  nerves  enter 
and  leave  the  brain.  The  brain  in  life  occupies  the  very  small  cavity  which  may 
be  seen  by  looking  into  the  foramen  magnum.  The  elongated  cavities  occupying 
the  interior  of  the  remainder  of  the  skull  and  extending  from  the  anterior  to  the 
posterior  nares  are  the  nasal  passages.  It  will  be  noted  that  the  posterior  nares 
have  been  moved  far  posteriorly  from  the  position  in  which  they  are  found  in 
Amphibia. 

2.  Membrane  bones  of  the  roof  of  the  skull. — At  the  anterior  end  of  the 
dorsal  surface  just  behind  the  anterior  nares  is  a  pair  of  long  bones,  the  nasals. 
At  each  side  of  their  posterior  ends  is  a  prefrontal  bone,  and  at  each  side  of  that 
a  small  lacrimal  bone.     Prefrontal  and  lacrimal  bones  form  the  anterior  rim 
of  the  orbit.    The  adlacrimal  or  supraorbital  is  a  small  bone  lying  loose  in  the 
eyelid  (and  hence  missing  in  many  skulls)  lateral  to  the  prefrontal.     Between 
the  two  orbits  dorsally  is  the  frontal  bone,  single  in  the  adult  but  double  in  the 
embryo.     In  the  median  line  posterior  to  the  frontal  is  the  parietal,  also  a  single 
bone  in  the  adult  but  paired  in  the  embryo.     The  postfrontal  bone  includes  the 
anterior  part  of  the  supratemporal  fossa  and  sends  down  a  process  which  forms 
the  upper  half  of  the  postorbital  bar.     Directly  behind  the  postfrontal  is  the 
squamosal  which  overhangs  the  external  auditory  meatus. 

3.  The  bones  of  the  upper  jaw. — The  upper  jaw  consists  of  the  original 
cartilaginous  half  of  the  mandibular  arch  (pterygoquadrate  cartilages)  plus  a 
number  of  membrane  bones.     Although  separate  in  elasmobranchs,  it  is  insep- 
arably fused  to  the  skull  in  land  vertebrates.     It  consists  of  two  parts,  a  maxillary 
arch  forming  the  lateral  parts  of  the  skull  both  above  and  below  and  composed 
entirely  of  membrane  bones,  and  a  median  region  developed  from  the  pterygo- 
quadrate cartilage,  and  consisting  partly  of  cartilage  bones  and  partly  of  mem- 
brane bones.     From  the  dorsal  view  identify  the  bones  of  the  maxillary  arch  as 
follows:  premaxillae,  in  front  of  and  at  the  sides  of  the  anterior  nares  and  bearing 


114       LABORATORY  MANUAL  FOR  VERTEBRATE  ANATOMY 

teeth;  maxillae,  the  large  bones  at  the  sides  of  the  nasal  bones,  and  also  bearing 
teeth;  jugal  or  malar,  the  elongated  bones  forming  the  lower  boundaries  of  the 
orbit  and  meeting  the  postf rentals  at  the  middle  of  the  postorbital  bar;  and 
quadratojugal,  a  slender,  obliquely  placed  bone  forming  the  posterior  boundaries 
of  the  lateral  temporal  fossae.  The  quadrate  is  a  stout  bone  obliquely  placed 
between  the  quadratojugal  and  the  exoccipital.  The  posterior  end  of  the  quad- 
rate has  a  concave  surface  for  articulation  with  the  lower  jaw. 

Turn  the  skull  over  and  identify  the  same  bones  of  the  maxillary  arch  from 
below.  Premaxillae  and  maxillae  form  the  anterior  half  of  the  ventral  surface, 
meeting  in  the  median  line.  Between  the  two  premaxillae  is  the  small  anterior 
palatine  vacuity.  Posterior  to  the  maxillae  in  the  median  region  and  forming 
the  inner  boundaries  of  the  posterior  palatine  vacuities  are  the  palatines.  Pos- 
terior to  the  palatines  are  the  broad  pterygoids  which  inclose  the  posterior  nares. 
Extending  from  the  sides  of  the  pterygoids  to  the  posterior  end  of  the  maxillae 
are  the  ecto pterygoids  or  trans  palatines. 

All  of  these  bones  of  the  upper  jaw  except  the  quadrate  are  membrane  bones. 
The  quadrate  represents  the  ossified  posterior  end  of  the  pterygoquadrate  carti- 
lage. The  quadrate  articulates  with  the  lower  jaw,  as  in  the  majority  of 
vertebrates. 

4.  The  occipital  region. — This  region  forms  the  posterior  end  of  the  skull 
and  consists  of  four  cartilage  bones  derived  from  the  parachordal  cartilages. 
Turn  the  skull  so  that  its  posterior  end  faces  you.     The  foramen  magnum  is 
bounded  by  three  bones,  one  on  each  side,  the  exoccipitals,  and  one  below,  the 
basioccipital.    The  basioccipital  bears  most  of  the  large  rounded  occipital  condyle 
but  both  exoccipitals  send  down  processes  which  take  part  in  the  condyle.     Above 
and  between  the  two  exoccipitals  is  the  triangular  supraoccipital,  which  articulates 
with  the  posterior  end  of  the  parietal. 

5.  The  bones  of  the  otic  capsule. — There  are  three  of  these  surrounding  the 
external  auditory  meatus  and  inclosing  the  ear.     They  are  named  the  epiotic, 
the  opisthotic,  and  the  prootic.     The  first  of  these  is  fused  with  the  supraoccipital ; 
the  second  with  the  exoccipital;    the  third  alone  remains  distinct  in  the  adult. 
They  are  difficult  or  impossible  to  see  in  external  view.     To  find  the  prootic, 
turn  the  skull  sidewise  so  as  to  obtain  a  profile  view,  and  identify  a  large  foramen, 
the  foramen  ovate,  just  below  the  supratemporal  fossa.     The  posterior  wall  of 
this  foramen  is  formed  by  the  prootic  bone,  which  articulates  with  the  quadrate 
bone  behind,  the  suture  between  the  two  occurring  at  the  external  rim  of  the 
foramen.     The  otic  bones  are  of  course  cartilage  bones,  formed  in  the  otic  capsule. 

6.  The  posterior  sphenoid  region. — This  region  is  anterior  to  the  occipital 
region  and  forms  the  floor  of  the  cavity  occupied  by  the  brain.     Turn  the  pos- 
terior  end  of  the  skull  toward  you   and   identify   the  V-shaped   end  of  the 
basisphenoid,  between  the  basioccipital  and  the  pterygoids.     Most  of  the  basi- 
sphenoid  is  concealed  from  view  by  the  pterygoids  and  the  quadrates,  but  its 


THE  ENDOSKELETON:    SKULL  AND  VISCERAL  SKELETON  115 

anterior  end  projects  as  a  thin  blunt  process,  the  rostrum,  into  the  space  between 
the  two  orbits.  Find  it  in  side  view  of  the  skull.  Above  the  rostrum  on  each 
side  is  an  alisphenoid  bone,  extending  upward  to  the  roof  of  the  skull  and  forming 
part  of  the  walls  of  the  foramen  ovale  and  the  supratemporal  fossa.  In  the 
interior  of  the  skull  is  a  cavity  extending  from  the  foramen  magnum  to  the  ali- 
sphenoid bones;  this  cavity  contains  the  brain  in  life,  and  the  ventral  surface  of 
the  brain  therefore  rests  upon  the  occipital  and  sphenoid  bones,  derived  from  the 
parachordals  and  prechordals. 

7.  The  anterior  sphenoid  and  ethmoid  regions. — These  regions  remain  in  the 
cartilage  stage  in  the  alligator,  forming  a  cartilaginous  partition  between  the 
two  nasal  passages  and  between  the  orbits.     The  cartilage  is  of  course  wanting 
in  the  dried  skull  so  that  the  nasal  passages  and  orbits  appear  to  be  connected. 

Draw  a  ventral  view  of  the  skull,  showing  the  sutures  accurately. 

8.  The  lower  jaw. — The  lower  jaw  or  mandible  is  composed  of  two  halves  or 
rami  united  in  front  by  a  symphysis.    Each  ramus  consists  of  six  separate  bones. 
Near  the  posterior  end  of  the  jaw  is  a  large  vacuity,  the  external  mandibular 
foramen,  and  in  front  of  this  on  the  inner  side  a  smaller  internal  mandibular 
foramen.    The  bones  of  each  ramus  are:  the  dentary,  bearing  teeth  and  forming 
the  outer  surface  of  the  anterior  two-thirds  of  the  ramus;  the  splenial,  of  about 
the  same  shape  and  size  as  the  dentary  and  in  the  same  position  on  the  inner 
surface;    the  angular,  below  the  external  mandibular  foramen,  and  separated 
from  the  splenial  by  the  internal  mandibular  foramen;    the  supra-angular  or 
surangular,  above  the  external  mandibular  foramen;   the  coronoid,  a  small  bone 
on  the  inner  surface,  between  the  anterior  ends  of  the  angular  and  the  supra- 
angular;  and  the  articular,  above  the  posterior  end  of  the  angular  and  bearing 
a  concavity  for  articulation  with  the  quadrate.    A  cavity  exists  in  the  interior 
of  the  ramus  between  the  dentary  and  the  splenial.     This  cavity  is  occupied  in 
life  by  Meckel's  cartilage,  which  it  may  be  recalled  is  the  original  lower  half  of 
the  first  gill  arch.     The  posterior  end  of  this  cartilage  has  ossified  into  the  articular, 
which  is  therefore  the  only  cartilage  bone  in  the  lower  jaw.     The  articulation 
between  upper  and  lower  jaw  is  by  way  of  the  articular  and  the  quadrate,  a 
condition  found  in  the  majority  of  land  vertebrates.     Draw  the  lower  jaw  from 
both  inner  and  outer  views. 

9.  The  hyoid  apparatus. — As  has  already  been  explained,  this  is  derived 
from  the  hyoid  arch  and  remaining  gill  arches.     The  hyoid  apparatus  is  generally 
missing  on  dried  skeletons,  and  for  its  study  preserved  specimens  are  necessary. 
It  consists  of  a  broad  cartilaginous  plate,  the  body  of  the  hyoid,  and  a  pair  of 
processes  or  horns  (cornua)  extending  posteriorly  and  dorsally  from  the  body, 
one  on  either  side.     The  cornua  are  partially  ossified.     The  body  of  the  hyoid 
apparatus  is  derived  trom  the  bases  of  the  second  and  other  arches  while  the 
horns  are  remnants  of  the  third  arch.     In  lieu  of  alligator  material,  the  hyoid 
apparatus  may  also  be  studied  on  turtle  skeletons.     In  the  turtles  there  is  a 


n6 


LABORATORY  MANUAL  FOR  VERTEBRATE  ANATOMY 


median  ventral  plate,  the  body  of  the  hyoid,  located  in  the  floor  of  the  mouth 
it  represents  the  ventral  ends  of  the  hyoid  and  other  gill  arches.  From  it  pro- 
ject posteriorly  two  pairs  of  slender  processes,  known  as  the  anterior  and  posteriot 
horns  of  the  hyoid.  These  are  portions  of  the  third  and  fourth  gill  arches. 
10.  The  teeth. — The  teeth  of  the  alligator  occur  in  a  single  row  on  the 
premaxillae,  maxillae,  and  dentary  bones.  Their  structure  is  the  same  as  that 
already  described  under  the  exoskeleton.  The  pulp  cavity  of  these  teeth  is 
widely  open  at  the  base,  and  the  teeth  can  therefore  be  replaced  an  indefinite 
number  of  times,  each  successive  tooth  having  the  same  pulp  cavity  as  its 


parietal 


squamosal 


frontal 


orbitosphenoid 
presphenoid 


upraoccipital 

petromastoid 

(hyomandibular) 


exoccipital 
basioccipital 


incus  fquadrate) 
malleus  (articular) 


dentary 


jugal 


Meckel's  cartilage 
pterygoid 


maxilla 


palatine 


FIG.  39. — Diagram  of  the  bones  of  the  mammalian  skull.  Membrane  bones  blank;  cartilage 
bones  stippled.  (After  Kingsley's  Comparative  Anatomy  of  Vertebrates,  copyright  by  P.  Blakiston's 
Son  and  Company.) 

predecessor.  The  teeth  are  set  into  sockets  or  alveoli  in  the  jaw;  teeth  so  placed 
are  said  to  be  thecodont.  The  teeth  are  all  alike  in  size  and  form;  hence  they  are 
said  to  be  homodont. 


G.   THE  BONES  OF  THE  MAMMALIAN  SKULL 

The  mammalian  skull  is  completely  ossified  with  the  exception  of  a  small 
part  of  the  ethmoid  region.  The  number  of  bones  which  it  contains  is,  however, 
considerably  less  than  that  found  in  reptile  skulls.  This  is  due  in  part  to  a  loss 
of  bones  and  in  part  to  a  fusion  of  persisting  bones.  This  fusion  is  most  marked 
in  the  human  skull.  Thus,  as  Williston  says,  "The  most  primitive  rep  tiles  had 
no  less  than  seventy-two  separate  bones  in  the  skull;  the  human  skull  has  but 
twenty-eight  inclusive  of  the  (six)  ear  bones."  The  accompanying  diagram  of 
the  mammalian  skull,  Figure  39,  should  be  studied  carefully  and  will  aid  in 
acquiring  an  understanding  of  the  construction  of  the  mammalian  skull.  The 


THE  ENDOSKELETON:   SKULL  AND  VISCERAL  SKELETON  117 

reduction  in  the  membrane  bones  of  the  roof  of  the  skull  is  illustrated  in 
Figure  36,  page  106. 

The  following  description  applies  to  the  skull  of  the  cat  and  rabbit.  That 
of  the  dog  is  so  similar  to  that  of  the  cat  that  the  same  description  applies  to  both. 
The  cat  skull  is  described  and  figured  in  R  and  J,  and  D,  the  rabbit  skull  in  B, 
and  P  and  H,  and  the  dog  skull  in  R.  The  student  must  learn  the  bones  of  the 
mammalian  skull  and  be  able  to  state  which  are  membrane  bones  and  which 
cartilage  bones.  In  studying  the  bones  locate  carefully  the  sutures  between 
the  bones  and  by  this  means  determine  accurately  the  extent  of  each  bone. 

i.  General  regions  and  features  of  the  skull. — The  skull  is  a  hard,  bony 
case,  in  which  the  limits  of  the  separate  bones  are  marked  by  wavy  or  jagged 
lines,  the  sutures.  Along  these  lines  the  bones  dovetail  into  each  other,  forming 
firm,  immovable  joints.  The  skull  may  be  divided  into  an  anterior  facial  portion 
supporting  the  nose  and  eyes  and  a  posterior  cranial  portion  inclosing  the  brain. 
At  the  anterior  end  of  the  facial  portion  are  the  two  nasal  openings  or  anterior 
nares  separated  in  life  by  a  cartilaginous  partition,  the  anterior  part  of  the 
septum  of  the  nose.  At  the  side  of  the  facial  portion  is  a  large  cavity,  partially 
separated  by  projecting  bony  processes  into  two  cavities:  an  anterior,  large, 
nearly  circular  one,  the  orbit  or  orbital  fossa,  which  contains  the  eye  in  life;  and 
a  posterior  one,  the  temporal  fossa,  filled  in  life  by  muscles.  The  temporal  fossa 
is  very  small  in  the  rabbit.  The  lower  boundary  of  these  fossae  is  formed  by  a 
projecting  arch  of  bone,  the  zygomatic  arch,  a  feature  very  characteristic  of  the 
mammalian  skull.  In  the  cat  an  orbital  process  extends  dorsally  from  the 
middle  of  the  zygomatic  arch  and  nearly  meets  a  zygomatic  process  descending 
from  the  roof  of  the  skull.  These  two  processes  form  the  posterior  boundary 
of  the  orbit.  In  the  rabbit  the  zygomatic  process  extends  backward  and  down- 
ward from  the  roof  of  the  orbit;  in  life  this  process  is  connected  to  the  zygomatic 
arch  by  a  ligament,  thus  marking  off  a  small  temporal  fossa  posterior  to  the 
ligament.  Dorsal  to  the  orbit  is  a  projecting  margin  of  bone,  the  supraorbital 
arch,  the  posterior  end  of  which  projects  as  the  zygomatic  process  already  men- 
tioned. In  the  rabbit  the  anterior  end  of  this  arch  also  bears  a  projecting 
process, 

The  cranial  portion  of  the  skull  presents  the  following  features.  At  the 
posterior  end  is  the  large  foramen  magnum;  on  each  side  of  this  is  a  projection, 
the  occipital  condyle,  which  articulates  with  the  atlas.  Lateral  and  slightly 
anterior  to  each  occipital  condyle  is  a  conspicuous  hollow  expansion,  the  tympanic 
bulla,  which  contains  the  middle  ear.  On  the  posterior  surface  of  each  bulla 
are  two  processes,  an  anterior  mastoid  process  and  a  posterior  jugular  process. 
The  mastoid  process  will  be  seen  to  be  part  of  a  bone,  differing  in  its  rough  and 
pitted  surface  from  the  other  bones  of  the  skull;  this  bone,  the  petromastoid 
bone,  contains  the  internal  ear.  The  tympanic  bulla  opens  laterally  by  a  large 
opening,  the  external  auditory  meatus,  which  in  the  rabbit  is  bounded  by  a  bony 


n8       LABORATORY  MANUAL  FOR  VERTEBRATE  ANATOMY 

tube.  The  meatus  leads  into  the  cavity  of  the  bulla,  which  is  the  tympanic  cavity 
or  cavity  of  the  middle  ear.  From  the  dorsal  side  of  the  bulla  a  ridge  begins 
which  proceeds  across  the  posterior  part  of  the  skull  to  the  other  bulla;  this 
ridge  is  the  superior  nuchal  line  (or  lambdoidal  ridge}  and  is  the  most  anterior 
point  of  attachment  of  the  muscles  of  the  vertebral  column.  From  the  middle 
of  this  ridge  there  projects  posteriorly  the  external  occipital  protuberance,  slight 
in  the  cat  but  forming  a  rectangular  prominence  in  the  rabbit. 

On  the  ventral  surface  the  anterior  part  of  the  skull  is  occupied  by  the  hard 
palate.  Dorsal  to  this  are  the  nasal  passages  which  open  at  the  posterior  end  of 
the  hard  palate  by  the  posterior  nares  or  choanae.  The  hard  palate  contains  a 
pair  of  openings,  the  incisive  foramina  or  anterior  palatine  foramina;  these  are 
small  and  at  the  anterior  extremity  of  the  palate  in  the  cat  but  much  longer  and 
more  prominent  in  the  rabbit.  They  lead  into  the  nasal  cavities.  At  the 
posterior  end  of  the  zygomatic  arch  on  its  ventral  side  is  a  depression,  the  man- 
dibular  fossa,  for  the  reception  of  the  lower  jaw.  Medial  to  this  is  the  pterygoid 
fossa  for  the  attachment  of  certain  muscles.  The  pterygoid  fossa  in  the  cat  is 
continuous  with  the  temporal  and  orbital  fossae,  while  in  the  rabbit  it  is  included 
between  two  projecting  plates  of  bone  which  point  toward  the  tympanic  bulla. 

2.  Membrane  bones  of  the  roof  of  the  skull. — Beginning  just  behind  the 
anterior  nares  are  the  paired  nasals ,  roofing  the  nasal  cavities;  next  posterior, 
the  paired  frontals;   and  last,  the  paired  parietals,,  terminating  at  the  superior 
nuchal  line.     Between  the  posterior  ends  of  the  parietals  is  generally  a  small 
triangular  interparietal  bone,  the  boundaries  of  which  are  not  always  distinct. 
Each  frontal  bone  forms  the  dorsal  part  of  the  orbit  and  projects  out  above  the 
orbit  as  the  supraorbital  arch  which  terminates  posteriorly  in  the  zygomatic 
process  of  the  frontal  bone,  already  noted.     In  the  anterior  wall  of  the  orbit  is 
the  small  lacrimal  bone  (probably  homologous  with  the  prefrontal  bone  of 
reptiles).    At  the  anterior  end  of  the  lacrimal  bone  is  an  opening,  the  posterior 
end  of  the  nasolacrimal  duct,  by  means  of  which  the  tears  drain  into  the  nasal 
cavity. 

3.  The  bones  of  the  upper  jaw. — These  as  in  the  alligator  comprise  a  lateral 
maxillary  arch  on  each  side  and  a  median  series  of  bones.     The  maxillary  arch 
consists  on  each  side  of  the  following  elements:   premaxilla,  in  front  of  the 
anterior  nares  and  bearing  teeth;  maxilla,  forming  the  side  of  the  facial  region 
of  the  skull  and  also  bearing  teeth;  malar  Qijugal,  forming  most  of  the  zygomatic 
arch;  and  temporal,  completing  the  zygomatic  arch  and  covering  the  side  of  the 
cranial  part  of  the  skull  including  the  tympanic  bulla.     Further  details  concern- 
ing these  bones  may  be  noted.     The  premaxillae  send  frontal  processes  dorsally 
alongside  the  nasal  bones;  these  are  very  pronounced  in  the  rabbit.     The  pre- 
maxillae also  form  the  anterior  part  of  the  hard  palate  by  means  of  their  palatine 
processes,  which  meet  in  the  median  ventral  line  and  include  the  incisive  foramina. 
The  maxilla  is  the  main  bone  of  the  facial  region;  in  the  rabbit  it  is  much  fenes- 


THE  ENDOSKELETON:    SKULL  AND  VISCERAL  SKELETON  119 

trated.  It  forms  part  of  the  anterior  wall  of  the  orbit,  ventral  to  the  lacrimal 
bone,  by  its  orbital  process;  it  extends  to  the  frontal  bone  above  by  its  frontal 
process;  its  palatine  process  meets  its  fellow  in  the  median  ventral  line  continuing 
the  hard  palate;  its  alveolar  process  bears  teeth;  and  its  zygomatic  process  con- 
stitutes the  beginning  of  the  zygomatic  arch.  The  malar  or  jugal  bone  is  distinct 
in  the  cat  but  in  the  adult  rabbit  is  fused  to  the  zygomatic  process  of  the  maxilla. 
The  temporal  bone  is  a  compound  bone  characteristic  of  mammals.  It  consists 
of  a  squamous  portion  which  by  its  zygomatic  process  completes  the  zygomatic 
arch  and  which  also  contributes  to  the  cranial  wall,  ventral  to  the  parietal; 
of  the  tympanic  bulla  composed  of  a  tympanic  bone  of  uncertain  homology;  and 
of  the  periotic  or  petromastoid  bone  only  slightly  visible  on  the  surface  and  con- 
sisting of  the  fused  otic  bones  of  lower  vertebrates.  The  squamous  part  of  the 
temporal  bone  is  homologous  with  the  squamosal  bone  of  the  lower  vertebrates 
and  is  the  most  posterior  bone  of  the  maxillary  arch.  All  of  the  bones  of  the 
maxillary  arch  are  membrane  bones. 

The  median  portion  of  the  upper  jaw  forms  part  of  the  ventral  surface  of 
the  skull.  We  have  already  noted  that  the  anterior  portion  of  the  hard  palate 
is  composed  of  the  palatine  processes  of  the  premaxillae  and  maxillae.  Posterior 
to  the  latter  are  the  palatines,  membrane  bones  which  include  the  posterior  nares. 
Dorsal  to  the  palatines  the  roof  of  the  nasal  passages  is  completed  by  the  vomer 
of  which  only  the  extreme  posterior  tip  is  visible  in  the  roof  of  the  choanae. 
The  vomer  forms  the  floor  of  the  nasal  cavities  and  is  best  seen  in  a  sagittal  section 
of  the  skull.  The  posterior  parts  of  the  palatines  extend  up  into  the  orbit, 
there  being  in  the  rabbit  a  very  deep  cleft  between  them  in  the  midventral  line. 

Observe  that  the  quadrate  is  wanting.  The  lower  jaw  consequently  articu- 
lates with  the  squamosal  (temporal)  by  means  of  a  depression,  the  mandibular 
fossa,  on  the  under  surface  of  the  zygomatic  process  of  the  temporal  bone.  This 
feature  distinguishes  mammalian  skulls  from  those  of  all  other  vertebrates. 
Owing  to  the  absence  of  the  quadrate  all  of  the  bones  of  the  upper  jaw  are  mem- 
brane bones. 

4.  The  occipital  region. — This  region  surrounds  the  foramen  magnum  and 
consists  of  a  single  occipital  bone  extending  from  the  superior  nuchal  line  to  a 
point  between  the  anterior  ends  of  the  tympanic  bullae.  It  is  really  composed 
of  the  four  occipital  bones  present  in  reptiles;  these  are  distinct  in  embryonic 
and  young  mammals,  as  shown  in  Figure  ^oA ,  page  127,  but  are  fused  in  the  adults. 
The  occipital  bone  bears  the  two  occipital  condyles,  whose  number  serves  to  dis- 
tinguish mammalian  skulls  from  those  of  all  other  vertebrates  except  Amphibia, 
where  there  are  also  two  condyles.  The  dorsal  part  of  the  occipital  may  appear 
to  extend  anteriorly  between  the  posterior  ends  of  the  two  parietals;  this  is 
due  to  the  fact  that  interparietal  bone  is  commonly  fused  to  the  occipital.  This 
is  the  case  in  man.  The  occipital  bone  bears  the  superior  nuchal  line  and  the 
jugular  process  resting  on  the  bulla. 


120       LABORATORY  MANUAL  FOR  VERTEBRATE  ANATOMY 

5.  The  otic  capsules. — As  already  explained,  the  bones  of  the  otic  capsules 
are  all  fused  together  and  fused  with  the  squamosal  and  the  tympanic  bulla  to 
form  the  temporal  bone,  a  bone  very  characteristic  of  mammals  (Fig.  406',  p.  127). 
That  part  of  the  temporal  bones  which  is  composed  of  the  otic  bones  is  named  the 
periotic  or  petromastoid  bone.     The  separate  otic  bones  of  which  the  petromas- 
toid  is  composed  can  be  seen  only  in  early  embryonic  stages.     The  mastoid  portion 
of  this  bone  is  visible  on  the  external  surface  of  the  skull  between  the  bulla  and 
the  occipital;   it  projects  over  the  bulla  as  the  mastoid  process,  very  prominent  in 
man  as  the  bump  behind  the  pinna.    The  petrous  portion  of  the  petromastoid  bone 
incloses  the  internal  ear  and  is  visible  only  from  the  inside  of  the  skull.     The 
tympanic  bulla  consists  of  one  or  two  membrane  bones  of  uncertain  homology; 
it  incloses  the  middle  ear  and  the  three  middle  ear  bones.     These  can  be  seen  in 
well-cleaned  cat  skulls  as  a  delicate  chain  of  bones  extending  across  the  anterior 
part  of  the  tympanic  cavity.     If  separate  specimens  are  available,  identify 
them  as  follows:  the  malleus  or  hammer,  a  slender  bone  terminating  in  a  knob; 
the  incus  or  anvil,  a  smaller  bone  with  two  pointed  processes;  and  the  stapes  or 
stirrup,  shaped  like  a  stirrup.    There  is  good  reason  to  believe  that  the  malleus 
represents  a  very  much  reduced  articular;  that  the  incus  is  the  reduced  quadrate; 
and  the  stapes  is  the  hyomandibular. 

6.  The  posterior  sphenoid  region. — On  the  ventral  surface  of  the  skull  in 
front  of  the  ventral  part  of  the  occipital  bone  is  the  basisphenoid.     The  basi- 
sphenoid  extends  laterally  in  front  of  the  tympanic  bullae  as  processes,  the  wings 
of  the  basisphenoid.     These  wings  are  in  reality  the  alisphenoids  and  are  separate 
in  young  stages  (see  Fig.  40$).     The  alisphenoids  meet  the  squamous  part  of 
the  temporal  bones  dorsally.     The  alisphenoids  extend  forward  as  the  pterygoid 
processes  which  meet  the  posterior  ends  of  the  palatines.     In  the  rabbit  the 
pterygoid  process  presents  two  backwardly  projecting  thin  plates  of  bone,  the 
lateral  and  medial  lamellae.     These  inclose  between  them  the  pterygoid  fossa. 
The  medial  lamella  in  the  rabbit  and  the  pterygoid  process  itself  in  the  cat  bear 
a  pointed  process,  the  hamulus.    The  pterygoid  processes  correspond  to  the 
pterygoid  bones  of  lower  forms. 

7.  The  anterior  sphenoid  region. — In  the  median  ventral  line  in  front  of 
the  basisphenoid  is  the  presphenoid,  a  slender  bone.     In  the  rabbit  this  is  at  the 
bottom  of  the  deep  cleft  between  the  palatines.     The  presphenoid  sends  up 
wings  into  the  orbit  which  meet  the  frontal  bones  above  and  contain  the  large 
optic  foramen  for  the  passage  of  the  optic  nerve.     In  the  cat  this  foramen  is 
the  most  anterior  of  a  row  of  four  foramina.     To  see  the  wings  of  the  presphenoid 
turn  the  skull  to  obtain  a  lateral  view.     The  wings  are  in  reality  the  orbitosphenoid 
bones  of  lower  vertebrates  and  are  separate  in  young  stages  (Fig.  40^,  p.  127) 

Draw  a  ventral  view  of  the  skull,  showing  the  sutures  accurately. 

8.  The  ethmoid  region  and  the  sagittal  section  of  the  skull. — The  ethmoid 
is  nearly  completely  ossified  in  mammals  but  can  be  studied  only  in  sagittal 


THE  ENDOSKELETON:    SKULL  AND  VISCERAL  SKELETON  121 

sections,  since  it  is  in  the  interior  of  the  nose.  Obtain  a  sagittal  section  of  the 
skull.  These  sections  are  cut  slightly  to  one  side  of  the  median  line  so  that  the 
septum  of  the  nose  is  included  in  one  half  and  missing  on  the  other  half.  Students 
should  see  both  halves.  The  section  shows  that  the  interior  of  the  cranial  portion 
of  the  skull  is  occupied  by  a  large  cranial  cavity,  divisible  into  three  regions  of 
unequal  size.  The  most  posterior  region  inclosed  within  the  occipital  and  tem- 
poral bones  is  the  posterior  or  cerebellar  fossa  of  the  skull.  Its  anterior  boundary 
is  marked  by  a  prominent  (cat)  or  slight  (rabbit)  ridge  or  shelf  of  bone,  the  tentor- 
ium,  which  in  life  is  completed  by  a  membrane.  In  the  lateroventral  wall  of  the 
cerebellar  fossa  is  a  rounded  area  of  very  hard  compact  bone  bearing  two  openings ; 
this  is  the  petrous  part  of  the  petromastoid  bone  and  incloses  the  internal  ear.  The 
greater  part  of  the  cranial  cavity  comprises  the  middle  or  cerebral  fossa,  extending 
forward  from  the  tentorium.  Its  roof  and  walls  are  formed  by  the  frontal, 
parietal,  and  temporal  bones,  its  floor  by  the  sphenoids.  In  the  floor  of  the 
cerebral  fossa,  located  in  the  basisphenoid  bone,  is  a  marked  saddle-shaped 
depression,  the  sella  turcica,  in  which  in  life  the  pituitary  body  is  lodged.  The 
presphenoid  bone  contains  a  cavity,  the  sphenoidal  sinus.  In  the  anterior  part 
of  the  frontal  bone,  cavities,  the  frontal  sinuses,  are  also  present.  The  anterior 
end  of  the  cranial  cavity  is  the  small  anterior  or  olfactory  fossa  located  between 
the  anterior  parts  of  the  two  frontal  bones.  The  olfactory  fossa  is  separated 
from  the  nasal  cavities  which  lie  in  front  of  it  by  a  nearly  vertical  plate  of  bone, 
perforated  by  numerous  holes,  the  cribriform  plate  of  the  ethmoid.  This 
plate  is  best  seen  in  the  intact  skull  of  the  cat  by  looking  through  the  foramen 
magnum.  The  plate  pierced  by  holes  like  a  sieve  is  then  seen  closing  the  anterior 
end  of  the  cranial  cavity.  Our  study  of  the  sagittal  section  shows  that  the  floor 
of  the  skull  is  composed  of  a  chain  of  cartilage  bones,  occipital,  sphenoids,  and 
ethmoid,  on  which  the  brain  rests.  These  bones  as  has  already  been  explained 
are  derived  from  the  chondrocranium. 

The  nasal  cavities  or  nasal  fossae  are  inclosed  partly  in  cartilage  bones- 
derived  from  the  ethmoid  plate — and  partly  by  membrane  bones.  The  roof  of 
the  cavities  consists  of  the  nasal  bones  and  a  small  part  of  the  frontals.  The 
two  cavities  are  separated  by  a  median,  vertical,  bony  partition,  the  perpendicular 
plate  of  the  ethmoid;  in  the  living  state  this  is  continued  to  the  anterior  nares 
by  a  cartilaginous  plate.  The  two  together  constitute  the  septum  of  the  nose. 
The  bony  part  of  the  septum,  that  is,  the  perpendicular  plate  of  the  ethmoid, 
is  small  in  the  rabbit.  Dorsally  the  septum  meets  the  nasal  and  (cat)  frontal 
bones;  ventrally  it  meets  the  vomer,  an  elongated  bone  dorsal  to  the  maxillae 
and  palatines.  The  posterior  end  of  the  septum  meets  the  cribriform 
plate. 

On  the  half  of  the  skull  where  the  septum  is  missing  the  turbinated  bones  or 
conchae  may  be  studied.  They  are  peculiar,  delicate,  grooved  and  folded  bones 
which  occupy  the  lateral  walls  of  the  nasal  cavities  and  fill  most  of  the  interior. 


122       LABORATORY  MANUAL  FOR  VERTEBRATE  ANATOMY 

The  most  posterior  of  these  bones  is  the  ethmoturbinal  or  ethmoid  labyrinth  sit- 
uated just  in  front  of  the  cribriform  plate.  It  is  a  greatly  folded  structure,  the 
folds  inclosing  spaces  known  as  the  ethmoid  cells.  In  front  of  the  ethmoturbinal 
is  another  but  smaller  mass,  the  maxilloturbinal,  borne  on  the  inner  surface  of 
the  maxilla.  Each  nasal  bears  on  its  inner  surface  a  single  elongated  ledge  of 
bone,  which  constitutes  the  nasoturbinal,  and  which  lies  above  the  uppermost 
scrolls  of  the  ethmoturbinals.  The  function  of  the  turbinals  is  to  increase  the 
respiratory  and  olfactory  surface  of  the  nose.  The  ethmoturbinals  are  covered 
by  the  olfactory  epithelium,  while  the  maxilloturbinals  serve  to  strain  and  moisten 
the  air.  The  latter  will  be  seen  to  project  into  the  passage  from  the  anterior  to 
the  posterior  nares. 

From  the  comparative  standpoint  the  septum  of  the  nose  is  the  mesethmoid, 
the  perpendicular  plate  of  the  ethmoid  being  its  ossified  portion,  while  the  ethmoid 
labyrinths  are  the  ectethmoids.  The  cribriform  plate  is  produced  by  the  exten- 
sion of  ossification  processes  between  the  mesethmoid  and  ectethmoids.  The 
perforations  in  the  plate  are  for  the  passage  of  the  olfactory  nerve.  Thus,  the 
three  ethmoid  bones  of  lower  vertebrates  are  fused  into  the  single  ethmoid  bone 
of  mammals. 

Draw  the  sagittal  section. 

9.  The  foramina  of  the  skull. — The  skull  is  pierced  by  numerous  openings  for 
the  passage  of  nerves  and  blood  vessels  and  sometimes  other  structures.     These 
are  listed  below  for  convenient  reference. 
Cat: 

a)  Incisive  foramina.    Anterior  end  of  ventral  side  of  maxillae;  connect  roof 
of  mouth  with  nasal  cavities. 

b)  Infraorbital  foramen.     Large  opening  in  the  maxilla  at  the  beginning  of 
the  zygoma  tic  arch ;  for  the  passage  of  certain  branches  of  the  fifth  cranial  nerve 
and  blood  vessels. 

c)  Nasolacrimal  canal.     Anterior  end  of  the  lacrimal   bone  and  passing 
through  the  maxilla  into  the  nasal  cavities;  for  the  draining  of  the  tears. 

d)  Spheno palatine  foramen.     In  that  part  of  the  palatine  bone  which  extends 
into  the  orbit,  posterior  to  the  lacrimal;  for  the  passage  of  branches  of  the  fifth 
nerve  into  the  nasal  cavity. 

e)  Posterior  palatine  canal.     The  posterior  end  of  this  is  immediately  ventral 
to  the  sphenopalatine  foramen;   its  anterior  end  is  in  about  the  middle  of  the 
palatine  process  of  the  maxilla;   for  the  passage  of  a  branch  of  the  fifth  nerve 
to  the  palate. 

f)  Optic  foramen.     In  the  orbitosphenoid  part  of  the  presphenoid,  in  the 
posterior  part  of  the  orbit,  most  anterior  of  a  row  of  four  foramina;    for  the 
passage  of  the  optic  nerve. 

g)  Orbital  fissure.     Second  and  largest  of  the  row  of  four;  through  it  pass  the 
third,  fourth,  and  sixth  nerves  to  the  muscles  of  the  eyeball,  and  a  part  of  the  fifth. 


THE  ENDOSKELETON:    SKULL  AND  VISCERAL  SKELETON  123 

h)  Foramen  rotundum.  Third  of  the  row;  in  the  alisphenoid  bone;  trans- 
mits part  of  the  fifth  nerve. 

i)  Foramen  ovate.  Last  of  the  four;  in  the  alisphenoid;  transmits  part  of  the 
fifth  nerve. 

j)  Canal  for  the  Eustachian  tube.  In  the  anterior  wall  of  the  bulla,  its  roof 
formed  by  the  alisphenoid;  for  the  passage  of  the  Eustachian  tube  from  the 
pharynx  into  the  bulla. 

k)  Pterygoid  canal.  Each  bulla  terminates  anteriorly  in  a  point  (styliform 
process)  lying  on  the  basisphenoid;  this  point  is  directed  to  a  minute  opening, 
the  pterygoid  canal,  lying  in  the  suture  between  the  basisphenoid  and  the  ptery- 
goid  process;  for  the  passage  of  a  branch  of  the  fifth  nerve  into  the  bulla. 

/)  Jugular  foramen.  Large  foramen  on  the  medial  side  of  the  posterior  end 
of  the  bulla,  for  the  passage  of  the  ninth,  tenth,  and  eleventh  nerves. 

m)  Hypoglossal  foramen.  In  the  medial  side  of  the  preceding  foramen  for 
the  passage  of  the  twelfth  nerve. 

n)  Stylomastoid  foramen.     At  the  ventral  tip  of  the  mastoid  process  for  the 
passage  of  the  seventh  nerve. 
Rabbit: 

a)  Incisive  foramina.     As  in  the  cat  but  larger. 

b)  Infraborbital  foramen.     As  in  the  cat  but  more  slitlike  and  elongated, 
forming  an  infraorbital  canal,  opening  into  the  orbit  above  the  expanded  part 
of  the  maxilla. 

c)  Nasolacrimal  canal.     As  in  the  cat,  situated  under  the  pointed  anterior 
end  of  the  supraorbital  arch. 

d)  Posterior  palatine  foramen.     On  the  ventral  side,  in  the  suture  between 
the  palatine  process  of  the  maxilla  and  the  palatines;  forms  the  anterior  open- 
ing of  the  palatine  canal.     The  posterior  opening  is  at  the  posterior  end  of  the 
expanded  mass  of  the  maxilla  located  in  the  orbit.     This  canal  is  for  the  passage 
of  a  branch  of  the  fifth  nerve. 

e)  Spheno  palatine  foramen.     In  common  with  the  posterior  end  of  the 
palatine  canal  just  described;    for  the  passage  of  branches  of  the  fifth  nerve. 

/)  Anterior  and  posterior  supraorbital  foramina.  The  projecting  anterior  and 
posterior  ends  of  the  supraorbital  arch  are  continued  in  life  by  ligaments,  thus 
forming  foramina,  through  which  branches  of  the  fifth  nerve  pass. 

g)  Optic  foramen.  Large  opening  in  the  center  of  the  orbit,  for  the  passage 
of  the  optic  nerve. 

h)  Orbital  fissure.  Posterior  and  ventral  to  the  preceding,  and  including 
the  foramen  rotundum  of  other  mammals.  For  the  third,  fourth,  and  sixth 
nerves  to  the  eyeball,  and  the  greater  part  of  the  fifth. 

i)  Anterior,  middle,  and  posterior  sphenoidal  foramina.  Three  foramina  in  a 
row  in  the  lateral  lamella  of  the  pterygoid  process  at  the  place  where  this  is 
continuous  with  the  alisphenoid.  For  the  passage  of  part  of  the  fifth  nerv*. 


124      LABORATORY  MANUAL  FOR  VERTEBRATE  ANATOMY 

j)  Foramen  lacerum.  In  front  of  the  tympanic  bulla  on  the  ventral  surface 
and  including  the  foramen  ovale  of  other  mammals;  for  the  passage  of  part  of 
the  fifth  nerve  and  an  artery. 

k)  External  carotid  foramen.  A  small  foramen  in  the  middle  of  the  medial 
surface  of  the  bulla;  for  the  passage  of  the  internal  carotid  artery,  the  other  end 
of  the  canal  lying  in  the  foramen  lacerum. 

0  Jugular  foramen.  Just  posterior  to  the  preceding,  in  the  depression 
between  occipital  condyle  and  bulla;  for  the  ninth,  tenth,  and  eleventh  cranial 
nerves,  and  a  vein. 

m)  Eypoglossal  canal.  Including  small  apertures  posterior  to  the  preceding 
for  the  passage  of  the  twelfth  (hypoglossal)  nerve. 

n)  Stylomastoid  foramen.  In  front  of  the  middle  of  the  mas  toid  pro  cess,  for 
the  passage  of  the  seventh  nerve. 

10.  The  lower  jaw. — The  lower  jaw  or  mandible  consists  of  a  single  pair  of 
bones,  the  dentaries,  fused  in  front  by  a  symphysis.    All  other  bones  seen  in  the 
lower  jaw  of  the  alligator  have  vanished  (except  the  articular  which  is  supposed 
to  be  the  malleus  of  the  middle  ear).     The  horizontal  part  of  the  mandible  is 
named  the  body,  the  vertical  part,  the  ramus.     (In  the  lower  vertebrates  each 
half  of  the  lower  jaw  is  also  named  ramus.)     The  posterior  end  of  the  mandible 
(cat)  extends  dorsally  into  a  strong  coronoid  process,  which  in  the  natural  position 
projects  into  the  temporal  fossa.     In  the  rabbit  this  is  reduced  to  a  slight  projec- 
tion which  forms  the  lateral  boundary  of  a  deep  groove.     The  articulating 
surface  of  the  mandible  is  borne  on  the  condyloid  process.     The  depressed  areas 
in  the  posterior  part  of  the  mandible  are  for  the  insertion  of  the  muscles  of 
mastication.    Near  the  anterior  tip  of  the  mandible  on  the  outer  surface  is  the 
mental  foramen  (or  two  in  the  cat)  through  which  the  nerve  of  the  lower  jaw 
exits.    Near  the  caudal  end  of  the  inner  surface  is  the  mandibular  foramen 
through  which  the  nerve  enters  and  pursues  a  course  in  the  interior  of  the  man- 
dible to  the  mental  foramen.     In  the  rabbit  there  is  an  additional  foramen 
just  above  the  mandibular  foramen,  for  the  passage  of  a  vein. 

Owing  to  the  absence  of  the  quadrate  and  of  all  of  the  bones  of  the  lower 
jaw  except  the  dentary,  the  articulation  of  the  lower  jaw  to  the  skull  is  between 
the  dentary  and  the  squamosal.  This  feature  distinguishes  mammals  from  all 
other  vertebrates,  for  in  the  latter  the  articulation  is  between  the  articular  and 
the  quadrate.  The  condition  found  in  mammals  is,  however,  approached  by 
those  reptiles  directly  ancestral  to  mammals. 

11.  The  teeth. — The   teeth  of  mammals  possess  several   characteristics: 
they  are  set  in  sockets  or  alveoli  in  the  jaws,  a  condition  known  as  thecodont; 
they  are  heterodont,  that  is,  differentiated  into  several  different  kinds;   they  can 
generally  be  replaced  only  once;   and  some  of  them  are  complicated  in  form. 
The  teeth  of  mammals  furnish  important  taxonomic  characters.     Those  of  the 
cat  and  rabbit  are  very  different,  the  former  having  teeth  characteristic  of 


THE  ENDOSKELETON:    SKULL  AND  VISCERAL  SKELETON  125 

carnivores,  the  latter  with  the  chisel-like  front  teeth  and  grinding  back  teeth 
common  to  rodents. 

Cat:  At  the  tip  of  the  jaws  are  six  small  simple  teeth,  named  incisors.  On 
either  side  of  the  incisors  is  a  canine,  a  long,  sharp  but  simple  tooth.  Back  of 
the  canine  on  each  side  are  four  teeth  in  the  upper  jaw,  three  in  the  lower.  These 
teeth  are  mostly  more  complicated  than  the  preceding,  having  more  than  one 
cusp  or  pointed  projection  and  more  than  one  root.  These  teeth  are  known  as 
premolars  and  molars.  In  the  upper  jaw  the  first  three  on  each  side  are  pre- 
molars,  and  the  first  two  in  the  lower  jaw;  the  last  tooth  on  each  side  in  each 
jaw  is  a  molar.  Note  that  the  upper  incisors  are  borne  on  the  premaxillae;  the 
other  teeth  of  the  upper  jaw  on  the  maxilla.  Between  the  canines  and  the 
premolars  is  more  or  less  of  a  gap,  known  as  a  diastema. 

Rabbit:  At  the  tip  of  the  premaxillae  are  borne  four  chisel-like  incisor  teeth, 
a  small  pair  behind  a  larger  anterior  pair.  The  chisel-like  form  of  the  incis- 
ors is  characteristic  of  rodents  and  is  due  to  the  fact  that  the  enamel  is 
present  on  the  front  face  of  the  tooth  only;  the  posterior  face  being  composed 
of  the  softer  dentine  wears  away,  leaving  a  sharp  edge  to  the  enamel.  The 
incisors  of  rodents  further  continue  to  grow  indefinitely  so  that  the  loss  from  use 
at  the  tip  is  replaced  by  growth  at  the  root.  At  the  tip  of  the  lower  jaw  are  two 
similar  incisors.  Posterior  to  the  incisors  is  a  very  wide  gap  or  diastema.  The 
canine  teeth,  found  in  most  mammals  posterior  to  the  incisors,  are  missing  in 
the  rabbit  and  rodents  in  general.  On  the  alveolar  process  of  the  maxilla  are 
borne  six  teeth  in  the  upper  jaw  and  five  in  the  lower,  on  each  side.  Of  these 
the  first  three  in  the  upper  and  first  two  in  the  lower  jaw,  are  premolars,  the  last 
three  molars.  The  teeth  are  ridged  crosswise,  an  adaptation  for  the  grinding 
of  vegetable  food;  the  ridges  consist  of  enamel  with  dentine  between  them. 

The  teeth  of  mammals  are  generally  designated  by  a  dental  formula,  which 
expresses  the  number  of  each  different  kind  of  tooth  in  each  half-jaw  from  the 
anterior  to  the  posterior  extremity  of  the  jaw.  The  teeth  of  the  upper  jaw  are 
placed  in  the  numerator  of  the  formula,  those  of  the  lower  jaw  in  the  denomina- 
tor. The  complete  dentition  for  mammals  consists  of  three  incisors,  one  canine, 
four  premolars,  and  three  molars  in  each  half  of  each  jaw;  the  dental  formula 
is  then  3/3,  i/i,  4/4,  3/3.  In  the  cat  the  formula  is  3/3,  i/i,  3/2,  i/i;  in  the 
rabbit  2/1,  o/o,  3/2,  3/3;  in  man  2/2,  i/i,  2/2,  3/3.  There  is  no  structural 
difference  between  premolars  and  molars,  but  those  teeth  with  complicated 
crowns  which  occur  in  both  the  "milk"  and  the  permanent  dentitions  are 
designated  premolars,  while  those  which  are  permanent  from  the  first  and  never 
replaced  are  designated  as  molars.  It  is  not  known  whether  the  complicated, 
apparently  compound  structure  of  premolars  and  molars  is  due  to  their  having 
arisen  through  the  fusion  of  several  simple  teeth  or  to  the  subdivision  of  the 
original  simple  conical  teeth.  The  structure  of  teeth  and  their  origin  from  placoid 
scales  was  discussed  in  Section  V. 


126       LABORATORY  MANUAL  FOR  VERTEBRATE  ANATOMY 

12.  The  hyoid  apparatus. — This  is,  as  already  explained,  the  remnant  of  the 
hyoid  and  other  gill  arches.  It  is  generally  absent  on  prepared  skeletons  and 
isolated  specimens  will  be  provided  for  its  study. 

The  hyoid  of  the  cat  consists  of  a  bony  bar  placed  in  the  root  of  the  tongue 
just  in  front  of  the  larynx;  this  bar  is  called  the  body  of  the  hyoid.  From  it 
extend  two  pairs  of  processes  or  horns,  an  anterior  pair  and  a  posterior  pair. 
The  anterior  horns  consist  of  four  separate  pieces,  of  which  the  terminal  piece 
is  attached  to  the  tympanic  bulla  just  ventral  to  the  stylomastoid  foramen 
of  the  skull.  The  groove  which  it  occupies  can  generally  be  seen  on  the  side  of 
the  bulla.  The  posterior  horns  consist  each  of  a  single  piece  which  is  united  to 
the  larynx.  The  body  and  anterior  horns  belong  to  the  hyoid  arch;  the  posterior 
horns  to  the  third  gill  arch.  The  remaining  gill  arches  are  represented  in  the 
larynx  which  will  be  studied  later. 

The  hyoid  apparatus  of  the  rabbit  consists  of  a  stout  bone,  the  body  of  the 
hyoid,  situated  at  the  base  of  the  tongue  in  front  of  the  larynx.  It  bears  two 
pairs  of  processes  or  horns.  The  anterior  horn  is  a  short  piece  connected  by  a 
muscle  with  the  jugular  process  of  the  skull.  The  posterior  horn  is  a  longer 
piece  connected  by  ligament  with  the  larynx  and  by  muscle  with  the  jugular 
process.  The  body  and  anterior  horn  are  remnants  of  the  hyoid  arch,  the  posterior 
horn  of  the  third  gill  arch. 

H.      GENERAL   SUMMARY  OF   THE   SKULL  AND  VISCERAL   SKELETON 

1.  The  skull  begins  as  a  cartilaginous  case,  the  chondrocranium,  whose  method  of 
formation    has   already    been    described.      This    case    includes    the    olfactory    and    otic 
capsules. 

2.  The  gill  arches  become  closely  associated  with  the  chondrocranium.     There  are 
usually  seven  of  them,  the  first  forming  the  upper  and  lower  jaws,  the  second  the  hyoid  arch. 
The  arches  are  reduced  in  number  and  altered  in  function  in  land  vertebrates. 

3.  Bones  derived  from  the  chondrocranium  and  gill  arches  are  the  cartilage  bones  of 
the  skull. 

4.  In  addition  to  these  there  are  membrane  bones  added.    They  come  from  the  dermis 
of  the  skin  and  were  originally  dermal  scales. 

5.  The  cartilage  bones  of  the  skull   and   sense  capsules  are  the  occipitals,  sphenoids, 
ethmoids,  otics,  and  turbinals.    There  are  four  occipitals  (supraoccipital,  two  exoccipitals, 
basioccipital) ;    a  posterior  group  of  three  sphenoids  (basisphenoid,  two  alisphenoids) ;    an 
anterior  group  of  three  sphenoids  (presphenoid,  two  orbitosphenoids) ;  and  a  group  of  three 
ethmoids  (mesethmoid,  two  ectethmoids).    The  otic  bones  ossified  in  the  ear  capsule  are 
generally  three  in  number  but  may  be  as  many  as  five;  in  mammals  they  are  fused  to  form 
a  periotic  or  petromastoid  bone.    The  nasal  capsules  may  furnish  additional  cartilage  bones. 
The  occipitals  and  sphenoids  form  the  posterior  end  and  floor  of  the  brain  cavity,  while  the 
ethmoids  inclose  the  nasal  cavities. 

6.  The  first  gill  arch  generally  gives  rise  to  two  cartilage  bones,  a  quadrate  derived  from 
the  posterior  end  of  the  upper  jaw  cartilages  or  pterygoquadrate  cartilages;  and  an  articular 
derived  from  the  posterior  end  of  the  lower  jaw  cartilages  or  Meckel's  cartilages.     In  some 
forms  the  pterygoquadrate  cartilages  may  give  rise  to  palatines  and  pterygoids  in  addition. 


THE  ENDOSKELETON:    SKULL  AND  VISCERAL  SKELETON 


127 


7.  The  second  or  hyoid  gill  arch  gives  rise  to  a  cartilage  bone,  the  hyomandibular,  and 
the  remainder  of  it  takes  part  in  the  formation  of  the  hyoid  apparatus. 

8.  All  other  bones  of  the  skull  and  jaws  are  membrane  bones. 

9.  The  number  of  membrane  bones  of  the  skull  is  greatest  in  the  lower  vertebrates,  and 
the  membrane  bones  present  the  most  typical  arrangement  in  extinct  and  primitive  groups 
of  Amphibia  and  reptiles.    The  number  of  membrane  bones  diminishes  in  the  higher  verte- 
brates, chiefly  by  loss  but  also  by  fusion.    Thus,  we  may  note  that  several  membrane  bones 
present  in  the  alligator  are  missing  in  the  cat  and  rabbit.     This  loss  of  membrane  bones  is 
illustrated  in  Figure  36,  page  106. 

10.  The  number  of  cartilage  bones  is  likewise  greatly  reduced  in  the  higher  forms  by  a 
process  of  fusion  of  originally  separate  bones.    Thus,  the  occipital  bone  of  mammals  consists 
of  four  originally  separate  bones;   the  sphenoids  of  mammals  are  more  or  less  fused  and  in 
man  unite  to  a  single  sphenoid  bone  which  consists  of  six  originally  separate  cartilage  bones 


FIG.  40. — Some  bones  of  the  human  skull  at  an  early  age,  showing  their  compound  nature.  A,  the 
occipital  bone  at  birth,  showing  the  five  elements  of  which  it  is  composed;  a,  interparietal;  b,  supra- 
occipital;  c,  exoccipital;  d,  is  the  foramen  magnum;  e,  basioccipital.  B,  the  sphenoid  bone  in  an 
embryo  of  four  months,  showing  its  components;  /,  center  for  the  presphenoid;  g,  center  for  the  orbito- 
sphenoid;  h,  alisphenoid;  i,  center  for  the  basisphenoid;  j,  center  for  the  lingula;  k,  center  for  the 
pterygoid.  C,  the  temporal  bone  at  birth,  showing  its  three  components;  /,  squamosal;  w,  tympanic; 
n,  petromastoid  or  periotic.  Membrane  bones  blank;  cartilage  bones  open  stippling;  cartilage,  close 
stippling.  The  subsequent  ossification  of  the  cartilage  obliterates  the  boundaries  between  the  com- 
ponents. (A  and  C  from  specimens  loaned  by  the  anatomy  department;  B  after  McMurrich's  Develop- 
ment of  the  Human  Body,  copyright  by  P.  Blakiston's  Son  and  Company.) 

and  includes  some  membrane  bones  in  addition;   the  ethmoid  of  mammals  is  composed  of 
three  ethmoid  bones;  the  petromastoid  of  several  otic  bones,  etc.  (Fig.  40). 

11.  The  lower  jaw  undergoes  reduction  to  the  mammalian  condition,  where  it  consists 
of  but  a  single  pair  of  bones.    In  most  vertebrates  the  articulation  of  the  lower  jaw  is  between 
the  quadrate  and  the  articular  bones,  while  in  mammals  it  is  between  the  dentary  and  the 
squamosal  bones. 

12.  The  quadrate,  the  articular,  and  the  hyomandibular  are  believed  to  be  represented 
in  mammals  in  the  tiny  bones  found  in  the  middle  ear.    The  remnant  of  the  quadrate  forms 
the  incus,  of  the  articular,  the  malleus,  and  of  the  hyomandibular,  the  stapes. 

13.  The  hyoid  and  other  gill  arches  are  gradually  reduced  in  land  vertebrates  but  persist 
in  part  in  the  hyoid  apparatus  and  the  cartilages  of  the  larynx, 


IX.     THE   COMPARATIVE   ANATOMY   OF   THE    MUSCULAR   SYSTEM 

A.      GENERAL   CONSIDERATIONS 

i.  The  kinds  and  origin  of  muscle. — The  muscles  of  the  vertebrate  body  may  be  divided 
into  two  genera]  classes,  the  involuntary  and  the  voluntary.  The  involuntary  or  smooth  muscles 
occur  in  the  walls  of  the  digestive  tract  and  other  viscera,  and  in  the  skin  and  certain  deriva- 
tives thereof.  They  originate  through  the  transformation  of  mesenchyme  cells,  which  may 
be  of  various  origins.  The  majority  of  the  smooth  musculature,  however,  is  produced  by  the 
mesenchyme  of  the  hypomere,  since  in  development  the  hypomere  closes  around  the  archen- 
teron  and  its  derivatives  (see  Fig.  10,  p.  43).  The  voluntary  or  striated  muscles,  on  the  other 
hand,  with  certain  exceptions  specified  below,  arise  from  the  myotomes.  The  myotomes,  it 
will  be  recalled,  are  those  portions  of  the  epimeres  remaining  after  the  epimeres  have  given 
rise  to  the  sclero  tomes  and  derma  tomes.  From  their  original  dorsal  positions  the  myotomes 
grow  down  between  the  hypomere  and  the  skin  and,  those  of  opposite  sides  meet  in  the  median 
ventral  line.  See  Figure  10  and  re-read  Section  IV  of  the  manual.  In  this  way  there  is 
produced  a  complete  coat  of  voluntary  muscles,  lying  beneath  the  skin.  This  muscle  coat 
is  divided  into  dorsal  and  ventral  parts  by  the  horizontal  skeletogenous  partition  which 
intersects  the  skin  at  the  lateral  line.  The  muscles  dorsal  to  this  septum  are  called  the 
epaxial  muscles,  those  below  the  septum,  the  hypaxial  muscles  (Figs.  15^!,  p.  58,  20,  p.  62). 

All  of  the  muscles  originating  from  the  myotomes  are  voluntary  muscles  and  are  designated 
as  parietal  or  somatic  muscles.  Not  all  of  the  voluntary  muscles  are,  however,  of  this  kind. 
In  the  gill  region  of  vertebrates  a  system  of  voluntary  muscles  is  developed  for  moving  the 
gill  arches.  Since  the  gills  and  related  parts  are  of  entodermal  origin,  the  muscles  in  the 
walls  in  the  gill  region  are  homologous  with  the  muscles  of  the  rest  of  the  digestive  tract 
and  are,  in  fact,  derived  from  the  mesenchyme  of  the  hypomeres.  These  gill  arch  muscles  are 
consequently  designated  as  visceral  muscles,  although,  unlike  the  muscles  of  the  viscera,  they 
are  striated  and  voluntary.  There  are  consequently  two  kinds  of  voluntary  muscles,  iden- 
tical in  structure  but  different  in  origin — the  parietal  or  somatic  muscles  derived  from  the 
myotomes  and  distributed  widely  over  the  body  and  the  visceral  muscles  derived  from 
the  hypomeres  and  found  only  in  the  gill  region. 

The  student  should  note  that  the  terms  muscle  and  muscular  system,  when  used  without 
further  qualification,  refer  only  to  the  voluntary  muscles.  In  dissecting  a  vertebrate  only  the 
voluntary  muscles  are  studied,  as  the  study  of  the  involuntary  muscles  properly  belongs  to 
histology.  It  should  further  be  always  borne  in  mind  that  when  the  expression  visceral  muscles 
is  employed  this  refers  not  to  the  involuntary  muscles  of  the  viscera  but  to  the  voluntary 
muscles  of  the  gill  arches.  It  is  assumed  that  the  student  understands  the  histological  differ- 
ence between  smooth  and  striated  muscle. 

Consult  also  the  accounts  of  the  comparative  anatomy  of  the  muscular  system  in  K, 
W,  and  Wd. 


B.   THE  MUSCLES  OF  THE  DOGFISH 

i.  The  parietal  or  somatic  muscles.— Strip  off  the  skin  from  the  dogfish  at 
the  base  of  the  tail,  in  the  neighborhood  of  the  pelvic  fins.  In  doing  this  make 
a  cut  through  the  skin,  grasp  the  cut  edge  with  the  fingers,  and  strip  off  a  piece  of 

128 


COMPARATIVE  ANATOMY  OF  THE  MUSCULAR  SYSTEM  129 

skin  without  the  further  use  of  the  knife.  If  you  attempt  to  cut  the  skin  from 
the  body  by  means  of  a  scalpel  you  will  slash  into  the  muscles.  After  removing 
a  considerable  area  of  skin  note  that  the  body  wall  under  the  skin  is  composed 
of  a  coat  of  muscles  completely  sheathing  the  body.  These  are  the  parietal 
muscles.  Observe  that  the  parietal  muscles  consist  of  a  series  of  zigzag  myotomes, 
each  separated  from  its  neighbor  by  a  white  sheet  of  connective  tissue,  the 
myocomma  or  myoseptum.  In  the  middle  of  the  side  of  the  body  observe  a 
white  line  running  lengthwise.  This  is  the  outer  edge  of  the  horizontal  skele- 
togenous  septum,  which  intersects  the  skin  at  the  lateral  line.  This  septum 
divides  the  myotomes  into  dorsal  portions — the  epaxial  muscles — and  ventral 
portions — the  hypaxial  muscles.  On  the  ventral  side  note  that  there  is  a  white 
partition  in  the  median  ventral  line.  This  is  the  linea  alba;  it  separates  the 
myotomes  of  the  two  sides  of  the  body.  No  muscle  ever  crosses  the  dorsal  or 
ventral  median  lines,  and  hence  all  of  the  muscles  are  paired. 

Draw  from  the  side  a  portion  of  the  body  to  show  the  myotomes. 

2.  The  modification  of  the  parietal  muscles  by  the  presence  of  appendages. — 
Strip  off  the  skin  from  the  bases  of  the  pelvic  fins  and  observe  that  additional 
muscles  are  present  in  the  fins  at  their  bases  for  the  purpose  of  moving  the  fins. 
A  mass  of  muscle  springs  from  the  myotomes  and  is  inserted  on  the  fin  on  both 
dorsal  and  ventral  sides.     Cut  through  the  middle  of  the  dorsal  mass,  carefully 
separate  the  cut  ends  from  the  body  wall,  and  note  that  the  myotomes  are  re- 
vealed underneath  the  fin  muscles.     The  dorsal  ends  of  the  fin  muscles  will  be  seen 
to  spring  from  the  myotomes  with  which  they  are  continuous,  and  it  is  important 
to  note  that  several  myotomes  contribute  in  this  way  to  the  fin  muscles.     We 
thus  learn  that  in  the  neighborhood  of  an  appendage  the  myotomes  bud  off 
muscle  slips  for  the  appendages.     In  this  manner  the  muscles  of  the  appendages 
originally  arose.     It  should  be  stated,  however,  that  in  the  higher  vertebrates  the 
intrinsic  appendicular  muscles  can  no  longer  be  seen  to  originate  in  this  fashion 
but  are  formed  in  place  in  the  limbs. 

3.  The  visceral  muscles  of  the  dogfish. — Make  a  median  ventral  incision  in 
the  skin  of  the  ventral  side  of  the  head  between  the  gill  slits  and  strip  off  the  skin 
in  an  upward  direction  over  the  gill  slits  and  up  to  the  median  dorsal  line.    Note 
that  dorsally  above  the  gill  slits  the  myotomes  are  present  and  as  typical  in  form 
and  arrangement  as  in  more  posterior  parts  of  the  body.     In  the  region  of  the  gill 
slits  and  on  the  ventral  surface  of  the  head,  however,  entirely  new  sets  of  muscles 
are  found  which  serve  to  move  the  gill  arches  and  the  jaw.     These  muscles  are 
the  visceral  muscles,  and  they  are  derived  from  the  hypomeres.    There  are  a 
great  many  of  them,  and  it  is  not  our  purpose  to  identify  them  in  detail,  but 
the  following  may  be  noted:  trapezius,  the  long  muscle  above  the  gill  slits,  and 
below  the  myotomes;  the  dorsal  constrictors,  between  the   dorsal  portions  of 
the  gill  arches,  and  with  their  fibers  directed  obliquely  forward;   the  ventral 
constrictors,  between  the  ventral  portions  of  the  gill  arches,  and  covering  most  of 


130       LABORATORY  MANUAL  FOR  VERTEBRATE  ANATOMY 

the  ventral  surface,  extending  forward  to  the  mouth,  with  fibers  directed  back- 
ward; and  the  adductor  mandibularis,  the  thick  muscle  at  the  angle  of  the  jaws, 
used  to  close  the  lower  jaw.  The  constrictors  open  the  gill  slits  by  diminishing  the 
distance  between  the  gill  arches  by  their  contraction.  These  muscles  of  the 
dogfish  persist  in  higher  vertebrates  after  the  gills  have  been  lost  in  association 
with  those  structures  which  are  derived  from  the  gill  arches,  that  is,  the  jaws,  the 
hyoid,  and  the  cartilages  of  the  larynx. 

C.      THE   MUSCLES   OF   NECTURUS 

Animals  that  have  been  preserved  for  some  time  in  formalin  are  preferable  for 
the  study  of  the  muscles.  Make  a  median  dorsal  incision  extending  the  length 
of  the  head  and  trunk.  Loosen  the  cut  edges  of  skin  with  the  fingers,  noting  in 
the  cut  surfaces  the  flask-shaped  cutaneous  glands  which  secrete  slime.  Then 
with  the  fingers  separate  the  skin  from  the  muscles  in  a  ventral  direction  until  you 
have  removed  the  skin  from  the  head,  trunk,  and  appendages.  The  gills  are  to  be 
left  in  place.  The  white  fibrous  material  between  the  skin  and  muscles  is  the 
subcutaneous  connective  tissue  or  superficial  fascia. 

1.  The  muscles  of  the  trunk  and  tail. — These  muscles  preserve  the  generalized 
arrangement  typical  of  primitive  and  embryonic  vertebrates.     They  consist,  as 
in  the  dogfish,  of  a  series  of  myo tomes,  separated  by  myosepta.     The  myo tomes 
are  long,  nearly  rectangular  blocks  extending  from  the  mid-dorsal  to  the  mid- 
ventral  line.    Their  narrowed  dorsal  ends  slant  forward.     Note  their  division 
into  epaxial  and  hypaxial  portions  by  the  horizontal  septum.     Although  the 
trunk  muscles  appear  to  be  unmodified,  they  are  in  reality  already  separating 
into  layers.     On  cutting  into  the  epaxial  muscles  they  will  be  seen  to  constitute 
a  mass  whose  fibers  are  all  directed  forward;  this  mass  corresponds  to  the  lon- 
gissimus  dorsi  muscle  of  higher  forms.     On  cutting  into  the  hypaxial  muscles,  on 
the  other  hand,  they  will  be  found  divisible  into  three  distinct  layers.     The 
outer  layer,  or  external  oblique  muscle,  is  composed  of  fibers  directed  obliquely 
ventrad  and  caudad.     The  middle  layer,  or  internal  oblique  muscle,  is  composed 
of  fibers  directed  craniad  and  ventrad.     On  cutting  through  the  internal  oblique 
a  third  layer — the  transverse — next  to  the  body  cavity  will  be  found.     On  either 
side  of  the  median  ventral  line  the  fibers  of  the  myotomes  are  directed  parallel 
to  the  longitudinal  axis  of  the  body  and  form  a  narrow  band,  the  rectus  abdominis 
muscle,  which  is  rather  distinct  just  anterior  to  the  pelvic  girdle. 

2.  The  muscles  of  the  pelvic  girdle  and  hind  limb. — In  this  region  of  the 
body,  as  in  the  case  of  the  dogfish,  the  series  of  myotomes  is  interrupted  by 
the  presence  of  limb  muscles,  extending  from  the  trunk  into  the  limb.     Examine 
the  ventral  face  of  the  pelvic  girdle.     It  is  covered  by  muscles  which  may  be  sepa- 
rated into  two  muscles.     The  anterior  muscle  covering  the  pubic  cartilage  extends 
from  the  median  line  of  the  girdle  to  the  femur;  it  may  be  named  the  pubofemora- 
lis  externus.    The  posterior  muscle  extends  from  the  ischial  region  of  the  girdle 


COMPARATIVE  ANATOMY  OF  THE  MUSCULAR  SYSTEM  131 

to  the  tibia  and  is  called  the  ischiotibialis .  From  the  anterolateral  margin  of 
the  girdle  a  muscle,  partly  concealed  under  the  pubofemoralis  externus,  projects 
forward,  and  its  fibers  become  continuous  with  the  ventral  portions  of  the 
myotomes.  This  is  the  posterior  end  of  the  rectus  abdominis  noted  above. 
Around  the  anus  is  a  large  mass,  the  anal  gland.  On  removing  this  gland  two 
muscles  are  revealed  on  each  side.  The  medial  one  is  the  ischiocaudalis,  extend- 
ing from  the  posterior  margin  of  the  ischium  to  the  tail,  and  the  lateral  one  is 
the  pyriform,  its  anterior  end  fastened  to  the  ischiotibialis,  its  posterior  end  pro- 
ceeding into  the  tail.  Follow  these  two  muscles  into  the  tail.  Turn  the  animal 
sidewise,  bend  the  leg  forward,  and  note  that  the  pyriform  splits  near  its  anterior 
end  and  gives  off  another  muscle  which  passes  to  the  femur.  This  muscle 
is  the  femorocaudalis .  The  three  muscles  just  mentioned  are  tail  or  caudal 
muscles. 

Certain  definitions  may  now  be  made.  Each  muscle  is  attached  at  its  two 
ends  and  more  or  less  free  in  the  middle.  The  attached  end  which  is  fixed  and 
immovable  is  named  the  origin;  the  attached  end  which  moves  when  the  muscle 
contracts  is  the  insertion.  The  best  method  of  naming  muscles  is  to  combine  the 
origin  and  insertion  into  a  compound  word,  the  origin  preceding.  The  names  of 
the  majority  of  the  muscles  given  above  are  of  this  kind,  but  muscles  are  also 
frequently  named  from  their  shapes,  positions,  etc.  In  the  case  of  many  muscles 
either  end  may  serve  as  the  origin  or  insertion,  depending  on  which  end  is  held 
fixed;  thus,  the  caudal  muscles  of  Necturus  will  move  the  tail  if  the  pelvic  girdle 
is  fixed  or  will  move  the  girdle  if  the  tail  remains  stationary.  The  muscles  of 
the  girdle  already  named  serve  chiefly  to  bend  the  limb  toward  the  median 
ventral  line,  a  movement  known  as  adduction. 

On  the  ventral  surface  of  the  thigh  four  muscles  may  be  identified.  These 
are :  the  pubofemoralis  internus,  the  most  anterior  one,  originating  on  the  anterior 
rim  of  the  acetabulum  and  inserted  on  the  distal  end  of  the  femur;  the  pubotibialis, 
next  posterior  to  the  preceding,  extending  from  the  acetabulum  to  the  proximal 
end  of  the  tibia ;  next,  the  distal  portion  of  the  ischiotibialis,  this  part  being  some- 
times designated  the  gracilis  muscle;  and  most  posteriorly,  a  slender  muscle,  the 
femorofibularis,  originating  on  the  preceding  muscle  and  inserted  on  the  fibula. 
Some  of  these  muscles  act  to  bend  the  shank  toward  the  thigh,  an  act  known  as 
flexion. 

On  the  dorsal  side  of  the  thigh  in  addition  to  some  of  the  muscles  already 
mentioned,  which  appear  also  in  dorsal  view,  there  is  found  chiefly  the  ilioexten- 
sorius,  a  broad  band  originating  on  the  ilium  (which  may  readily  be  found  as  a 
curved  bone  between  the  myotomes)  and  inserted  on  the  distal  end  of  the  femur 
by  a  tendon  (sheet  of  connective  tissue)  which  also  passes  onto  the  shank.  This 
muscle  abducts  the  thigh,  that  is,  draws  it  dorsally,  and  straightens  or  extends  the 
shank.  Between  the  ilioextensorius  and  the  pubofemoralis  internus,  partly 
concealed  by  the  latter,  is  the  iliofemoralis ,  extending  from  the  ilium  to  the  femur. 


132       LABORATORY  MANUAL  FOR  VERTEBRATE  ANATOMY 

The  homology  of  these  muscles  with  those  of  mammals  is  somewhat  doubtful. 
Some  suggestions  are  given  by  W,  page  229.  A  figure  which  may  aid  in  identify- 
ing the  muscles  will  be  found  in  W,  page  228. 

3.  The  muscles  of  the  pectoral  girdle  and  fore  limb. — Study  the  ventral 
surface  of  the  pectoral  girdle.     Covering  the  coracoid  cartilage  is  a  fan-shaped 
muscle,  the  supracoracoideus,  inserted  on  the  humerus.     On  either  side,  overly- 
ing the  elongated  procoracoid  cartilage,  is  the  procoracohumeralis,  inserted  on  the 
humerus  just  anterior  to   the  preceding.     Behind   the  supracoracoideus  and 
practically  continuous  with  it  is  the  large  fan-shaped  pectoral  muscle,  which 
originates  on  the  linea  alba  and  some  of  the  myosepta.     Covering  the  ventral 
surface  between  the  two  procoracohumerales,  is  the  broad  sternohyoid  muscle, 
differentiated  out  of  the  ventral  portions  of  the  most  anterior  myotomes.     At  its 
anterior  end  the  two  halves  of  the  sternohyoid  muscle  fork,  another  pair  of 
muscles  (geniohyoids)  being  inserted  between  the  two  parts  of  the  fork.     The 
sternohyoid  should   be  traced  posteriorly;  it  has   some  attachments  to  the 
pectoral  girdle,  passes  dorsal  to  the  pectoral  muscles  which  should  be  lifted  to 
see  it,  and  becomes  continuous  with  the  rectus  abdominis.     The  sternohyoid 
and  rectus  abdominis  preserve  almost  perfectly  the  segmented  condition. 

On  the  dorsal  side  of  the  pectoral  girdle  identify  the  latissimus  dorsi,  the  most 
posterior  and  largest  of  the  dorsal  girdle  muscles.  It  originates  by  about  five 
separate  slips  from  the  myosepta  of  adjacent  myotomes,  and  these  slips  converge 
to  the  humerus.  This  muscle  illustrates  very  well  the  compound  origin  of 
limb  muscles  by  slips  or  buds  from  several  myotomes.  Anterior  to  the  latissimus 
dorsi  and  covering  the  surface  of  the  scapula  is  the  dor  sails  scapulae,  also  inserted 
on  the  humerus.  Anterior  to  this  is  the  trapezius  (or  cucullaris)  originating  by 
two  slips  or  heads  and  inserted  on  the  scapula.  Between  the  trapezius  and  the 
procoracohumeralis  is  the  omohyoid  muscle,  running  parallel  to  the  latter,  from 
the  hyoid  to  the  girdle.  Cut  through  the  middle  of  the  latissimus  dorsi  and 
find  beneath  it  the  typical  myotomes.  These  become  in  higher  forms  the 
serratus  muscle. 

In  the  upper  arm  identify  on  the  ventral  side  two  muscles:  an  anterior  one, 
the  biceps,  extending  from  the  humerus  to  radius;  and  a  posterior  coracobrachialis 
originating  from  the  coracoid  cartilage  and  passing  to  the  distal  end  of  the 
humerus.  The  dorsal  side  of  the  upper  arm  is  occupied  by  the  large  triceps 
muscle,  with  origins  on  coracoid,  scapula,  and  humerus,  and  insertion  on  the 
ulna.  The  muscles  of  the  forearm  are  in  two  chief  masses,  a  dorsal  extensor 
which  extends  the  hand  and  a  ventral  flexor  which  bends  the  hand. 

4.  The  visceral  and  head  muscles. — The  visceral  muscles  or  muscles  of  the 
gill  arches  are  considerably  modified  in  relation  to  the  greater  separation  of  head 
from  trunk  and  the  degeneration  of  the  gill  arches.     Skin  the  head  completely. 
Across  the  ventral  surface  from  one  half  of  the  lower  jaw  to  the  other  passes 
the  mylohyoid  muscle.     It  originates  on  the  jaws  and  is  inserted  on  the  median 


COMPARATIVE  ANATOMY  OF  THE  MUSCULAR  SYSTEM  133 

connective  tissue  partition  (raphe).  Slit  up  the  raphe  and  find  beneath  it  two 
elongated  parallel  muscles,  the  geniohyoids,  arising  from  the  tip  of  the  mandible 
and  inserted  on  the  fascia  of  the  sternohyoid.  The  sternohyoid  parts  on  either 
side  of  this  place  and  passes  around  and  dorsal  to  the  posterior  ends  of  the  genio- 
hyoids  to  be  inserted  on  the  hyoid  arch.  On  each  side  of  the  geniohyoid  a  large 
muscle,  the  external  ceratohyoid,  originates  from  the  hyoid  arch  and  passes  pos- 
teriorly around  the  angle  of  the  jaw.  The  posterior  end  of  the  ceratohyoid  is 
covered  by  the  posterior  part  of  the  mylohyoid,  which  is  inserted  on  the  surface 
fascia  of  the  ceratohyoid  and  on  the  gill  region,  on  which  the  ceratohyoid  is  also 
inserted.  At  the  posterior  end  of  the  ceratohyoid  is  a  fan-shaped  muscle  which 
is  inserted  on  the  gill  arches.  This  muscle,  the  levator  branchiarum,  serves  to 
lift  the  gills,  while  the  ceratohyoid  draws  them  forward. 

On  the  dorsal  side  of  the  head  on  either  side  of  the  median  line  is  the  temporal 
muscle.  Lateral  to  this  is  a  large  mass,  the  masseter  muscle.  Both  are  inserted 
on  the  lower  jaw  at  the  angle  of  the  jaw  (the  insertion  of  the  temporal  being  con- 
cealed by  the  masseter)  and  serve  as  elevators  of  the  lower  jaw.  Separate  the 
masseter  and  ceratohyoid  widely  and  find  between  and  beneath  them  a  smaller 
muscle,  the  digastric,  which  extends  from  the  skull  to  the  angle  of  the  lower  jaw. 
It  is  the  depressor  or  opener  of  the  lower  jaw.  Behind  the  digastric  under  the 
posterior  end  of  the  ceratohyoid  is  a  muscle  slightly  smaller  than  the  digastric, 
the  levator  arcuum,  which  passes  to  the  gill  arches  and  serves  to  lift  them. 

Draw  dorsal  and  ventral  views  of  the  head,  shoulder  girdle  and  gill  arch 
muscles.  This  region  of  urodeles  is  illustrated  in  K,  page  137,  Figure  146,  and 
W,  page  247,  Figure  58. 

D.   THE  MUSCLES  OF  THE  CAT  AND  RABBIT 

In  mammals  those  processes  of  change  in  the  muscular  system  which  we  saw 
beginning  in  the  dogfish  and  progressing  in  Necturus  reach  a  maximum.  Owing 
to  the  increase  in  size  and  strength  of  the  limbs  and  the  adoption  of  the  habit  of 
elevating  the  body  above  the  ground,  the  muscles  associated  with  the  limbs  be- 
come correspondingly  increased  in  size  and  importance.  We  have  already  learned 
that  the  girdle  and  limb  muscles  originate  as  small  buds  from  the  myotomes. 
In  mammals  they  have  so  increased  in  size  and  so  spread  from  their  original 
locations  that  the  original  myotomes  are  scarcely  recognizable  in  the  adult, 
although  present  in  the  embryo  in  the  typical  primitive  arrangement.  Further, 
owing  to  the  transformation  of  the  gill  arches  the  visceral  muscles  have  under- 
gone great  changes.  We  shall  attempt  to  point  out  to  some  extent  the  homology 
between  the  muscles  of  the  mammals  and  those  of  the  lower  forms.  Muscles 
become  altered  from  the  primitive  segmented  condition  by  processes  of  splitting, 
fusion,  and  extension. 

For  more  complete  details  concerning  the  muscles  than  are  given  below  see, 
for  the  cat,  R  and  J,  and  for  the  rabbit,  B.  The  directions  which  follow  are 


134       LABORATORY  MANUAL  FOR  VERTEBRATE  ANATOMY 

applicable  to  either  animal,  differences  between  them  being  specifically  noted 
wherever  necessary. 

i.  The  dermal  or  integumental  muscles. — Skin  the  animal  and  in  doing  so 
note  the  structures  named  in  the  following  paragraph.  (If  the  animal  has 
already  been  skinned,  this  part  of  the  work  will  have  to  be  omitted.)  In  skin- 
ning, make  a  median  dorsal  incision  from  the  base  of  the  tail  to  the  back  of  the 
head.  Be  sure  that  this  and  following  incisions  cut  through  the  skin  only. 
Make  an  incision  through  the  skin  around  the  throat,  around  ankles  and 
wrists,  and  incisions  along  the  outer  surface  of  the  limbs.  Connect  these  with 
the  median  incision.  Loosen  the  skin  along  the  incisions  and  gradually  work 
the  skin  loose  from  the  muscles,  using  fingers  and  back  of  the  scalpel.  Avoid 
as  far  as  possible  the  method  of  cutting  the  skin  from  the  body,  as  one  is  liable 
to  cut  into  the  muscles  by  this  procedure.  Work  from  the  dorsal  side  toward  the 
ventral  side.  Leave  the  skin  on  the  head  and  on  the  perineal  region  for  the  present. 

The  following  points  should  be  noted  during  the  skinning.  The  skin  is 
connected  with  the  underlying  muscles  by  a  loose  weblike  material,  the  subcuta- 
neous connective  tissue  or  superficial  fascia,  often  impregnated  with  fat.  Below 
this  is  the  much  firmer  and  tougher  connective  tissue  on  the  surface  of  the  muscles, 
forming  the  deep  fascia.  Passing  from  among  the  muscles  into  the  skin  will  be 
seen  at  regular  intervals,  which  represent  the  segments  of  the  body,  a  slender 
cord,  composed  of  an  artery,  a  vein,  and  a  sensory  nerve.  These  may  be  severed. 
Other  blood  vessels,  not  segmentally  arranged,  will  also  be  seen  passing  onto  the 
under  surface  of  the  skin,  from  anterior  and  posterior  regions  toward  the  middle. 
The  arteries  are  colored  by  an  injection  mass  and  are  readily  recognized.  The 
veins  are  usually  of  a  very  dark  reddish-brown  color.  All  vessels  to  the  skin 
should  be  severed. 

When  the  skin  has  been  loosened  to  the  sides  of  the  animal,  there  will  be  noted 
a  thin  layer  of  muscle  fibers  on  its  under  surface,  appearing  like  a  fine  striping. 
Toward  the  chest  and  shoulder  region  this  assumes  the  form  of  a  thin  sheet. 
This  muscle  is  a  dermal  or  skin  muscle,  the  panniculus  carnosus  or  cutaneous 
maximus.  It  covers  the  entire  lateral  surface  of  the  thorax  and  abdomen,  being 
more  prominent  anteriorly.  On  continuing  to  skin  forward  and  ventrally 
the  muscle  will  be  found  to  take  its  origin  from  the  outer  surface  of  a  muscle 
(latissimus  dorsi)  situated  posterior  to  the  shoulder,  and  from  the  axilla  in  the 
cat,  the  medial  side  of  the  humerus  (rabbit),  and  from  the  linea  alba  and  various 
points  on  the  ventral  side  of  the  thorax  in  both  animals.  These  points  of  origin 
should  be  cut  through  and  the  cutaneous  maximus  removed  with  the  skin  to 
which  it  generally  adheres.  The  muscle  is  inserted  on  the  skin  and  serves  to 
shake  the  skin.  In  man  this  muscle  is  degenerate.  It  is  an  outgrowth  of  the 
latissimus  dorsi  muscle. 

There  is  one  other  dermal  muscle,  the  platysma.  This  will  be  found  on  the 
under  surface  of  the  skin  of  the  neck  and  head,  and  consists  of  many  different 


COMPARATIVE  ANATOMY  OF  THE  MUSCULAR  SYSTEM  135 

parts,  which  have  received  separate  names.  Some  of  these  will  be  seen  later, 
but  the  study  of  the  parts  of  the  platysma  is  almost  impossible  in  any  but  freshly 
killed  specimens.  The  platysma  muscle  is  inserted  on  the  skin  of  the  ears,  eyelids, 
lips,  etc.,  and  serves  to  move  them.  In  man  it  constitutes  the  muscles  of  facial 
expression.  The  platysma  is  a  visceral  muscle,  derived  from  the  muscles  of  the 
hyoid  arch  by  extension. 

In  females  on  the  under  surface  of  the  skin  of  the  ventral  side  the  mam- 
mary or  milk  glands  will  be  noted  spread  out  as  a  thin  irregular  layer. 

The  skin  having  been  removed  and  discarded,  clean  away  fascia  and  fat 
from  the  surface  of  the  muscles.  There  is  generally  a  large  mass  of  fat  at  the 
base  of  the  hind  legs.  It  will  now  be  seen  that  the  exposed  surface  in  part 
consists  of  muscles,  pinkish  masses  composed  of  parallel  fibers,  and  in  part  of  the 
deep  fascia  which  forms  very  strong  white  sheets.  The  posterior  half  of  the  back 
is  covered  by  such  a  sheet,  known  as  the  lumbodorsal  fascia.  In  the  median 
ventral  line  is  the  linea  alba.  The  angle  between  the  base  of  the  thigh  and 
the  abdominal  wall  is  known  as  the  inguinal  region.  At  the  bottom  of  this  will 
be  found  in  the  rabbit  a  stout  white  shining  cord,  the  inguinal  ligament,  which 
stretches  from  the  pubic  symphysis  to  the  crest  of  the  ilium.  It  is  absent  in  the 
cat.  The  angle  between  the  upper  arm  and  chest  is  called  the  axilla ,  or  axillary 
fossa. 

In  studying  the  muscles  it  is  necessary  to  separate  each  muscle  from  its 
neighbors.  This  is  done  by  searching  carefully  for  the  white  lines  of  connective 
tissue  which  mark  the  boundaries  of  muscles  and  slitting  along  these  lines  with 
the  point  of  the  scalpel.  Observing  the  direction  in  which  the  fibers  run  will 
also  aid  in  separating  muscles,  since  the  fibers  in  one  muscle  generally  run  in 
the  same  direction,  which  is  usually  different  from  that  of  the  neighboring  muscles. 
After  freeing  the  margins  of  a  muscle  the  fingers  should  be  worked  under  the  muscle 
until  it  is  separated  from  its  fellows.  As  each  muscle  is  inclosed  in  a  connective 
tissue  sheath,  it  will  separate  smoothly  from  its  neighbors;  the  presence  of  rough 
edges  indicates  that  the  muscle  itself  has  been  cut.  Avoid  the  use  of  sharp 
instruments  in  freeing  the  muscles.  In  case  it  is  necessary  to  cut  through  a 
muscle  in  order  to  reveal  another  muscle  beneath  it,  always  cut  through  the  center 
of  the  muscle. 

2.  The  parts  of  a  muscle. — We  shall  first  study  the  parts  of  some  convenient 
muscle  in  order  to  become  acquainted  with  the  terminology  applied  to  muscles. 
For  this  purpose  we  shall  select  the  external  oblique.  The  external  oblique  is 
the  large  muscle  covering  the  sides  of  the  abdomen.  The  superficial  fascia  should 
De  cleaned  away  from  its  surface.  On  closely  inspecting  the  cleaned  surface  of 
the  external  oblique,  the  muscle  will  be  observed  to  consist  of  numerous  pinkish 
stripes  directed  obliquely  ventrad  and  caudad. 

The  pinkish  stripes  are  bundles  of  muscle  fibers,  called  fasciculi.  The  white 
material  between  the  fasciculi  which  binds  them  together  into  a  muscle  and 


136       LABORATORY  MANUAL  FOR  VERTEBRATE  ANATOMY 

which  dips  down  between  them  is  part  of  the  deep  fascia.  It  forms  a  sheath,  the 
perimysium,  for  each  fasciculus.  The  part  of  the  muscle  which  is  composed  of 
fasciculi  with  their  perimysia,  is  known  as  the  fleshy  part  of  the  muscle,  or  the 
belly.  The  fasciculi  do  not  extend  completely  to  the  ends  of  a  muscle,  but  the 
deep  fascia  does.  Consequently  the  ends  of  muscles  are  non-fleshy,  composed 
of  connective  tissue  only.  These  connective  tissue  ends  of  muscles  commonly 
form  tough  shining  bands  or  cords,  known  as  tendons.  When  the  tendon  is  very 
broad  and  flat,  it  is  often  called  an  aponeurosis,  or  the  name/asaa  may  be  retained 
for  such  broad  tendons.  The  attachments  of  muscles  are  always  by  means  of 
tendons,  aponeuroses  or  fasciae,  never  by  the  muscle  fibers.  Muscles  are 
attached  to  bones  or  to  tendons,  aponeuroses,  or  fasciae,  which  themselves 
are  attached  to  bones.  The  purpose  of  the  voluntary  muscles  is  to  move  the 
bones  of  the  skeleton. 

The  anterior  part  of  the  external  oblique  is  concealed  under  a  large  flat 
muscle,  the  latissimus  dor  si,  which  covers  the  anterior  part  of  the  back  and  slopes 
toward  the  upper  arm.  The  posterior  boundary  of  this  muscle  should  be  located 
and  slit,  and  it  should  then  be  lifted  from  the  surface  of  the  external  oblique  by 
thrusting  the  fingers  between.  If  fat  is  present  between  these  two  muscles,  it 
must  be  cleaned  away.  The  most  posterior  chest  muscles  also  cover  the  anterior 
part  of  the  external  oblique  and  should  be  lifted  off  in  a  similar  manner.  The 
external  oblique  will  then  be  found  to  be  attached  to  the  posterior  ribs  by  separate 
slips.  The  fixed  points  of  attachment  of  a  muscle  are  called  its  origin.  When 
there  is  more  than  one  origin,  each  one  is  known  as  a  head.  When  there  are 
a  number  of  points  of  attachments  segmentally  arranged,  they  are  generally 
designated  as  slips.  The  origin  of  the  external  oblique  is  from  the  posterior 
ribs  by  separate  slips  and  from  the  lumbodorsal  fascia.  Its  fibers  (fasciculi) 
pass  obliquely  downward  and  backward,  and  in  the  rabbit  the  more  dorsal 
ones  pass  nearly  straight  caudad.  The  movable  points  of  attachment  of  a 
muscle  on  which  it  exerts  its  effect  are  called  its  insertion.  The  insertion  of  the 
external  oblique  is  by  way  of  an  extensive  aponeurosis  which  passes  to  the 
median  ventral  line.  The  insertion  is:  rabbit,  on  the  linea  alba  by  its  aponeu- 
rosis and  on  the  inguinal  ligament,  which  is  in  turn  attached  to  the  ilium  and  the 
pubic  symphysis;  cat,  on  the  linea  alba  and  the  pubis  by  its  aponeuroses,  and 
on  the  median  ventral  line  (raphe)  of  the  thorax.  The  function  of  a  muscle 
is  called  its  action.  The  action  of  the  external  oblique  is  constrictor  of  the 
abdomen. 

In  the  following  dissection  of  the  muscles  the  dissection  is  to  be  confined 
strictly  to  the  left  side,  leaving  the  right  side  intact  for  the  dissection  of  other 
systems. 

3.  The  muscles  of  the  abdominal  wall. — The  abdominal  wall  is  composed 
of  three  layers  of  muscles  with  their  aponeuroses.  The  aponeuroses  of  these 
muscles  are  cmite  extensive.  The  three  layers  are:  an  external  layer,  the 


COMPARATIVE  ANATOMY  OF  THE  MUSCULAR  SYSTEM  137 

external  oblique;  a  middle  internal  oblique;  and  an  internal  layer,  the  transversus 
abdominus. 

a)  External  oblique:  This  is  the  outermost  of  the  muscle  layers  of  the  ab- 
dominal wall.     It  was  described  above. 

b)  Internal   oblique:   Very  carefully  cut  through  the  middle  of  the  belly  of 
the  external  oblique,  in  a  longitudinal  direction,  and  separate  it  from  the  under- 
lying muscle,  which  is  the  internal  oblique.     This  separation  is  often  difficult 
The  internal  oblique  is  a  short  muscle  lying  beneath  the  more  dorsal  portion  of 
the  external  oblique.     Its  fibers  are  directed  obliquely  downward  and  forward, 
and  are  continued  by  a  very  broad  aponeurosis.    Origin:  rabbit — second  sheet  of 
the  lumbodorsal  fascia,  posterior  ribs  and  inguinal  ligament;  cat — second  sheet  of 
the  lumbodorsal  fascia  and  border  of  the  pelvic  girdle.     Insertion,  on  the  linea 
alba  by  the  extensive  aponeurosis;  action,  compressor  of  the  abdomen. 

c)  Transverse:   On  cutting  through  the  preceding  and  separating  the  edges, 
a  third  muscle  layer,  very  thin,  will  be  found.     This  is  the  transversus  abdominis. 
Its  fibers  are  directed  ventrally  and  slightly  posteriorly.     Origin,  insertion,  and 
function  similar  to  the  preceding. 

d)  Rectus  abdominis:  This  is  a  long  slender  muscle  on  each  side  of  the  linea 
alba,  extending  from  the  pubic  symphysis  to  the  anterior  part  of  the  thorax. 
It  is  found  inside  of  and  between  the  aponeuroses  of  the  preceding  muscles. 
Slit  open  these  aponeuroses  along  each  side  of  the  linea  alba  and  expose  the  rectus 
abdominis.     Its  fibers  run  longitudinally  and  in  the  cat  are  crossed  at  regular 
intervals  by  transverse  white  lines.     Origin,  anterior  end  of  the  pubic  symphysis; 
insertion,   sternum   and   costal  cartilages;    action,   retracts  ribs  and  sternum 
and  constricts  the  abdomen.     Underneath  the  transverse  and  the  rectus  ab- 
dominis is  the  peritoneal  membrane,  or  lining  of  the  coelom. 

The  foregoing  muscles  are  hypaxial  muscles,  formed  into  sheets  by  the  side- 
wise  fusion  of  myotomes.  The  rectus  abdominis  represents  the  ventral  ends  of 
the  myotomes.  The  three  layers  on  the  sides  of  the  abdomen  have  resulted 
from  a  splitting  process. 

4.  The  epaxial  muscles. — Remove  the  lumbodorsal  fascia  over  the  posterior 
part  of  the  back,  finding  beneath  it  a  great  thick  mass  of  muscle  inclosed  in  a 
tough  shining  fascia.  Note  attachment  of  the  lumbodorsal  fascia  in  the  median 
line  to  the  neural  spines  of  the  vertebrae.  The  mass  of  muscle  is  divisible 
into  a  slender  narrow  median  portion,  the  multifidus,  next  to  the  median  dorsal 
line,  and  a  very  thick  lateral  portion,  the  sacrospinalis.  The  latter  is  in  the  cat 
readily  divisible  into  three  longitudinal  portions.  These  muscles  continue  up 
into  the  thoracic  region,  where  they  will  be  seen  later,  to  the  back  of  the  head, 
their  various  portions  receiving  different  names.  They  are  the  epaxial  muscles 
and  are  sharply  separated  from  the  hypaxial  muscles  in  the  abdominal  region 
by  a  furrow  which  corresponds  to  the  position  of  the  horizontal  skeletogenous 
septum  of  fishes.  These  muscles  are  the  most  powerful  muscles  in  the  body. 


138 


LABORATORY  MANUAL  FOR  VERTEBRATE  ANATOMY 


They  are  attached  to  the  sacrum,  the  ilium,  the  vertebrae  and  ribs  at  various 
points,  and  to  the  back  of  the  skull,  and  serve  to  move  the  vertebral  column 
as  a  whole  or  in  part,  raise  the  head,  etc.  All  parietal  muscles  not  included  in 
this  mass  are  hypaxial  muscles,  which  have  in  some  cases  spread  dorsally  so  as 
to  cover  the  epaxial  muscles. 


genioglossus 
styloglossus 
geniohyoid 


mandible 
molar  gland 


digastric 
mylohyoid 
parotid  gland 
submaxillary  gland 
lymph  glands 


sternomastoid, 


cut  edge  of 
clavotrapezius 
scaJenes 
biceps  brachii 
subscapularis 
teres  major 


clavobrachialis 


thyrohyoid 
cut  edge  of 
sternomastoid 
sternohyoid 
sternothyroid 
cleidomastoid 


pectoantibrachialis 

pectpralis  major 

cut  edge  of 


long  head  of 
the  triceps 


transversus  costarum 
serratus  ventralis 


rectus  abdominis 
latissimus  dorsi 

external  oblique 


FIG.  41. — Ventral  view  of  the  anterior  part  of  the  cat  to  show  the  muscles.  All  dermal  muscles 
have  been  removed.  Superficial  muscles  on  the  right  side,  deeper  layer  of  muscles  on  the  left  side, 
after  removal  of  the  pectoral  muscles,  sternomastoid,  mylohyoid,  and  digastric.  The  nerves  and 
blood  vessels  which  cross  the  axilla  have  been  omitted.  The  view  of  the  axilla  is  different  from  that 
revealed  in  the  dissection  given  in  the  text.  The  epitrochlearis  is  called  extensor  antibrachii  in  the  text. 


5.  The  muscles  of  the  chest. — (Fig.  41.)  Turn  the  animal  on  its  back  and 
expose  the  chest  by  spreading  and  fastening  the  fore  limbs.  The  great  muscles 
covering  the  chest  or  ventral  side  of  the  thorax  are  the  pectoral  muscles.  They 
are  divisible  into  several  portions  which  are  not  very  definitely  separable  from 


COMPARATIVE  ANATOMY  OF  THE  MUSCULAR  SYSTEM  139 

each  other.     According  to  B  there  are  five  parts  in  the  rabbit,  and  according  to 
R  and  J,  four  parts  in  the  cat.     Of  these  we  may  consider  the  following. 
Rabbit: 

a)  Pectoralis  major.     This  is  a  large  muscle  originating  from  the  whole  length 
of  the  sternum  and  inserted  on  the  humerus.     It  covers  most  of  the  surface  of 
the  chest,  but  the  insertion  is  concealed  by  a  muscle  coming  down  from  the  head 
(clavodeltoid).     Action,  draws  the  arm  toward  the  chest. 

b)  Pectoralis  primus.    A  slender  muscle  at  the  anterior  end  of  the  pre- 
ceding and  covering  its  anterior  fibers.     Origin,  manubrium  of  the  sternum; 
insertion,  humerus;  action,  like  the  preceding.     Its  anterior  border  is  in  contact 
with  the  clavodeltoid,  which  also  partly  covers  its  insertion. 

c)  Pectoralis  minor.     Cut  through  the  middle  of  the  belly  of  the  pectoralis 
major,  and  upon  deflecting  the  cut  edges  note  internal  to  it  a  similar  muscle, 
the  pectoralis  minor.     Origin,  manubrium;  insertion,  clavicle,  and  spine  of  the 
scapula.    To  find  the  insertion  loosen  up  the  clavodeltoid  and  locate  in  its 
fibers  at  the  shoulder  a  small  slender  bone,  the  clavicle.    The  clavicle  is  on  the 
inner  surface  of  the  muscle.     Then  loosen  the  muscle  next  lateral  to  the  clavodel- 
toid, a  long  muscle  coming  down  from  the  back  of  the  neck  (anterior  trapezius). 
The  pectoralis  minor  is  inserted  by  some  fibers  on  the  clavicle,  but  most  of  its 
fibers  sweep  over  the  shoulder  internal  to  the  clavodeltoid  and  anterior  trapezius 
and  are  inserted  on  the  spine  of  the  scapula.     Action,  draws  arm  and  shoulder 
toward  the  chest. 

Cat: 

a)  Pectoantibrachialis.     Anterior  and  most  superficial  of  the  chest  muscles. 
Origin,  manubrium;  insertion,  by  a  flat  tendon  on  the  fascia  of  the  forearm; 
action,  draws  the  arm  toward  the  chest. 

b)  Pectoralis  major.     Next  posterior  to  the  preceding  and  extending  ante- 
riorly dorsal  to  the  preceding  which  should  be  cut  across;  originating  on  the 
sternum  and  median  ventral  raphe  and  inserted  on  the  humerus.     Action,  like 
the  preceding. 

c)  Pectoralis  minor.    Next  posterior  to  the  preceding  and  covered  in  large 
part  by  the  pectoralis  major.     The  latter  should  be  cut  through  and  the  extent 
of  the  pectoralis  minor  noted.     The  pectoralis  minor  is  divisible  into  several 
parts.     Origin,  sternum;   insertion,  humerus;   action,  like  the  preceding.     The 
insertion  cannot  be  fully  traced  at  the  present  stage  of  the  dissection. 

d)  Xiphihumeralis.     The  last  of  the  chest  muscles,  a  thin  flat  long  muscle, 
passing  from  the  xiphoid  process  of  the  sternum,  its  anterior  part  passing 
dorsal  to  the  posterior  part  of  the  pectoralis  minor,  and  inserted  on  the  humerus. 
The  insertion  is  covered  by  a  mass  of  fat  in  the  axilla.     Remains  of  the  cutaneous 
maximus  are  probably  present  on  its  surface.    Action,  like  the  preceding. 

6.  The  muscles  of  the  neck  and  throat.— (Figs.  41  or  42.)     Slit  the  skin  up 
the  center  of  the  throat  to  the  tip  of  the  lower  jaw  and  loosen  it  so  as  to  expose 


140       LABORATORY  MANUAL  FOR  VERTEBRATE  ANATOMY 

fully  the  lower  jaw.  Note  in  doing  so  parts  of  the  platysma  muscle  on  the  under 
side  of  the  skin.  It  sweeps  from  the  median  dorsal  line  of  the  neck  around  the 
sides  of  the  head  to  face  and  ears,  and  portions  of  it  generally  are  attached  near 
the  anterior  end  of  the  sternum.  In  dissecting  the  throat  muscles  work  on 
one  side  only,  leaving  the  other  intact  for  the  study  of  other  parts.  Avoid 
cutting  any  blood  vessels.  A  large  vein,  the  external  jugular  vein,  runs  in 
the  superficial  muscles  of  the  throat.  At  the  angle  of  the  jaw  is  a  rounded 
pinkish  body,  the  submaxillary  gland,  one  of  the  salivary  glands.  Other  small 
bodies  are  lymph  glands. 
Rabbit: 

a)  Special  portion  of  the  platysma.     A  broad  thin  sheet  of  dermal  muscle 
extends  from  the  manubrium  of  the  sternum  forward,  forking  like  the  letter  V, 
each  half  inserting  at  the  base  of  the  ear.     This  is  the  depressor  conchae  posterior 
and  is  the  most  superficial  muscle  on  the  ventral  surface  of  the  neck.     The 
external  jugular  vein  runs  in  it.     It  is  a  part  of  the  platysma.     It  should  be 
well  separated  from  the  underlying  muscles  and  the  posterior  end  severed  and 
turned  forward,  without,  however,  injuring  the  vein. 

b)  Sternohyoid.    This  is  the  long  muscle  in  the  median  line  of  the  neck, 
the  two  members  of  the  pair  being  closely  fused  in  the  median  ventral  line. 
Origin,  manubrium  of  the  sternum;  insertion,  anterior  horn  of  the  hyoid.    Follow 
the  muscle  up  to  the  throat  and  feel  with  the  fingers  the  bony  hyoid  on  which 
the  muscle  is  inserted.     Action,  draws  the  hyoid  posteriorly  or  raises  the  sternum. 

c)  Sternomastoid.    The  long  muscle  on  each  side  of  the  preceding,  the  two 
members  of  the  pair  converging  toward  the  manubrium  of  the  sternum  from 
which  they  originate  ventral  to  the  origin  of  the  preceding.     Insertion,  mastoid 
process  of  the  skull;  action,  singly  turn  the  head,  together  depress  the  head  on 
the  neck. 

d)  Cleidomastoid  and  basiodavicularis .  These  two  long  strap-shaped  muscles 
are  next  lateral  to  the  preceding  and  unite  at  the  clavicle  with  the  clavodeltoid. 
The  cleidomastoid  is  the  more  medial  one  and  lies  lateral  and  somewhat  dorsal 
to  the  Sternomastoid.  Origin,  mastoid  region  of  the  skull;  insertion,  clavicle; 
action,  elevates  clavicle  or  turns  the  head.  The  basiodavicularis  is  slightly 
lateral  to  the  preceding  at  its  cranial  end  but  crosses  ventral  to  it  caudally  so 
that  its  insertion  on  the  clavicle  is  medial  to  that  of  the  cleidomastoid.  Origin, 
occipital  bone;  insertion,  clavicle;  function,  like  preceding.  The  origins  of  these 
muscles  cannot  be  followed  out  conveniently. 

e)  Clavodeltoid.  Continuation  of  the  two  preceding  muscles.  Origin, 
clavicle;  insertion,  humerus;  action,  raises  the  humerus. 

/)  Masseter.  The  great  mass  of  muscle  covering  the  angle  of  the  jaws,  its 
outer  surface  with  a  very  tough  shining  fascia.  Origin,  zygomatic  arch;  inser- 
tion, outer  surface  of  the  posterior  end  of  the  mandible;  action,  closes  the  lower 
iaw  (elevator  of  the  law.) 


COMPARATIVE  ANATOMY  OF  THE  MUSCULAR  SYSTEM  141 

g)  Digastric.  The  muscle  along  the  ventral  surface  of  each  half  of  the 
jaw  bone,  terminating  in  a  slender  tendon.  Origin,  occipital  bone;  insertion, 
ventral  surface  of  the  mandible;  action,  opens  the  jaw  (depressor  of  the 
jaw). 

h)  Mylohyoid.  The  thin  sheet  of  muscle  crossing  transversely  between 
and  dorsal  to  the  two  digastrics.  Origin,  mandible;  insertion,  median  ventral 
line  (raphe)  and  the  hyoid;  action,  raises  the  floor  of  the  mouth  and  brings 
the  hyoid  forward. 

i)  Sternothyroid.  Divide  the  two  sternohyoids  in  the  median  ventral  line. 
This  exposes  the  trachea  or  windpipe,  a  tube  stiffened  by  rings  of  cartilage.  At 
the  top  of  the  trachea  at  a  level  about  between  the  two  submaxillary  glands 
there  is  an  enlarged  chamber,  the  larynx  or  Adam's  apple,  whose  walls  are  sup- 
ported by  cartilage.  The  chief  cartilage  of  the  larnyx  is  the  large  shield-shaped 
cartilage,  which  forms  the  ventral  wall;  this  is  called  the  thyroid  cartilage.  The 
Sternothyroid  muscle  will  be  found,  one  on  each  side  of  the  trachea,  dorsal  to 
the  sternohyoid,  originating  on  the  sternum  and  inserted  on  the  thyroid  cartilage. 
Action,  pulls  the  larynx  posteriorly. 

j)  Thyrohyoid.    A  thin  muscle  at  each  side  of  the  larynx,  extending  from 
'the  thyroid  cartilage  to  the  hyoid.    Action,  raises  the  larynx. 
Cat: 

a)  Sternomastoid.    This   is    the  superficial  muscle  of  the  ventral  side  of 
the  neck.     A  large  vein,  the  external  jugular  vein,  crosses  its  surface  at  an  angle 
to  the  direction  of  its  fibers.     Origin  by  two  parts,  from  the  median  raphe  and 
themanubrium  of  the  sternum,  the  first-named  origin  lying  ventral  to  the  second, 
so  that  the  muscle  appears  divisible  into  two  muscles.     From  the  origins  the 
muscle  passes  obliquely  away  from  the  median  ventral  line  around  the  sides  of 
the  neck  and  is  inserted  on  the  skull  from  the  lambdoidal  ridge  onto  the  mastoid 
process.     The  muscle  passes  internal  to  the  submaxillary  gland  and  the  parotid 
gland;  the  latter  is  a  mass  at  the  base  of  the  ear.     The  insertion  on  the  mastoid 
process  is  by  means  of  a  thick  tendon.     Action,  singly  turn  the  head,  together 
depress  head  on  neck. 

b)  Sternohyoid.    The  anterior  ends  of  these  muscles  are  visible  between 
the  two  sternomastoids,  as  the  latter  diverge  from  the  median  raphe.     Slit  the 
raphe  of  the  sternomastoids  to  the  manubrium  of  the  sternum,  thus  exposing 
the  full  length  of  the  sternohyoids.     They  extend  in  the  median  ventral  line 
from  the  first  costal  cartilage  to  the  body  of  the  hyoid  bone,  the  two  being  closely 
united  in  the  median  line.     Action,  draw  the  hyoid  posteriorly. 

c)  Cleidomastoid.     Lateral  to  the  sternomastoid  is  a  long  muscle  passing 
from  the  head  to  the  upper  arm.     Loosen  this  up  and  find  internal  to  it  a  narrow 
flat  muscle,  the  cleidomastoid.     Origin,  clavicle,  which  will  be  found  as  a 
slender  bone  on  the  internal  surface  of  the  long  muscle  just  mentioned  at  the 
level  of  the  shoulder;  insertion,  mastoid  process,  dorsal  to  the  insertion  of  the 


142       LABORATORY  MANUAL  FOR  VERTEBRATE  ANATOMY 

sternomastoid.      Action,  pulls  clavicle  craniad  or  turns  head,  acting  singly,  or 
lowers  head  on  neck. 

d)  Clavolrapezius  and  davobrachial.     The  long  muscle  on  the  side  of  the  neck 
and  passing  over  the  ventral  surface  of  the  shoulder  to  the  forearm  is  sometimes 
considered  as  one  muscle,  the  cephalobrachial,  or  as  two.     In  the  latter  case  the 
upper  part  is  known  as  the  clawtrapezius  and  extends  from  the  skull  to  the  clavicle. 
It  will  be  considered  later.     The  lower  part  from  the  clavicle  to  the  fore- 
arm is  the  clawbrachialis .    Origin,  clavicle  and  fibers  of  the  clavotrapezius; 
insertion,  ulna;   action,  flexor  of  the  forearm.     The  clavicle  will  be  found  on 
the  inner  surface  of  the  muscle  in  the  shoulder    region,   imbedded  in  the 
muscle. 

e)  Masseter.    The  great  thick  muscle  covering  the  angle  of  the  jaws,  situated 
in  front  of  the  submaxillary  and  parotid  glands.     It  is  covered  by  a  very  tough 
shining  fascia.     Origin,  zygomatic  arch;   insertion,  posterior  half  of  the  lateral 
surface  of  the  mandible;  action,  elevator  of  the  lower  jaw. 

f)  Temporal.  Remove  the  skin  from  the  side  of  the  head  up  to  the  median 
dorsal  line.  A  great  mass  of  muscle  covered  by  a  strong  shining  fascia  will  be 
seen  occupying  the  temporal  fossa  of  the  skull,  dorsal  to  the  ear.  Origin,  from  the 
side  of  the  skull  from  the  superior  nuchal  line  to  the  zygomatic  process  of  the 
frontal  bone,  and  from  part  of  the  zygomatic  arch;  insertion,  coronoid  process 
of  the  mandible;  action,  elevator  of  the  jaw,  in  common  with  the  masseter. 

g)  Digastric.  The  muscle  lying  along  the  medial  surface  of  each  half  of  the 
mandible.  It  extends  posteriorly  internal  to  the  submaxillary  gland.  Origin, 
jugular  and  mastoid  processes  of  the  skull;  insertion,  mandible;  action,  depres- 
sor of  the  lower  jaw. 

h)  Mylohyoid.  The  thin  transverse  sheet  passing  across  between  the  two 
digastrics  from  one  half  of  the  mandible  to  the  other.  Origin,  mandible,  the 
origin  concealed  by  the  digastrics;  insertion,  median  raphe;  action,  raises  floor 
of  the  mouth  and  brings  hyoid  forward. 

i)  Genio hyoid.  Cut  through  the  median  raphe  of  the  mylohyoid.  This 
exposes  a  pair  of  long  slender  muscles,  the  geniohyoids,  lying  in  the  median  line. 
Origin,  mandible  near  the  symphysis;  insertion,  body  of  the  hyoid;  action, 
draws  the  hyoid  forward. 

j)  Sternothyroid.  Separate  the  two  sternohyoids  in  the  median  line.  This 
exposes  the  trachea  or  windpipe,  a  tube  stiffened  by  rings  of  cartilage.  At  the 
top  of  the  trachea  is  a  chamber  with  cartilaginous  walls,  the  larynx.  The  chief 
cartilage  of  the  larynx  is  the  large  shield-shaped  thyroid  cartilage,  forming  the 
ventral  walls  of  the  larynx.  Just  in  front  of  the  thyroid  cartilage  the  body  of  the 
hyoid  is  felt  as  a  bony  bar.  The  Sternothyroid  muscles  are  located,  one  on  each 
side  of  the  trachea,  dorsal  to  the  sternohyoids.  Origin,  sternum  in  common 
with  the  sternohyoid;  insertion,  thyroid  cartilage  of  the  larynx;  action  pulls 
the  larynx  posteriorly. 


COMPARATIVE  ANATOMY  OF  THE  MUSCULAR  SYSTEM  143 

k)  Thyrohyoid.  Short  narrow  muscle  on  each  side  of  the  thyroid  cartilage 
from  which  it  takes  its  origin;  insertion,  posterior  horn  of  the  hyoid;  action, 
raises  the  larynx. 

The  majority  of  the  muscles  mentioned  in  this  section  are  the  visceral 
muscles.  They  correspond  to  the  muscles  found  in  the  dogfish  associated  with  the 


branch  of  facial  nerve 
parotid  duct 
masseter 


digastric 

mylohyoid 
submaxillary  gland 
sternohyoid 


lateral  head  of  the  triceps 
long  head  of  the  triceps 


sternomastoid 
cleidomastoid 
basioclavicularis 

cephalohumeral 
first  deltoid 


parotid  gland 

rhomboideus  capitis 

splenius 

external  jugular  vein 
levator  scapulae  ventralis 


pectoralis  minor 
anterior  trapezius 


second  deltoid 


posterior  trapezius 


latiss!-uus  dora 


external  oblique 


FIG.  42. — Lateral  view  of  the  anterior  part  of  the  rabbit  to  show  the  muscles.     The  head  is  turned 
slightly  to  give  a  ventral  view  of  the  throat.    All  dermal  muscles  have  been  removed. 

gill  arches.  They  are  the  muscles  of  the  jaws,  the  hyoid,  and  the  cartilages  of  the 
larynx,  all  of  which  structures  are,  as  learned  in  the  study  of  the  skull,  homolo- 
gous with  the  gill  arches  of  fishes.  The  masseter,  the  temporal,  the  mylohyoid, 
and  part  of  the  digastric  are  muscles  of  the  mandibular  arch;  the  platysma  and 
the  rest  of  the  digastric  are  muscles  of  the  hyoid  arch;  while  the  muscles  of  the 
remaining  arches  become  muscles  of  the  pharynx  and  larynx,  which  were  not 
studied. 


144       LABORATORY  MANUAL  FOR  VERTEBRATE  ANATOMY 

7.  The  muscles  of  the  upper  back  and  shoulder  and  back  of  the  neck.— 
(Fig.  42.) 
Rabbit: 

a)  Latissimus  dorsi.    Turn  the  animal  on  one  side,  so  that  the  side  on  which 
the  muscles  have  already  been  dissected  will  be  uppermost.     The  large  flat 
muscle  extending  obliquely  from  the  middle  of  the  back  to  the  fore  limb  is  the 
latissimus  dorsi.     Origin,  lumbodorsal  fascia  and  posterior  ribs;  insertion,  on 
the  crest  on  the  medial  side  of  the  humerus,  the  insertion  covered  by  the  chest 
muscles;  action,  draws  the  arm  caudad  and  dorsad. 

b)  Anterior  and  posterior  trapezius.     These  two  muscles  are  the  flat,  thin 
muscles  covering  the  upper  back  and  back  of  the  neck  anterior  to  the  latissimus 
dorsi.     The  posterior  trapezius  originates  from  the  lumbodorsal  fascia  and  the 
neural  spines  of  the  thoracic  vertebrae,  and  is  inserted  on  the  spine  of  the  scapula. 
Action,  draws  the  scapula  dorsally.     The  anterior  trapezius  originates  on  the 
external  occipital  protuberance  of  the  skull  and  ligament  in  the  mid-dorsal  line 
and  is  inserted  on  the  metacromion  process  (which  is  very  long  in  the  rabbit)  and 
nearby  muscles  and  fascia;  action,  draws  the  scapula  and  limb  upward  and  for- 
ward.    The  space  between  the  two  trapezius  muscles  is  filled   by   a   stout 
fascia. 

c)  Levator  scapulae  ventralis.    This  long  slender  muscle  runs  along  the  ventral 
border  of  the  anterior  trapezius  near  its  insertion,  then  diverges  to  its  origin 
from  the  ventral  surface  of  the  skull  at  the  suture  between  occipital  and  basisphe- 
noid;  insertion,  metacromion  process  in  common  with  the  anterior  trapezius; 
action,  pulls  the  scapula  anteriorly. 

d)  Rhomboideus.     Cut  across  the  middle  of  the  bellies  of  the  two  trapezius 
muscles  and  the  latissimus  dorsi.     The  large  thick  muscle  extending  from  the 
vertebral  border  of  the  scapula  to  the  mid-dorsal   line  is  the   rhomboideus. 
Origin,  mid-dorsal  ligament  of  the  neck  and  succeeding  neural  spines;  insertion, 
vertebral  border  of  the  scapula;  action,  draws  scapula  toward  vertebral  column. 

e)  Splenius.    A  fairly  broad  but  thin  muscle  on  the  back  of  the  anterior  part 
of  the  neck  under  the  anterior  trapezius.     (Running  along  its  external  surface 
is  a  narrow  straplike  muscle,  the  rhomboideus  capitis,  see  below.)     Origin  of 
splenius,  mid-dorsal  line  of  neck  and  adjacent  fascia;  insertion,  occipital  region 
of  the  skull  and  atlas;  action,  singly  turns  the  head,  together  raise  the  head. 

Under  the  splenius  are  the  epaxial  muscles,  continuations  of  those  already 
noted  in  the  lumbar  region. 

/)  Supraspinatus.  The  superficial  muscular  layer  of  that  part  of  the  scapula 
anterior  to  the  spine  consists  of  the  pectoralis  minor,  which  sweeps  over  the 
scapula  to  be  inserted  on  the  spine  and  vertebral  border.  Lift  up  its  anterior 
border  and  separate  it  from  the  muscle  beneath  it.  This  muscle  is  the  supra- 
spinatus,  filling  the  supraspinous  fossa  of  the  scapula.  Origin,  supraspinous 
fossa;  insertion,  greater  tuberosity  of  the  humerus;  action,  extends  the  humerus. 


COMPARATIVE  ANATOMY  OF  THE  MUSCULAR  SYSTEM  145 

g)  Deltoids.  There  are  three  deltoids  in  the  rabbit,  of  which  one,  the 
clavodeltoid,  has  already  been  considered.  The  second  deltoid  is  a  small  triangu- 
lar muscle  lateral  to  the  clavodeltoid.  Origin,  acromion  process;  insertion, 
humerus;  action,  raises  the  humerus.  The  third  deltoid  is  lateral  to  the  second 
and  is  a  longer  muscle.  It  passes  under  the  long  metacromion  process  and 
takes  its  origin  from  the  fascia  of  the  muscle  which  fills  the  infraspinous  fossa. 
Insertion  and  action  like  the  preceding. 

h)  Infraspinatus.  The  muscle  partly  covered  by  the  third  deltoid  which  is 
attached  to  its  surface.  The  deltoid  may  be  removed  to  see  it.  Origin,  infra- 
spinous fossa  and  spine;  insertion  and  action  like  the  supraspinatus. 

i)  Teres  major.  The  stout  muscle  along  the  axillary  border  of  the  scapula 
behind  the  preceding.  Origin,  dorsal  half  of  the  axillary  border  of  the  scapula; 
insertion,  on  the  humerus  in  common  with  the  latissimus  dorsi;  action,  draws 
humerus  against  body  and  rotates  it. 

j)  Teres  minor.  Separate  the  teres  major  well  from  the  infraspinatus  and 
look  in  between  them.  On  the  inner  surface  of  the  latter  will  be  found  a  small 
but  stout  muscle.  Origin,  ventral  half  of  the  axillary  border  of  the  scapula; 
insertion,  greater  tuberosity  of  the  humerus;  action,  like  the  preceding. 

k)  Rhomboideus  capitis  (levator  scapulae  minor  of  B).  Cut  through  the 
rhomboideus.  A  slender  bandlike  muscle  lies  in  contact  with  the  inner  surface 
of  the  rhomboideus  and  passes  along  the  external  surface  of  the  splenius  to  be 
connected  with  the  skull.  Origin,  side  of  the  skull  above  the  tympanic  bulla; 
insertion,  posterior  end  of  the  vertebral  border  on  the  medial  side;  action,  draws 
scapula  craniad  and  rotates  it. 

/)  Subscapularis.  Lift  the  scapula.  A  large  muscle,  the  subscapularis,  com- 
pletely covers  the  medial  or  inner  surface  of  the  scapula,  its  fibers  disposed 
in  several  directions.  The  muscle  has  a  shining  fascia ;  its  posterior  end  is  more 
or  less  continuous  with  teres  major.  Origin,  medial  surface  of  the  scapula; 
insertion,  lesser  tuberosity  of  the  humerus;  action,  pulls  the  humerus  toward 
the  median  ventral  line. 

m)  Serratus  ventralis  (anterior).  On  raising  with  the  finger  the  vertebral 
border  of  the  scapula  a  large  fan-shaped  muscle  will  be  seen  extending  anteriorly 
and  posteriorly  from  the  scapula  to  the  walls  of  the  thorax.  This  is  the  serratus 
ventralis ;  it  is  readily  divisible  into  anterior  and  posterior  portions.  The  anterior 
or  cervical  portion  originates  on  the  transverse  processes  of  the  cervical  vertebrae 
by  separate  slips  and  on  the  first  two  ribs.  The  posterior  or  thoracic  portion  takes 
its  origin  by  seven  slips  from  the  ribs.  Insertion,  vertebral  border  of  the  scapula 
above  the  subscapularis;  action,  draws  scapula  forward,  backward,  or  against 
the  body. 

n)  Scalenes.  The  scalenes  are  several  long  flat  muscles  extending  from 
the  transverse  processes  of  the  cervical  vertebrae  to  the  ribs.  They  will  be 
seen  by  lifting  up  the  scapula  and  looking  on  the  ventral  side  of  the  origin  of  the 


146       LABORATORY  MANUAL  FOR  VERTEBRATE  ANATOMY 

serratus  ventralis.  They  lie  internal  to  the  sternomastoid,  cleidomastoid,  etc., 
previously  identified  and  farther  posteriorly  they  form  the  layer  next  internal 
to  the  pectoralis  muscles.  Action,  raise  the  ribs  and  bend  the  neck. 

0)  Serratus  dor  sails  (posterior).  The  dorsal  half  of  the  thorax  underneath 
the  latissimus  dorsi,  trapezius,  and  rhomboideus  muscles  is  covered  by  a  strong 
aponeurosis  (part  of  the  lumbodorsal  fascia),  in  the  ventral  part  of  which  muscle 
fibers  are  present  which  are  inserted  on  the  ribs  by  slips.  The  foremost  slips 
are  quite  fleshy  and  take  their  origin  by  a  tendon  from  the  median  dorsal 
line  of  the  neck.  This  muscle  is  the  serratus  dorsalis.  Action,  raises  the  ribs 
craniad. 

p)  Intercostals.  On  the  sides  of  the  chest  a  series  of  muscles  will  be  seen 
running  from  one  rib  to  the  next  one.  They  are  the  external  intercostals.  They 
extend  on  the  chest  wall  ventral  to  the  insertion  of  the  serratus  dorsalis,  which 
muscle  in  fact  covers  their  most  dorsal  portions.  Origin,  posterior  margins  of 
the  vertebral  ribs;  insertion,  anterior  margins  of  the  succeeding  vertebral  ribs; 
action,  pull  the  ribs  forward.  Observe  that  the  fibers  of  the  external  intercostals 
are  directed  obliquely  backward.  On  carefully  cutting  through  any  of  the 
external  intercostals,  a  layer  of  internal  intercostals  will  be  found  inside  of  them, 
their  fibers  being  directed  obliquely  forward.  The  internal  intercostals  are 
best  seen  in  the  ventral  thoracic  wall,  internal  to  the  scalenes,  which  may  be  cut 
through.  Here  between  the  costal  cartilages  the  internal  intercostals  are  not 
covered  by  the  external  intercostals.  Origin  and  insertion,  margins  of  the 
vertebral  and  sternal  ribs;  action,  lower  the  ribs.  The  intercostals  are  the  chief 
muscles  concerned  in  the  respiratory  movements  of  the  thorax.  The  scalenes, 
serratus,  and  other  muscles  assist. 

q)  Epaxial  muscles  of  the  thorax.  The  mass  of  epaxial  muscles  is  conspicuous 
running  along  the  dorsal  part  of  the  thorax.  This  mass  passes  internal  to  the 
serratus  dorsalis  which  should  be  cut  through;  it  lies  upon  the  dorsal  portions  of  the 
ribs  and  thus  conceals  the  dorsal  portions  of  the  intercostal  muscles.  It  is  covered 
by  the  tough  shining  lumbodorsal  fascia,  which  should  be  removed.  The  epaxial 
mass  is  easily  divisible  into  a  narrow  median  portion  next  to  the  median  dorsal 
line,  the  semispinalis  dorsi,  and  a  very  thick  lateral  portion,  the  longissimus. 
The  latter  gives  off  on  its  ventral  margin  the  narrow  iliocostalis  lying  on  the  ribs 
to  which  it  is  inserted.  The  longissimus  is  the  continuation  of  the  sacrospinalis 
and  is  inserted  on  the  ribs.  The  epaxial  mass  may  be  followed  along  the  neck 
by  cutting  the  splenius. 

The  attention  of  the  student  is  directed  to  the  fact  that  the  intercostal  muscles 
represent  the  original  layer  of  myotomes,  all  of  the  muscles  external  to  them 
having  been  derived  by  processes  of  budding  and  splitting.  The  serratus 
muscles  beautifully  illustrate  the  method  of  origin  of  a  muscle  by  buds  from  a 
number  of  myotomes,  since  the  slips  by  which  they  arose  from  fye  intercostals 
are  still  present. 


COMPARATIVE  ANATOMY  OF  THE  MUSCULAR  SYSTEM  147 

Cat: 

a)  Latissimus  dorsi.    Turn  the  animal  on  one  side  so  that  the  side  on  which 
the  muscles  have  already  been  dissected  will  be  uppermost.    The  large  flat 
muscle  extending  obliquely  forward  from  the  middle  of  the  back  to  the  upper  arm 
is  the  latissimus  dorsi.     Origin,  from  the  neural  spines  of  the  last  thoracic  and 
most  of  the  lumbar  vertebrae  and  from  the  lumbodorsal  fascia;  insertion,  by  a 
tendon  on  the  medial  surface  of  the  humerus;  action,  pulls  the  fore  limb  dor- 
sally  and  caudally. 

b)  Trapezius  muscles.    There  are  three  trapezius  muscles  in  the  cat;  they 
are  the  thin  flat  muscles  covering  the  back  and  neck  anterior  to  the  preceding. 
The  posterior  trapezius  or  spinotrapezius  takes  origin  from  the  spines  of  the 
thoracic  vertebrae  and  passes  obliquely  forward,  covering  part  of  the  latissimus 
to  be  inserted  on  the  fascia  of  the  scapula;  action,  draws  the  scapula  dorsad 
and   caudad.     In   front   of  this   is  the  middle  trapezius  or  acromiotrapezius. 
Origin,  neural  spines  of  cervical  and  first  thoracic  vertebrae;  insertion,  meta- 
cromion  process  and  spine  of  the  scapula  and  fascia  of  the  preceding  muscle; 
action,  draws  scapula  dorsad  and  holds  the  two  scapulae  together.    The  anterior 
trapezius  or  clawtrapezius  is  the  anterior  part  of  the  long  muscle  already  described 
as  cephalobrachial.     Origin,  superior  nuchal  line  and  median  dorsal  line  of  neck; 
passes  obliquely  ventrally  to  be  inserted  on  the  clavicle  which  is  imbedded 
on  its  inside  surface;  it  is  continuous  with  the  clavobrachial  muscle.    Action, 
draws  the  clavicle  dorsad  and  craniad. 

c)  Levator  scapulae  ventralis.     Carefully  free  the  three  trapezius  muscles. 
Along  the  ventral  border  of  the  acromiotrapezius  and  apparently  continuous 
with  it  is  seen  a  flat  bandlike  muscle  which  passes  anteriorly  diverging  from  the 
acromiotrapezius  and  passing  internal  to  the  clavotrapezius  which  should  be 
cut  across.     Origin,  transverse  process  of  the  atlas  and  occipital  bone;  inser- 
tion, metacromion  process  and  neighboring  fascia;  action,  draws  the  scapula 
craniad. 

d)  Rhomboideus.     Cut  across  the  middle  of  the  bellies  of  the  spino-  and 
acromiotrapezius  muscles.     A  thick  muscle  will  be  seen  beneath  them  extending 
from  the  vertebral  border  of  the  scapula  to  the  mid-dorsal  line;    this  is  the 
rhomboideus.     Origin,  neural  spines  of  the  vertebrae  and  adjacent  ligaments; 
insertion,  vertebral  border  of  the  scapula;  action,  draws  scapula  dorsad.    The 
most  ventral  portion  of  this  muscle  is  a  practically  separate  muscle,  the  rhom- 
boideus capitis,  which  extends  as  a  slender  band  forward  to  originate  from  the 
superior  nuchal  line;   insertion,  scapula;   action,  draws  the  scapula  craniad  and 
rotates  it. 

e)  Splenius.     This  is  the  large  sheet  of  muscle  covering  the  back  of  the  neck 
in  front  of  the  rhomboideus,  internal  to  the  trapezii  and  crossed  by  the  rhom- 
boideus capitis.     Origin,  from  the  mid-dorsal  line  and  fascia;  insertion,  superior 
nuchal  line;  action,  raises  or  turns  the  head. 


148       LABORATORY  MANUAL  FOR  VERTEBRATE  ANATOMY 

/)  Supraspinatus.  On  turning  back  the  ventral  half  of  the  cut  acromio- 
trapezius  a  stout  muscle  is  seen  occupying  the  supraspinous  fossa  of  the  scapula. 
Origin,  whole  surface  of  the  supraspinous  fossa;  insertion,  greater  tuberosity 
of  the  humerus,  next  to  the  insertion  of  pectoralis  minor;  action,  extends  the 
humerus. 

g)  Deltoids.  There  are  two  deltoids  in  the  cat.  The  clavobrachialis  already 
described  is  sometimes  also  considered  as  a  deltoid  and  is  called  the  clavodeltoid 
by  some  authorities.  The  acromiodeltoid  is  a  short  thick  muscle  passing  ven- 
trally  from  the  acromion  process;  it  is  inserted  on  the  surface  of  other  muscles 
of  the  shoulder.  It  lies  lateral  to  the  clavobrachial.  Action,  with  the  next. 
Turn  the  flap  of  the  acromiotrapezius  forward  again.  Posterior  to  the  line 
marking  the  insertion  of  the  acromiotrapezius  and  the  levator  scapulae  ventralis 
is  a  muscle  which  passes  to  the  upper  arm,  across  the  upper  ends  of  the  muscles 
of  the  upper  arm.  This  is  the  spinodeltoid.  Origin,  spine  of  the  scapula; 
insertion,  ridge  (deltoid  ridge)  of  the  humerus,  the  insertion  being  concealed 
by  the  acromiodeltoid;  action,  the  deltoids  raise  and  rotate  the  humerus. 

h)  Infraspinatus.  Cut  across  the  belly  of  the  latissimus  dorsi.  Bring  the 
anterior  parts  of  the  latissimus  and  the  spinotrapezius  forward  so  as  to  expose 
the  posterior  part  of  the  scapula.  Two  large  muscles  are  here  seen.  The  ante- 
rior one  fills  the  infraspinous  fossa  of  the  scapula,  from  whose  surface  it  takes 
its  origin  and  is  inserted  on  the  greater  tuberosity  of  the  humerus,  the  insertion 
being  concealed  by  the  deltoids  which  may  be  cut  across  to  see  it.  This  muscle 
is  the  infraspinatus.  Action,  rotates  the  humerus. 

i)  Teres  major.  The  stout  muscle  immediately  behind  the  preceding,  its 
fibers  running  in  the  same  direction.  Origin,  axillary  border  of  the  scapula  and 
fascia  of  neighboring  muscles;  insertion,  in  common  with  latissimus  dorsi  on 
the  medial  surface  of  the  humerus;  action,  rotates  the  humerus,  and  lowers  it. 

j)  Teres  minor.  Carefully  separate  the  infraspinatus  from  teres  major  and 
separate  the  former  from  the  deltoids  and  the  muscles  of  the  upper  arm.  On 
the  posterior  border  of  the  infraspinatus  and  somewhat  covered  by  it  is  the  small 
teres  minor.  Origin,  axillary  border  of  the  scapula ;  insertion,  greater  tuberosity ; 
action,  assists  the  infraspinatus. 

k)  Subscapularis.  Place  the  finger  under  the  anterior  border  of  the  scapula 
and  clear  away  connective  tissue  from  the  under  surface  of  the  scapula.  The 
subscapular  fossa  is  seen  to  be  occupied  by  a  muscle,  the  subscapularis,  which 
covers  the  inner  or  medial  surface  of  the  scapula.  Origin,  subscapular  fossa; 
insertion,  lesser  tuberosity  of  the  humerus;  action,  pulls  the  humerus  medially. 
Posterior  to  the  subscapularis  will  be  found  part  of  teres  major  which  extends 
to  the  medial  surface  of  the  scapula. 

/)  Serratus  ventralis  (anterior).  Cut  through  the  rhomboideus  close  to  the 
vertebral  column.  The  scapula  then  swings  loose.  On  raising  the  scapula,  a 
large  fan-shaped  muscle  is  seen  extending  from  the  vertebral  border  of  the 


COMPARATIVE  ANATOMY  OF  THE  MUSCULAR  SYSTEM  149 

scapula  to  the  sides  of  the  thorax  and  neck.  Origin,  by  slips  from  the  first  nine 
or  ten  ribs  and  the  anterior  part  (called  by  R  and  J  levator  scapulae)  from  the 
transverse  processes  of  the  last  five  cervical  vertebrae;  insertion,  scapula  near 
the  vertebral  border;  action,  draws  the  scapula  craniad,  ventrad,  and  against  the 
thoracic  wall. 

m)  Serratus  dor  sails  (posterior).  The  anterior  part  of  this  muscle  arises  by 
a  number  of  fleshy  slips  from  the  ribs  near  their  angles.  The  short  slips  soon 
pass  into  a  thin  aponeurosis  which  overlies  the  epaxial  muscles  of  the  thorax,  and 
which  is  fastened  to  the  median  dorsal  line.  The  posterior  part  of  this  muscle 
consists  of  a  few  slips  lying  under  the  latissimus  dorsi  and  appearing  like  a  forward 
continuation  of  the  internal  oblique.  These  slips  originate  on  the  last  ribs  and 
are  inserted  by  means  of  an  aponeurosis  onto  the  median  dorsal  line.  Action, 
draw  the  ribs  forward. 

n)  Scalenes.  Raise  up  the  pectoralis  muscles  from  the  chest  wall  by  passing 
the  fingers  under  them.  If  necessary  their  posterior  parts  may  be  cut  into. 
Several  long  muscles  will  be  seen  in  the  chest  wall  ventral  to  the  origin  of  the 
serratus  ventralis  and  in  front  of  the  anterior  boundary  of  the  external  oblique. 
These  muscles  are  scalenes;  they  originate  on  the  ribs  and  pass  forward  in  a 
nearly  straight  course  to  be  inserted  on  the  transverse  processes  of  the  cervical 
vertebrae,  uniting  anteriorly  into  one  band  which  will  readily  be  seen  by  looking 
immediately  ventral  to  the  origin  of  the  anterior  part  of  the  serratus  ventralis. 
Insertion,  transverse  processes  of  the  cervical  vertebrae;  action,  draw  the  ribs 
forward  and  bend  the  neck. 

o)  Intercostals.  The  intercostals  are  a  set  of  muscles  extending  from  one 
rib  to  the  next.  The  external  layer  is  called  the  external  intercostals  and  will  be 
seen  in  part  by  looking  at  the  chest  wall  between  the  origins  of  the  serratus 
ventralis  and  dorsalis.  Their  fibers  run  obliquely  backward  and  downward. 
On  cutting  through  some  of  them  another  layer,  the  internal  intercostals,  will  be 
seen  inside  of  the  external  layer.  The  fibers  of  the  internal  intercostals  run 
obliquely  forward  and  downward.  Near  the  median  ventral  line  the  external 
intercostals  are  lacking,  so  that  the  internal  ones  are  exposed  by  cutting  through 
the  scalenes.  Action,  external  intercostals  bring  the  ribs  forward,  internal 
intercostals  draw  them  back  again.  The  intercostals  are  the  chief  respiratory 
muscles  of  the  thoracic  wall. 

p)  Epaxial  muscles  of  the  thorax.  On  cutting  through  the  aponeurosis  of  the 
serratus  dorsalis,  the  thick  mass  of  epaxial  muscle  is  exposed.  This  may  be 
followed  up  into  the  neck  by  cutting  through  the  splenius.  In  the  thoracic 
region  the  muscle  is  divisible  into  three  parts  of  about  equal  width,  a  dorsal  part 
next  the  median  dorsal  line,  the  semispinalis  dorsi;  a  median  part,  the 
longissimus;  and  a  ventral  part,  the  iliocostalis.  The  latter  is  composed  of  a 
number  of  separate  bundles  with  prominent  tendons  between  them,  and  lies  on 
the  ribs  to  which  it  is  attached.  The  longissimus  may  be  traced  forward  to  its 


150       LABORATORY  MANUAL  FOR  VERTEBRATE  ANATOMY 

insertion  on  the  transverse  processes  of  the  vertebrae  in  contact  with  the  origin 
of  the  serratus  ventralis.  The  mass  in  the  neck  is  divisible  into  a  medial 
biventer  cervicis  and  a  lateral  complexus.  All  of  the  epaxial  muscles  are  very 
complex  consisting  of  numerous  bundles.  They  are  attached  to  various  parts 
of  the  vertebrae,  ribs,  and  head,  and  have  the  general  function  of  moving  the 
vertebral  column  and  related  parts. 

8.  The  muscles  of  the  upper  arm. — Separate  these  muscles  from  each  other 
and  identify  them. 
Rabbit: 

a)  Triceps  brachii.     The  triceps  is  the  great  muscle  on  the  back  of  the  upper 
arm.     It  consists  of  three  heads  which  are  practically  distinct  muscles.     Free 
the  heads.     The /0wg  head  is  the  large  mass  on  the  back  of  the  upper  arm;  origin, 
scapula,  from  the  axillary  border;    insertion,  olecranon.     The  lateral  head  is 
on  the  lateral  surface  of  the  upper  arm,  ventral  to  the  preceding;  origin,  greater 
tuberosity  of  the  humerus.     The  medial  head  is  in  contact  with  the  humerus.     To 
see  it  spread  the  other  two  heads  apart  and  look  deep  between  them  or  cut  through 
the  middle  of  the  belly  of  the  lateral  head;  origin,  along  the  dorsal  surface  of 
the  humerus.     All  three  heads  insert  on  the  olecranon  and  are  the  great  extensors 
of  the  forearm. 

b)  Extensor  antibrachii.     On  the  medial  surface  of  the  long  head  of  the  tri- 
ceps is  a  fascia  from  the  lower  part  of  which  this  muscle  originates.     Insertion, 
olecranon;  action,  in  common  with  the  triceps. 

c)  Biceps  brachii.     On  the  anterior  surface  of  the  upper  arm  found  by  remov- 
ing the  insertions  of   the   pectoral  muscles  is  a  spindle-shaped  muscle,  the 
biceps.     Origin,  glenoid  fossa;  insertion,  ulna  and  radius;  action,  flexor  of  the 
forearm. 

d)  Brachialis.     This  muscle  is  next  lateral  to  the  biceps  and  in  contact  with 
the  humerus.     Origin,  ventral  and  lateral  surface  of  the  humerus;   insertion 
and  action,  in  common  with  the  biceps. 

Cat: 

a)  Triceps  brachii.     As  above  under  rabbit  with  three  heads. 

b)  Extensor  antibrachii.     On  the  medial  side  of  the  long  head,  a  thin  sheet 
of  muscle,  taking  origin  from  the  latissimus  dorsi  and  inserted  on  the  olecranon ; 
action,  in  common  with  the  triceps,  tending  also  to  rotate  the  ulna. 

c)  Biceps  brachii.     As  in  the  rabbit;  visible  only  after  cutting  the  insertions 
of  the  pectoral  muscles.     Origin,  glenoid  fossa;  insertion,  radius;  action,  flexor 
of  the  forearm. 

d)  Brachialis.     Lateral  to  the  biceps,  in  contact  with  the  lateral  head  of 
the  triceps.     Origin,  lateral  surf  ace  of  the  humerus ;  insertion,  ulna;  action,  with 
the  biceps. 

e)  Anconeus.     A  small  triangular  muscle  at  the  elbow  joint,  covered  by 
the  distal  end  of  the  lateral  head  of  the  triceps,  which  should  be  deflected. 


COMPARATIVE  ANATOMY  OF  THE  MUSCULAR  SYSTEM  151 

Origin,  distal  end  of  humerus;  insertion,  lateral  surface  of  the  ulna;  action, 
strengthens  the  elbow  joint,  which  it  covers,  and  possibly  rotates  the  ulna. 

9.  The  muscles  of  the  thigh. — (Fig.  43.) 
Rabbit: 

a)  Tensor  fasciae  latae.  The  anterior  half  of  the  lateral  surface  of  the 
thigh  is  covered  by  a  tough  fascia  or  aponeurosis  called  the  fascia  lata.  In  the 
dorsal  part  of  this  will  be  found  a  short  muscle,  the  tensor  fasciae  latae,  which 
is  more  or  less  continuous  with  adjacent  muscles.  Origin,  ilium;  insertion,  in 
the  fascia;  action,  tightens  the  fascia. 


fascia  lata 


sartorius 


rectus 
femoris 

vastus 
lateralis 
tensor  fascia 
latae 
vastus 
intermedi 
vastus 
medialis 
femur 
sartorius 
adductor 
longus 
biceps 
femoris 


semitendinosus 
adductor 
magnus 
semimembranosus 

gracilis 


POSTERIOR 


membranosus 
semitendinosus 


FIG.  43. — Cross-sections  through  the  thigh  of  A,  rabbit,  and  B,  cat  to  show  the  location  of  the 
muscles.  Black  spots  are  nerves,  small  circles,  blood  vessels,  a,  greater  saphencus  nerve,  artery, 
and  vein;  b,  peroneal  nerve;  c,  tibial  nerve;  d,  sciatic  vein;  e,  femoral  nerve,  artery,  and  vein;  /,  sciatic 
nerve.  (A  from  Bensley's  Practical  Anatomy  of  the  Rabbit,  University  of  Toronto  Press.) 

b)  Biceps  femoris.     Slit  open  the  fascia  lata.     Posterior  to  the  fascia  occupy- 
ing the  middle  portion  of  the  lateral  surface  of  the  thigh  is  the  large  biceps 
femoris.     Separate  it  from  the  muscle  posterior  to  it  which  forms  the  posterior 
margin  of  the  thigh  (semimembranosus).     On  removing  the  surface  fascia  from 
the  biceps  it  will  be  found  divisible  into  two  heads.     Follow  these  heads  dorsally 
to  their  origins,  clearing  away  the  tough  surface  fascia.     Origin  of  the  smaller 
anterior  head,  neural  spines  of  adjacent  vertebrae,  of  the  larger  posterior  head, 
ischial  tuberosity;  insertion,  by  a  tendon  on  the  kneecap  and  on  the  fascia  of 
the  shank;  action,  abductor  of  the  thigh  and  flexor  of  the  shank. 

c)  Glutens  maximus.    Remove   the  fascia  over  the    sacral   region.     The 
gluteus  maximus  is  under  the  fascia,  a  thin  muscle  whose  posterior  part  is  partly 
covered  by  the  biceps  femoris.     The  muscle  also  has  an  anterior  part  which  is 
continuous  with  the  tensor  fasciae  latae.     The  two  parts  of  the  gluteus  maximus 


152       LABORATORY  MANUAL  FOR  VERTEBRATE  ANATOMY 

are  united  by  an  aponeurosis.  Origin,  fascia  of  the  sacrum  and  part  of  the  ilium; 
insertion,  third  trochanter,  the  small  projection  below  the  great  trochanter; 
action,  abducts  the  thigh. 

d)  Glutens  medius.     The  larger,  thicker  muscle  anterior  to  the  preceding  and 
partly  covered  by  it.     Origin,  crest  and  anterior  part  of  the  ilium;  insertion, 
greater  trochanter;  action,  like  the  preceding.     When  the  two  gluteus  muscles 
have  been  well  separated,  the  origin  of  the  tensor  fasciae  latae  will-be  found  on 
the  ventral  edge  of  the  gluteus  medius  dipping  deep  down  to  the  ilium. 

e)  Vastus  lateralis.     Cut  through  the  fascia  lata  and  its  tensor  by  a  longi- 
tudinal slit  extending  to  the  knee.     Under  the  fascia  will  now  be  seen  the  stout 
vastus  lateralis.     Origin,  greater  trochanter  dorsal  to  the  insertion  of  the  gluteus 
medius. 

f)  Vastus  intermedius.     This  is  the  name  given  to  what  appears  to  be  the 
posterior  part  of  the  preceding  muscle.     It  is  partly  separable  from  the  vastus 
lateralis  and  lies  between  this  and  the  anterior  head  of  the  biceps  femoris.     Origin, 
greater  trochanter  and  surface  of  the  femur. 

g)  Rectus  femoris.    This  is  the  thin  muscle  which  is  folded  over  the  anterior 
margin  of  the  thigh;  it  extends  on  both  lateral  and  medial  surfaces  of  the  thigh. 
It  originates  on  the  ilium  and  the  fascia  lata  and  is  fused  with  the  fibers  of 
the  tensor  fasciae  latae.     The  rectus  femoris  consists  of  two  parts,  the  part  just 
described  and  a  second  part,  which  may  be  located  as  follows.     Separate  the 
rectus  femoris  from  the  vastus  lateralis  to  which  it  will  be  found  slightly 
fused,  continuing  this  separation  to  the  medial  side  of  the  thigh,  spreading  the 
legs  apart.     The  first  part  of  the  rectus  femoris  may  be  cut  through  at  the 
middle.     On  the  medial  side  of  the  thigh  internal  to  the  first  part  of  the  rectus 
femoris  and  ventral  to  the  vastus  lateralis  is  the  second  part  of  the  rectus  femoris, 
a  cylindrical  muscle.     Origin,  ilium  in  front  of  the  acetabulum. 

ti)  Vastus  medialis.  On  the  medial  side  of  the  thigh  posterior  to  the  first 
part  of  the  rectus  femoris  and  not  very  well  separable  from  it.  Origin, 
femur. 

The  rectus  femoris  and  the  three  vastus  muscles  together  constitute  the 
great  quadriceps  femoris  muscle.  The  origins  of  its  several  parts  have  already 
been  given.  Insertion,  on  the  tibia  and  the  patella  and  the  tendon  which  extends 
over  the  patella;  action,  powerful  extensor  of  the  shank. 

i)  Sartorius.  The  very  long  narrow  muscle  extending  like  a  band  along 
the  middle  of  the  medial  surface  of  the  thigh.  Origin,  inguinal  ligament;  inser- 
tion, tibia;  action,  adductor  of  the  thigh,  rotator  of  the  thigh,  extensor  of  the 
shank.  The  sartorius  lies  between  the  vastus  medialis  and  the  next  muscle  to 
be  described,  and  covers  some  large  blood  vessels. 

j)  Gracilis.  A  large  thin  muscle  over  the  posterior  half  of  the  medial  sur- 
face of  the  thigh.  Origin,  pubic  symphysis;  insertion,  fascia  of  the  distal  portion 
of  the  thigh  and  proximal  portion  of  the  shank:  action,  adductor  of  the  leg. 


COMPARATIVE  ANATOMY  OF  THE  MUSCULAR  SYSTEM  153 

k)  Adductor  longus  and  adductor  magnus.  Cut  through  the  middle  of  the 
gracilis  and  find  beneath  it  two  stout  muscles,  their  fibers  running  from  the 
median  ventral  line  to  the  femur.  The  anterior  muscle  is  the  adductor  longus; 
the  posterior  one,  the  adductor  magnus.  Origin,  various  parts  of  the  ischium; 
insertion,  femur;  action,  adductors  of  the  thigh. 

/)  Semitendinosus.  Split  open  the  adductor  magnus  and  find  inside  of  it 
a  cylindrical  muscle,  the  semitendinosus.  Origin,  ischial  tuberosity;  insertion, 
medial  condyle  of  the  tibia;  action,  flexor  of  the  shank. 

m)  Semimembranosus.    This  is  the  muscle  which  forms  the  posterior  margin 
of  the  thigh,  between  the  biceps  femoris  and  the  adductor  magnus.     Origin, 
fascia  over  the  biceps,  and  ischial  tuberosity;  insertion,  with  the  gracilis  in  the 
fascia  of  the  shank;  action,  flexor  of  the  shank, 
t: 

a)  Tensor  fasciae  latae.     Examine  the  lateral  (outer)  surface  of  the  thigh. 
The  anterior  part  of  this  is  covered  by  a  tough  fascia,  the  fascia  lata.     In  the 
dorsal  part  of  this  is  a  muscle,  the  tensor  fasciae  latae,  a  thick  triangular  muscle. 
Origin,  ilium  and  neighboring  fascia;  insertion,  fascia  lata;  action,  tightens  the 
fascia  lata. 

b)  Biceps  femoris.    This  is  the  large  muscle  on  the  lateral  surface  of  the 
thigh  posterior  to  the  fascia  lata  and  covering  more  than  half  of  the  surface  of 
the  thigh.     It  has  but  one  head  in  the  cat.     Origin,  tuberosity  of  the  ischium; 
insertion, patella  and  tibia  by  a  tendon,  and  the  fascia  of  the  shank;  action,  abduc- 
tor of  the  thigh  and  flexor  of  the  shank. 

c)  Caudofemoralis.     Clean  away  the  fascia  from  the  back  in  front  of  the 
base  of  the  tail,  as  far  forward  as  the  anterior  end  of  the  pelvic  girdle.     Muscles 
will  be  found  between  the  median  dorsal  line  and  the  thigh.     The  most  posterior 
of  these  is  the  narrow  flat  caudofemoralis,  passing  from  the  side  of  the  root  of 
the  tail  toward  the  dorsal  end  of  the  biceps  femoris.     Origin,  transverse  processes 
of  the  second  and  third  caudal  vertebrae;  insertion,  the  muscle  passes  ventrally 
concealed  by  the  anterior  margin  of  the  biceps  femoris.     This  should  be  lifted 
up  and  the  caudofemoralis  followed  to  its  tendon;  the  latter  is  very  long  and 
passes  to  the  patella  on  which  it  is  inserted.     Action,  abductor  of  the  thigh,  exten- 
sor of  the  shank. 

d)  Gluteus  maximus.    A  rather  thin  flat  muscle  immediately  anterior  to 
the  preceding.     It  is  imbedded  in  the  fascia  and  is  continuous  with  the  tensor 
fasciae  latae  anteriorly.     Origin,  from  the  fascia  and  from  the  transverse  processes 
of  the  last  sacral  and  first  caudal  vertebrae ;   insertion,  fascia  lata  and  to  a  slight 
extent  on  the  greater  trochanter;  action,  in  common  with  the  next. 

e)  Gluteus  medius.     The  very  large  triangular  muscle  immediately  in  front 
of  the  preceding  and  partly  covered  by  it.     The  gluteus  maximus  should  be 
cut  across  to  see  it.     Origin,  adjacent  fascia,  crest  of  the  ilium,  and  lateral  surface 
of  the  ilium,  and  transverse  processes  of  the  last  sacral  and  first  caudal  vertebrae; 


154       LABORATORY  MANUAL  FOR  VERTEBRATE  ANATOMY 

insertion,  by  a  strong  tendon  on  the  greater  trochanter  of  the  femur;  action, 
abductor  of  the  thigh.  Along  the  anterior  border  of  this  muscle  the  origin  of 
the  tensor  fasciae  latae  passes  internally  toward  the  ilium. 

f)  Sartorius.     This  muscle  forms  the  anterior  margin  from  the  lateral  view 
of  the  thigh.     It  is  folded  over  the  margin  and  on  following  it  to  the  medial 
or  inner  surface  of  the  thigh  will  be  found  to  cover  the  anterior  half  of  the  medial 
surface.     Origin,  crest,  and  ventral  border  of  the   ilium;    insertion,  proximal 
end  of  the  tibia  and  the  patella  and  the  fascia  and  ligaments  between;  action, 
adductor  and  rotator  of  the  thigh  and  extensor  of  the  shank. 

g)  Vastus  later  alls.     Cut  through  the  fascia  lata  by  a   longitudinal  slit 
extending  to  the  patella.     Separate  well  the  sartorius  from  underlying  parts. 
The  tensor  fasciae  latae  is  now  well  exposed.     The  large  stout  muscle  which  was 
covered  by  the  fascia  lata  is  the  vastus  lateralis.     Origin,  greater  trochanter  and 
surface  of  the  femur. 

ti)  Rectus  femoris.  At  its  anterior  margin  the  vastus  lateralis  will  be  found 
partly  separable  from  a  stout  muscle  lying  on  its  medial  side  and  covered  exter- 
nally by  the  sartorius.  The  sartorius  may  be  cut  across  the  middle.  The  muscle 
in  question  is  the  rectus  femoris.  Origin,  ilium  near  the  acetabulum. 

i)  Vastus  medialis.  This  is  on  the  medial  side  of  the  thigh  posterior  to 
the  rectus  femoris  which  its  anterior  margin  partly  covers.  It  also  is  covered 
externally  by  the  sartorius.  Origin,  femur. 

j)  Vastus  intermedius.  On  widely  separating  the  rectus  femoris  from  the 
vastus  lateralis  a  muscle  will  be  seen  deep  down  next  to  the  shaft  of  the  femur; 
Origin,  surface  of  the  femur. 

The  rectus  femoris  and  the  three  vastus  muscles  are  more  or  less  united  to 
each  other  and  constitute  the  great  quadriceps  femoris  muscle.  Its  origins  have 
been  described;  all  its  parts  are  inserted  on  the  patella  and  adjacent  ligaments. 
action,  extensor  of  the  shank. 

k)  Gracilis.  This  is  the  large  flat  muscle  forming  the  posterior  half  of  the 
medial  surface  of  the  thigh.  Origin,  ischial  and  pubic  symphyses;  insertion  by 
an  aponeurosis  which  passes  to  the  tibia;  action,  adductor  of  the  leg. 

/)  Adductor  longus  and  adductor  femoris.  Cut  through  the  middle  of  the 
gracilis  and  separate  each  half  from  the  underlying  muscles.  The  latter  consist 
of  three  muscles  passing  from  the  median  ventral  line  to  the  femur.  The  most 
anterior  of  the  three  is  quite  small;  this  is  the  adductor  longus.  Origin,  pubis; 
insertion,  femur;  action,  with  the  next.  The  middle  muscular  mass  is  the  large 
adductor  femoris  (corresponding  to  adductor  magnus  and  brevis  of  other  mam- 
mals). Origin,  pubis;  insertion,  femur;  action,  adductor  of  the  thigh. 

m)  Semimembranosus.  The  large  posterior  part  of  the  mass  which  was 
covered  by  the  gracilis.  Origin,  ischium;  insertion,  medial  epicondyle  of  the 
femur  and  proximal  end  of  the  tibia;  action,  extensor  of  the  thigh.  The  muscle 
is  more  or  less  divisible  into  two  parts. 


COMPARATIVE  ANATOMY  OF  THE  MUSCULAR  SYSTEM  155 

n)  Semitendinosus.  The  most  posterior  muscle  of  the  thigh,  posterior  to  the 
preceding.  Origin,  ischial  tuberosity ;  insertion,  tibia;  action,  flexor  of  the  shank. 

o)  Tenuissimus.  Turn  to  the  lateral  surface  of  the  thigh.  Cut  through 
the  middle  of  the  biceps  femoris.  Beneath  it  will  be  noted  a  very  narrow  long 
muscle,  the  tenuissimus.  Origin,  transverse  process  of  the  second  caudal  verte- 
brae, in  common  with  the  caudofemoralis;  insertion,  on  the  same  fascia  as  the 
insertion  of  the  biceps. 

On  separating  the  biceps  from  the  underlying  muscles  they  will  be  revealed 
as  extensions  of  muscles  already  identified  on  the  medial  surface.  The  adductor 
femoris  is  seen  in  contact  with  the  femur  posterior  to  the  vastus  lateralis;  the 
semimembranosus  comes  next,  and  the  semitendinosus  is  again  the  most  caudal 
of  the  thigh  muscles. 

10.  The  muscles  of  the  shank. — 
Rabbit: 

a)  Tibialis  anterior.    The  lateral  (outer,   dorsal)  surface  of  the  shank  is 
covered  by  the  distal  end  of  the  biceps  femoris  and  fascia.     These  should  be 
removed.    The  most  anterior  of  the  muscles  of  the  lateral  surface  is  the  tibialis 
anterior.     Origin,  lateral  condyle  and  tuberosity  of  the  tibia;  insertion,  second 
metatarsal;    action,  flexor  of  the  foot. 

b)  Peroneus.     Next  dorsal  to  the  preceding  on  the  lateral  surface,  consisting 
of  a  group  of  several  more  or  less  fused  muscles.     Origin,  tibia  and  fibula;  inser- 
tion, metatarsals;    action,  flexor  of  the  foot. 

c)  Gastrocnemius.    This  is  the  thin  but  broad  muscle  forming  the  caudal 
surface  of  the  shank,  divisible  into  two  nearly  separate  portions,  one  of  which  is 
on  the  lateral,  one  on  the  medial  surface  of  the  shank.     Origin,  lateral  and  medial 
condyles  of  the  femur  and  tibia;  insertion,  by  a  strong  tendon,  the  tendon  of 
Achilles,  which  passes  over  the  heel  (calcaneus)  on  which  it  is  inserted;  action, 
extensor  of  the  foot. 

d)  Soleus.    This  is  the  muscle  just  internal  to  that  part  of  the  gastrocnemius 
which  is  on  the  lateral  surface  of  the  thigh.     Origin,  head  of  the  fibula ;  insertion 
and  action  with  the  preceding. 

e)  Plantaris.    This  is  situated  internal  to  that  part  of  the  gastrocnemius 
which  is  medial.     Origin,  lateral  condyle  of  the  femur;    insertion  and  action 
with  the  preceding. 

/)  Other  muscles  of  the  shank  (optional).  There  are  three  more  muscles  of  the 
shank;  they  lie  in  contact  with  the  tibia.  They  are:  the  extensor  hallucis  longus 
exposed  on  the  medial  surface  of  the  tibia;  extensor  digitorum  longus,  covered  by 
the  tibialis  anterior;  said  flexor  digitorum  longus,  between  the  tibia  and  the  soleus 
and  plantaris.  As  their  names  imply,  these  muscles  are  inserted  by  long 
slender  tendons  on  the  digits  and  act  to  flex  and  extend  the  digits. 

It  should  be  noted  that  the  muscles  named  extensor  in  the  shank  and  foot 
are  really  flexor  in  their  action  and  those  named  flexor  are  extensor  in  their 


156       LABORATORY  MANUAL  FOR  VERTEBRATE  ANATOMY 

action,  following  a  custom  borrowed  from  human  anatomy.  The  custom  arises 
from  a  desire  to  retain  the  names  applied  to  the  muscles  of  the  fore  limb  for  the 
muscles  in  the  corresponding  positions  in  the  hind  limb.  Thus  in  the  supine 
position  of  the  forearm,  the  extensors  face  anteriorly  and  the  flexors  posteriorly. 
Similarly  the  muscles  on  the  anterior  side  of  the  leg  are  designated  extensors, 
although  they  really  flex  the  foot,  and  those  on  the  posterior  side,  flexors,  although 
they  extend  the  foot.  In  describing  the  action,  the  terms  flexor  and  extensor 
are  used  with  reference  to  the  movement  produced  and  not  with  reference  to 
the  position  of  the  muscle  on  the  limb. 
Cat: 

a)  Tibialis  anterior.     Clean  away  the  tough  fascia  of  the  shank  and  also 
the  insertions  of  the  biceps  and  the  gracilis.     Examine  the  lateral  (outer)  surface 
of  the  shank.     The  most  ventral  muscle  whose  ventral  border  is  in  contact  with 
the  tibia  is  the  tibialis  anterior.     Origin,  proximal  parts  of  tibia  and  fibula; 
insertion  by  a  strong  tendon  which  should  be  traced  into  the  foot  where  it  will 
be  found  to  pass  obliquely  to  the  medial  side  of  the  foot  to  be  inserted  on  the 
first  metatarsal;    action,  flexor  of  the  foot. 

b)  Extensor  digitorum  longus.    This  is  the  muscle  next  dorsal  to  the  preceding 
on  the  lateral  surface  of  the  shank.     It  is  so  closely  placed  to  the  preceding  as  to 
appear  as  part  of  it,  but  the  line  of  separation  will  be  found  by  a  little  searching. 
Origin,  lateral  epicondyle  of  the  femur;   insertion,  by  a  stout  tendon  which  if 
followed  into  the  foot  is  found  to  diverge  into  four  tendons,  one  of  which  is 
inserted  on  each  digit;  action,  extensors  of  the  digits. 

c)  Peroneus  muscles.     These  are  next  dorsal  to  the  preceding,  originating 
on  the  fibula.     There  are  three  of  them  more  or  less  fused  to  each  other.     The 
three  end  each  in  a  tendon;  the  three  tendons  pass  over  the  lateral  surface  of 
the  lateral  malleolus  of  the  tibia  and  over  the  calcaneus,  and  are  inserted  on 
the  metatarsals  and  digits.     Action,  extensors  and  flexors  of  the  foot. 

d)  Gastrocnemius.    This  is  the  large  muscle  forming  the  posterior  or  caudal 
surface  of  the  shank.     It  is  divisible  into  two  large  portions,  one  on  the  medial 
surface,  the  other  on  the  lateral  surface  of  the  shank.     The  lateral  head  is  sub- 
divisible into  four  heads.     Origins,  from  the  surface  fascia,  the  femur,  and  the 
tendon  and  fascia  of  the  plantaris  muscle  (see  below);  insertion,  by  a  strong 
tendon,  which  passes  to  the  heel  bone  (calcaneus)  on  which  it  is  inserted.     Action, 
extensor  of  the  foot. 

e)  Soleus.    On  carefully  separating  the  lateral  head  of  the  gastrocnemius, 
a  muscle,  the  soleus.  will  be  found  internal  to  it.     It  is  a  flat  muscle  in  contact  with 
the  peroneus  muscles  ventrally;  it  tapers  abruptly  to  a  tendon  which  joins  the 
tendon  of  the  gastrocnemius.     Origin,  fibula;  insertion,  calcaneus;  action,  with 
the  gastrocnemius,  of  which  it  is  sometimes  considered  a  part. 

/)  Plantaris.  On  carefully  separating  the  medial  head  of  the  gastrocnemius 
a  large  muscle  will  be  found  internal  to  it,  lying  between  the  two  heads  of  the 
gastrocnemius  which  practically  inclose  it.  It  is  fused  to  a  considerable  extent 


COMPARATIVE  ANATOMY  OF  THE  MUSCULAR  SYSTEM  157 

to  the  lateral  head,  but  quite  separable  from  the  medial  head,  being  covered  on 
the  medial  side  by  a  shining  aponeurosis.  Origin,  patella  and  femur;  insertion, 
by  a  thick  tendon  which  passes  in  the  middle  of  a  sort  of  tube  formed  by  the 
tendon  of  the  gastrocnemius  and  soleus  onto  the  ventral  surface  of  the  calcaneus. 
Here  it  broadens  and  finally  divides  into  four  slips,  each  attached  to  a  digit. 
Action,  flexor  of  the  digits. 

g)  Flexor  digitorum  longus.  On  turning  to  the  medial  side  of  the  shank 
and  clearing  away  the  surface  fascia,  the  following  may  be  identified.  Most 
ventrally  will  be  seen  the  tibialis  anterior;  next  comes  the  exposed  surface  of  the 
tibia.  Immediately  dorsal  to  the  bone  is  the  flexor  digitorum  longus  which 
consists  of  two  parts,  somewhat  separated.  The  other  part  is  more  lateral  in 
contact  with  the  peroneus  muscles.  Separate  the  part  of  the  flexor  which  appears 
on  the  medial  surface  from  the  tibia  by  a  cut,  and  lift  it  up.  Internal  to  it  is 
seen  a  long  tendon,  and  on  the  other  side  of  this  tendon  is  the  other  part  of  the 
flexor,  this  part  corresponding  to  the  flexor  hallucis  longus  of  man.  Both  parts 
of  the  flexor  terminate  in  slender  tendons  which  unite  distally  into  a  broad 
tendon,  which  eventually  divides  into  four  tendons  inserted  on  the  digits. 
Origin,  tibia,  fibula,  and  adjacent  fascia;  action,  flexor  of  the  digits. 

h)  Tibalis  posterior.  The  long  tendon  between  the  two  parts  of  the  preceding 
muscle  was  noted  above.  It  is  the  tendon  of  the  tibialis  posterior  and  on  follow- 
ing this  tendon  proximally  the  belly  of  the  muscle  will  be  located.  Origin, 
fibula,  tibia,  and  fascia;  insertion,  scaphoid  and  medial  cuneiform  of  the  ankle; 
action,  extensor  of  the  foot. 

E.      SUMMARY 

1.  The  voluntary  muscles  are  of  two  kinds — the  somatic  or  parietal  muscles  which  are 
produced  by  the  epimeres  and  the  visceral  muscles  which  arise  from  the  hypomeres.     Voluntary 
muscles  of  hypomeral  origin  occur  only  in  connection  with  the  gill  arches,  from  which,  however, 
they  may  spread  to  cover  considerable  areas. 

2.  In  primitive  vertebrates  the  somatic  muscles  exist  in  the  form  of  muscle  segments  or 
myotomes  which  are  repeated  at  regular  intervals  along  the  longitudinal  axis.    Each  myotome 
extends  from  the  mid-dorsal  to  the  mid-ventral  line.    The  myotomes  are  bounded  by  connec- 
tive tissue  partitions,  the  myosepta. 

3.  The  myotomes  are  divided  into  dorsal  or  epaxial  halves  and  ventral  or  hypaxial  halves 
by  the  horizontal  skeletogenous  septum. 

4.  The  girdles  and  paired  appendages  interrupt  the  series  of  myotomes.    The  myotomes 
adjacent  to  the  appendages  send  out  muscle  buds  which  produce  the  muscles  of  the  girdles  and 
paired  appendages. 

5.  In  higher  animals  these  muscles  of  the  girdles  and  appendages  increase  in  size  and  impor- 
tance and  spread  over  the  segmented  musculature  until  the  latter  is  scarcely  recognizable. 
The  myotomes  are  further  altered  by  processes  of  splitting  and  fusion  so  that  the  primitive 
arrangement  no  longer  exists  in  the  adults  of  higher  land  vertebrates. 

6.  The  derivatives  of  the  hypaxial  muscles  form  the  larger  part  of  the  mammalian  muscu- 
lature.   The  epaxial  muscles  remain  as  a  thick  mass  on  either  side  of  the  vertebral  column, 
concealed  from  surface  view  by  dorsal  extensions  of  the  hypaxial  muscles. 

7.  With  the  adoption  of  the  air-breathing  habit  the  visceral  or  gill  musculature  is  greatly 
changed  and  is  found  in  mammals  associated  with  the  larynx,  pharynx,  hyoid  apparatus, 
upper  and  lower  jaws,  and  the  skin  of  the  neck  and  face.    The  muscles  of  the  head  are  prac- 
tically «UJ  visceral  muscles  (except  the  muscles  of  the  eyeball). 


X.     THE  COMPARATIVE  ANATOMY  OF  THE  COELOM,  THE  DIGESTIVE, 
AND  RESPIRATORY  SYSTEMS 

A.      THE    ORIGIN  AND   PARTS   OF   THE   COELOM  AND   THE   MESENTERIES 

i.  Origin. — The  coelom  or  body  cavity  of  vertebrates  is  the  cavity  of  the  hypomere.  It  is 
never  at  any  stage  segmented.  The  outer  wall  of  the  hypomere  comes  in  contact  with  the 
inner  surface  of  the  layer  of  voluntary  muscles  and  forms  the  lining  of  the  body  wall.  This 
lining  is  known  as  the  parietal  peritoneum.  The  inner  walls  of  the  hypomeres  of  the  two  sides 
come  in  contact  in  the  median  plane  folding  around  the  intestine  and  inclosing  the  intestine 
between  their  two  walls.  The  inner  walls  of  the  hypomere  thus  become  the  covering  layer  of 
the  intestine  and  other  viscera,  and  are  then  named  the  visceral  peritoneum  or  serosa.  Above 


FIG.  44. — Diagrams  to  show  the  relations  of  certain  viscera  to  the  mesenteries.  A,  showing  intes- 
tine d,  supported  by  the  dorsal  mesentery  c,  and  the  heart  g,  inclosed  in  the  ventral  mesentery  e  and  h. 
B,  showing  the  liver  n,  inclosed  in  the  ventral  mesentery,  part  of  which,  the  lesser  omentum  /,  extends 
between  the  intestine  k  and  the  liver,  and  part  oi  which,  the  falciform  ligament  p,  extends  between  the 
liver  and  the  ventral  body  wall.  C,  showing  relation  of  the  intestine  q  to  the  dorsal  mesentery  c. 
a,  neural  tube;  b,  notochord;  c,  dorsal  mesentery  of  the  digestive  tract;  d,  esophagus;  e,  dorsal  mesen- 
tery of  the  heart  or  dorsal  mesocardium;  /,  pericardial  cavity;  g,  heart;  h,  ventral  mesentery  of  the 
heart  or  ventral  mesocardium;  i,  dorsal  aorta;  j,  posterior  cardinal  vein;  k,  duodenum;  I,  lesser  omen- 
tum or  hepatoduodenal  ligament;  m,  serosa  or  visceral  peritoneum;  n,  liver;  o,  parietal  peritoneum; 
p,  falciform  ligament  of  the  liver;  q,  small  intestine;  r,  peritoneal  cavity;  s,  bile  duct.  In  A,  e  and  h, 
and  in  B,  I  and  p,  form  the  ventral  mesentery  of  the  digestive  tract  which  incloses  the  heart  and  liver; 
in  C  the  ventral  mesentery  is  absent.  (From  Prentiss  and  Arey's  Textbook  of  Embryology,  courtesy 
of  the  W.  B.  Saunders  Company.) 


and  below  the  intestine  the  two  walls  of  the  hypomere  are  in  contact  and  form  a  double-walled 
membrane,  which  is  designated  as  a  mesentery.  That  portion  of  the  mesentery  between  the 
dorsal  wall  of  the  coelom  and  the  intestine  is  called  the  dorsal  mesentery;  that  between  the 
ventral  wall  and  the  intestine  is  the  ventral  mesentery.  Different  portions  of  these  mesenteries 
receive  special  names,  which  will  be  given  in  the  course  of  the  dissection.  The  dorsal  mesentery 
is  intact  for  its  entire  length  in  most  vertebrates,  but  the  ventral  mesentery  very  early 
disappears  except  in  certain  regions  which  will  be  noted  later.  These  matters  have  already  been 

158 


THE  COELOM,  DIGESTIVE,  AND  RESPIRATORY  SYSTEMS  159 

described  in  connection  with  the  section  on  chordate  development,  Section  IV.  This  should 
be  re-read  and  Figures  7,  8,  and  10  studied.  (See  also  Fig.  44(7;  K,  Figs.  8  and  9,  pp.  14  and 
15;  W,  Fig.  15,  p.  65.) 

2.  Divisions  of  the  coelom. — At  first  the  coelom  consists  of  a  continuous  cavity  extending 
the  entire  length  of  the  trunk  region,  divided  into  two  longitudinal  halves  by  the  dorsal  and 
ventral  mesenteries.  With  the  partial  disappearance  of  the  ventral  mesentery,  the  two 
halves  of  the  coelom  are  connected  ventral  to  the  intestine  (Fig.  44(7).  In  the  adults  of  all 
vertebrates  the  coelom  is  divided  into  at  least  two  compartments  by  the  formation  of  a  par- 
tition. This  partition,  called  the  transverse  septum,  develops  at  the  posterior  end  of  the  heart 
and  cuts  off  the  heart  from  all  of  the  other  viscera.  The  transverse  septum  thus  divides  the 
coelom  into  a  small  anterior  compartment,  the  pericardial  cavity,  which  contains  only  the 
heart,  and  a  very  large  posterior  compartment,  the  pleuroperitoneal  cavity,  which  contains  all 
of  the  other  viscera  (see  Fig.  45^).  The  pericardial  cavity  in  fishes  and  urodeles  is  anterior 
to  the  pleuroperitoneal  cavity,  and  the  transverse  septum  in  those  groups  passes  transversely 
across  the  body  (Fig.  45^  and  B).  In  the  Anura  and  all  vertebrates  above  Anura  the  peri- 
cardial cavity  has  descended  posteriorly  so  that  it  comes  to  lie  ventral  to  the  anterior  part  of 
the  pleuroperitoneal  cavity;  the  transverse  septum  then  assumes  an  oblique  position  (Fig.  456"). 
In  that  portion  of  the  pleuroperitoneal  cavity  which  in  consequence  of  the  descent  of  the 
pericardial  cavity  lies  dorsal  to  the  pericardial  cavity,  the  lungs  are  situated.  This  condition 
of  the  coelom,  as  in  Figure  456",  is  found  in  Anura  and  reptiles.  In  birds  and  mammals,  the 
pleuroperitoneal  cavity  is  divided  into  anterior  and  posterior  parts  by  the  formation  of  a  parti- 
tion which  descends  from  the  dorsal  body  wall  and  unites  with  the  transverse  septum  (Fig.  45/2 
and  E).  This  partition  is  known  as  the  oblique  septum  in  birds  and  as  the  diaphragm  in  mammals. 
In  birds  it  is  non-muscular  while  in  mammals  it  is  infiltrated  with  striated  muscles  derived  from 
adjacent  myotomes  of  the  body  wall. 

The  oblique  septum  or  diaphragm  forms  immediately  posterior  to  the  lungs.  That 
portion  of  the  pleuroperitoneal  cavity  which  is  cut  off  anterior  to  the  oblique  septum  or 
diaphragm  consequently  contains  the  lungs.  It  consists  of  the  two  pleural  cavities  or  pleural 
sacs,  each  inclosing  a  lung.  The  two  pleural  cavities  are  completely  separated  from  each 
other,  the  pericardial  cavity  containing  the  heart  being  situated  in  the  median  line  between 
their  ventral  portions.  That  part  of  the  pleuroperitoneal  cavity  cut  off  posterior  to  the 
oblique  septum  or  diaphragm  is  called  the  peritoneal  or  abdominal  cavity;  it  incloses  the 
greater  part  of  the  digestive  tract  and  the  urogenital  system.  It  will  be  seen  from  this 
account  that  in  birds  and  mammals  the  coelom  is  divided  into  four  compartments — the 
pericardial  cavity,  the  two  pleural  cavities,  and  the  peritoneal  cavity. 

It  is  convenient  to  speak  of  the  viscera  as  being  inclosed  in  or  contained  in  the  coelomic 
cavities.  This  is  not,  however,  a  correct  expression,  since,  owing  to  the  fact  that  the  viscera 
are  covered  by  the  visceral  peritoneum  or  serosa,  they  are  not  really  inside  of  the  coelom  in  the 
same  sense  that  a  chair  could  be  said  to  be  inside  of  a  room.  They  are  outside  of  it  and  have 
the  same  relations  to  it  as  if  they  were  pushed  into  the  coelom  carrying  the  coelomic  wall  before 
them.  To  illustrate  farther,  one  cannot  get  into  the  inside  of  a  tent  or  a  balloon  by  pushing 
against  the  wall;  one  carries  the  tent  or  balloon  wall  before  him  and  always  remains  in  reality 
on  the  outside  of  the  tent  or  ba!loon.  Similarly,  the  viscera  are  outside  of  the  coelom  although 
they  appear  to  be  contained  within  its  cavity. 

B.      THE   DIGESTIVE   TRACT  AND   ITS   DERIVATIVES 

i.  The  origin  of  the  digestive  tract. — The  primitive  intestine  or  archenteron,  as  we 
learned  in  the  section  on  development,  is  produced  by  the  process  of  invagination  or  other 
processes  in  the  gastrula  stage  of  the  embryo.  It  is  at  first  a  simple  tube  of  entoderm  with  one 


i6o 


LABORATORY  MANUAL  FOR  VERTEBRATE  ANATOMY 


opening  to  the  exterior,  the  blastopore,  which  is  situated  at  the  future  posterior  end  of  the 
embryo.  This  entodermal  tube  persists  as  the  lining  of  the  adult  digestive  tract  and  of  all  of 
its  derivatives;  to  it  there  are  added  other  layers  (connective  tissue  and  muscular  layers) 
derived  from  the  splanchnic  mesoderm  of  the  hypomere,  which,  it  will  be  recalled,  folds  around 
the  archenteron.  The  adult  digestive  tract  thus  consists  of  a  thick-walled  tube,  composed  of 


FIG.  45. — Diagrams  to  illustrate  the  divisions  of  the  coelom  in  the  various  vertebrate  classes. 
The  transverse  septum  and  its  derivatives  are  indicated  by  thick  lines.  A,  fishes,  showing  the  division 
of  the  coelom  into  pericardial  cavity  a  and  pleuroperitoneal  cavity  g  by  means  of  the  transverse  septum  d. 
B,  urodeles,  similar  to  fishes  with  the  addition  of  the  lung  h  which  projects  into  the  pleuroperitoneal 
cavity  g.  C,  turtle;  the  pericardial  cavity  a  has  descended  posteriorly  until  it  lies  ventral  to  the  anterior 
part  of  the  pleuroperitoneal  cavity  g;  the  anterior  face  of  the  transverse  septum  d  has  now  become  part 
of  the  wall  of  the  pericardial  sac;  the  lung  h  is  retroperitoneal.  D,  early  stage  of  mammals,  showing  the 
beginning  of  the  coelomic  fold  (pleuroperitoneal  membrane)  j  descending  from  the  dorsal  body  wall,  and 
the  liver/  inclosed  within  the  transverse  septum  d.  E,  later  stage  of  mammals,  showing  union  of  the 
coelomic  fold  j  with  the  transverse  septum  d,  the  two  together  forming  the  diaphragm  which  separates 
the  pleural  cavity  k  from  the  peritoneal  cavity  m;  the  liver  has  constricted  from  the  main  part  of  the 
transverse  septum,  the  constriction  becoming  the  coronary  ligament  i.  a,  pericardial  cavity;  b,  heart; 
G,  parietal  pericardium  or  pericardial  sac;  d,  transverse  septum;  e,  serosa  of  the  liver,  this  being  a  part 
of  the  transverse  septum  originally;  /,  liver;  g,  pleuroperitoneal  cavity;  h,  lung;  i,  coronary  ligament 
of  the  liver;  j,  coelomic  fold  which  forms  part  of  the  diaphragm;  k,  pleural  cavity;  /,  pleuropericardial 
membrane  or  anterior  continuation  of  the  transverse  septum;  m,  peritoneal  cavity. 

both  entodermal  and  mesodermal  elements,  the  latter  predominating.  The  anterior  and 
posterior  ends  of  the  digestive  tube  are  formed  by  the  invagination  of  the  surface  ectoderm, 
that  at  the  anterior  end  which  becomes  the  lining  of  the  mouth  cavity  being  called  the  stomo- 
daeum,  that  at  the  posterior  end,  lining  the  anus,  the  proclodaeum. 


THE  COELOM,  DIGESTIVE,  AND  RESPIRATORY  SYSTEMS 


161 


pharynx 


visceral  pouch 
gill  slit 
thyroid  gland 


anterior  lobe 
of  hypophysis 

oral  cavity 


small  intestine 


2.  The  differentiation  of  the  digestive  tube. — Along  its  course,  the  tube  soon  differentiates 
into  various  regions  with  different  functions.  These  regions,  beginning  at  the  anterior  end,  are : 
mouth  or  oral  cavity,  pharynx,  esophagus,  stomach,  small  intestine,  large  intestine  or  colon, 
cloaca.  The  esophagus  may  be  provided  with  an  enlargement,  the  crop,  as  in  birds.  The 
stomach  may  be  subdivided  into  two  or  more  compartments,  each  with  special  functions,  as  in 
birds  and  cud-chewing  mammals.  The  first  part  of  the  small  intestine  is  named  the  duodenum; 
the  remainder,  if  of  sufficient  length,  is  subdivided  into  jejunum  and  ileum.  There  is  generally 
a  blind  pouch,  the  caecum  (sometimes  more  than  one),  at  the  junction  of  small  and  large  intes- 
tine. The  colon  may  be  divided  into  several  regions.  Its  terminal  portion  is  often  named 
the  rectum.  The  rectum  passes  into 
the  cloaca  which  opens  to  the  exterior 
by  the  anus.  The  cloaca  receives  not 
only  the  intestine  but  also  the  genital 
and  urinary  ducts. 

3.  The  outgrowths  of  the  diges- 
tive tube. — A  number  of  outgrowths 
arise  from  the  tube  at  various  levels 
(see  Fig.  46). 

a)  The  oral  glands:  The  mouth 
cavity  is  commonly  provided  with 
glands  which  consist  of  evaginations 
of  the  lining  epithelium.  These  glands 
are  of  two  kinds:  mucous  glands, 
which  secrete  a  slimy  fluid  used  to 
moisten  the  mouth  cavity  and  the 
food  and  in  some  forms  as  an  aid  in 
capturing  prey;  and  salivary  glands, 

characteristic  of  mammals,  in  which  'r~^ ^    /vV  —^ doaca 

the  secretion  contains,  not  only  mucus, 
but  also  digestive  enzymes.  The 
names  of  the  oral  glands  indicate 
their  positions  in  the  walls  of  the 
mouth  cavity. 

6)  The  anterior  lobe  of  the  hypoph- 
ysis: From  the  roof  of  the  mouth 

cavity  an  evagination  occurs  in  the  embryo,  producing  a  blind  pouch  which  extends  toward 
and  comes  in  contact  with  the  floor  of  the  brain  in  a  certain  region.  This  pouch,  known  as 
the  anterior  lobe  of  the  hypophysis,  fuses  with  a  portion  of  the  brain  wall,  the  compound  struc- 
ture thus  formed  being  designated  the  hypophysis  or  pituitary  body.  It  is  one  of  the  glands  of 
internal  secretion. 

c)  The  thyroid  gland:  This  is  an  outgrowth  from  the  floor  of  the  pharynx  midway  between 
the  second  gill  slits.    It  is  homologous  with  the  endostyle  of  lower  chordates  and  is  a  gland  of 
internal  secretion,  producing  a  secretion  necessary  for  health  and  growth. 

d)  The  visceral  pouches  and  gills:  The  visceral  pouches  are  paired  evaginations  from  the 
wall  of  the  pharynx,  typically  five  or  six  in  number  in  vertebrates.    Opposite  each  visceral 
pouch,  the  ectoderm  invaginates,  forming  the  visceral  furrow.    Visceral  pouches  and  furrows 
meet  at  their  extremities  and  the  point  of  fusion  breaks  through,  the  opening  being  known  as  a 
gill  slit  or  visceral  cleft  (see  Fig.  47).    The  entoderm  lining  the  visceral  pouch  grows  out  into 
thin  plates  or  delicate  filaments,  the  gill,  used  for  respiration.    The  tissue  between  successive 
visceral  pouches  and  slits  is  called  the  visceral  arch.    In  the  center  of  each  visceral  arch  is  a 


FIG.  46. — Diagram  to  illustrate  the  chief  derivatives  of 
the  digestive  tract.  (From  McMurrich's  Development  of 
the  Human  Body,  after  His,  copyright  by  P.  Blakiston's 
Son  and  Company.) 


1 62       LABORATORY  MANUAL  FOR  VERTEBRATE  ANATOMY 

cartilaginous  or  bony  gill  arch,  already  described  in  connection  with  the  skeleton.  Certain  blood 
vessels,  the  aortic  arches,  also  traverse  the  visceral  arches.  The  visceral  muscles,  already 
described,  develop  from  the  tissue  of  the  visceral  arches.  During  the  change  from  the  aqua- 
tic to  the  terrestrial  habit  of  life  the  visceral  clefts,  pouches,  arches,  and  their  derivatives, 
undergo  profound  alterations. 

e)  The  tympanic  cavity  and  the  external  auditory  meatus:  Beginning  with  Amphibia  the 
first  visceral  pouch  forms  a  pouchlike  outgrowth  in  the  direction  of  the  internal  ear.  The  end 
of  this  outgrowth  expands  into  a  chamber,  the  tympanic  cavity  or  cavity  of  the  middle  ear.  The 
stalk  of  the  outgrowth  forms  the  auditory  or  Eustachian  tube,  connecting  the  pharyngeal  cavity 
with  the  tympanic  cavity.  In  some  reptiles,  in  birds,  and  mammals  an  invagination  occurs 
corresponding  to  the  position  of  the  first  external  gill  slit.  The  bottom  of  this  invagination 
meets  the  wall  of  the  tympanic  cavity  forming  at  the  place  of  contact  a  membrane  of  double 


FIG.  47. — Diagrams  to  illustrate  the  formation  of  the  visceral  pouches,  furrows,  arches,  and  clefts. 
A,  early  stage,  showing  the  evaginations  a  from  the  wall  of  the  pharynx  b,  which  form  the  visceral 
pouches.  B,  later  stage,  illustrating  formation  of  the  visceral  furrows  c  by  invagination  of  the  surface 
ectoderm.  C,  later  stage  showing  formation  of  the  external  gill  slits  g  by  the  union  of  the  visceral 
pouches  and  furrows  and  breaking  through  of  the  points  of  union,  a,  visceral  pouch;  b,  pharyngeal 
cavity;  c,  visceral  furrow;  d,  tympanic  cavity;  e,  thyroid  gland;  /,  internal  gill  slit;  g,  external  gill 
slit;  h}  visceral  arch;  i,  ectoderm;  j,  entoderm. 

origin,  the  tympanic  membrane  (ear  drum).    The  passage  formed  by  the  invagination  is  the 
external  auditory  meatus. 

/)  Glandlike  derivatives  of  the  visceral  pouches:  In  the  adults  of  land  vertebrates  the 
visceral  pouches  and  clefts  disappear,  leaving,  however,  certain  bodies  which  are  produced 
by  proliferation  of  the  lining  epithelium  of  the  visceral  pouches.  These  glandlike  bodies, 
remnants  of  the  visceral  pouches,  are  very  variable  in  number  and  mode  of  origin  in  different 
vertebrates.  Among  them  may  be  mentioned:  the  true  or  palatine  tonsils,  from  the  second 
visceral  pouch,  and  the  thymus,  parathyroids,  and  the  postbranchial  or  epithelial  bodies,  from 
a  variable  number  of  visceral  pouches.  The  three  last  named  are  glandlike  bodies  which  persist 
in  the  neck  region  and  appear  to  belong  to  the  category  of  the  glands  of  internal  secretion, 
although  their  function  is  uncertain.  These  glands  are  present  although  imperfectly  developed 
in  fishes. 

g)  The  trachea  and  lungs:  When  the  vertebrates  adopted  the  land  habitat,  the  gill  slits 
and  gills  disappeared  in  the  adult,  and  their  physiological  role  was  taken  over  by  a  new  out- 
growth from  the  pharynx.  This  outgrowth  is  a  tube  which  evaginates  from  the  midventral 


THE  COELOM,  DIGESTIVE,  AND  RESPIRATORY  SYSTEMS  163 

line  of  the  pharynx.  At  its  distal  end  it  divides  into  two  sacs.  The  tube  is  the  trachea  or  wind- 
pipe, and  the  two  sacs  at  its  end  are  the  lungs.  Both  trachea  and  lungs  subsequently  enlarge 
and  become  subdivided,  and  their  structure  is  further  complicated  by  the  addition  of  mesoder- 
mal  tissues  (connective  tissue,  smooth  muscle,  cartilage)  to  the  original  simple  entodermal 
layer. 

h)  The  swim  bladder:  This  is  an  outgrowth  from  the  digestive  tract,  occurring  only  in  the 
teleostome  fishes.  It  is  at  first  connected  with  the  digestive  tract,  generally  with  the  esophagus, 
on  either  dorsal  or  ventral  side  by  a  duct,  which  persists  throughout  life  in  ganoids.  The  swim 
bladder  lies  just  internal  to  the  dorsal  body  wall  of  teleostomes  and  is  supposed  to  have  hydro- 
static functions.  It  is  not  improbable  that  the  swim  bladder  is  the  forerunner  of  lungs. 

i)  The  liver:  The  liver  is  a  very  large  gland  which  arises  by  outgrowth  from  the  intestine. 
The  stalk  of  the  outgrowth  becomes  the  bile  duct,  which  enlarges  on  the  surface  of  the  liver  to 
form  the  gall  bladder.  The  liver  is  situated  between  the  two  layers  of  the  ventral  mesentery 
and  is  also  located  in  the  transverse  septum  but  has  grown  so  large  that  it  projects  extensively 
out  of  these  structures  (see  Figs.  44$  and  48). 

y)  The  pancreas:  This  gland  arises  by  one  to  four,  generally  three,  outgrowths,  from  the 
intestine  slightly  posterior  to  the  origin  of  the  liver.  The  stalks  of  the  outgrowths  become  the 
pancreatic  ducts.  The  outgrowths  combine  to  form  one  gland  of  various  form  in  different 
vertebrates.  The  pancreas  may  lie  between  the  two  layers  of  the  dorsal  mesentery,  the 
ventral  mesentery,  or  both. 

k)  The  yolk  sac:  In  the  embryos  of  most  vertebrates  a  yolk -filled  sac  projects  from  the 
intestine.  The  wall  of  this  sac  is  part  of  the  intestinal  wall,  which  may  be  regarded  as  having 
been  stretched  out  into  a  sac.  The  yolk  in  the  sac  is  used  for  the  growth  of  the  embryo,  and 
at  the  completion  of  the  embryonic  stage  the  yolk  sac  is  greatly  reduced  in  size  and  is  drawn 
into  the  intestinal  wall. 

I)  The  urinary  bladder:  This  is  a  large  sac  growing  out  from  the  ventral  wall  near  the 
posterior  termination  of  the  intestine.  In  the  embryos  of  reptiles,  birds,  and  mammals  it  is 
represented  by  a  large  sac,  the  allantois,  which  projects  beyond  the  limits  of  the  body.  The 
urinary  bladder  is  inclosed  between  the  two  layers  of  the  ventral  mesentery. 

The  structures  discussed  in  this  introduction  will  be  better  understood  after  they  have 
been  studied  in  the  dissections. 

Read  also  the  accounts  of  the  comparative  anatomy  of  the  coelem,  digestive,  and  respira- 
tory systems  in  K,  W,  or  Wd. 

C.     THE  COELOM,  DIGESTIVE,  AND  RESPIRATORY  SYSTEMS  OF  ELASMOBRANCHS 

The  following  directions  apply  chiefly  to  the  spiny  dogfish  but  may  also  be 
used  for  the  smooth  dogfish  and  skate,  the  minor  differences  between  these 
animals  being  specified  where  necessary. 

i.  The  body  wall  and  the  pleuroperitoneal  cavity. — Make  an  incision  from 
the  left  side  of  the  cloaca  forward  through  the  pelvic  girdle  slightly  to  the  left 
of  the  midventral  line  up  to  the  pectoral  girdle.  The  incision  will  probably 
cut  through  the  skin  first  and  should  then  be  extended  through  the  muscle  layer. 
To  assist  in  exposing  the  interior  a  transverse  incision  may  be  made  in  the 
middle  of  the  lateral  body  wall  on  each  side.  In  the  skate  cut  along  the  left  side 
of  the  cloaca  and  then  along  both  lateral  borders  of  the  body  cavity  but  not 
anteriorly,  leaving  the  flap  of  body  wall  adhering  to  the  pectoral  girdle.  The 
large  internal  cavity  is  the  pleuroperitoneal  cavity  and  constitutes  the  greater 


164       LABORATORY  MANUAL  FOR  VERTEBRATE  ANATOMY 

part  of  the  coelom.  It  is  lined  by  a  smooth  shining  membrane,  the  parietal  peri- 
toneum, which  adheres  closely  to  the  inside  of  the  body  wall.  The  body  wall 
is  seen  to  be  composed  of  three  layers:  skin,  muscles,  and  parietal  peritoneum. 
2.  The  viscera  of  the  pleuroperitoneal  cavity. — Within1  the  cavity  are  a 
number  of  organs  or  viscera,  most  of  which  belong  to  the  digestive  tract.  At 
the  anterior  end  of  the  cavity  is  the  large  brownish  or  grayish  liver.  This  con- 
sists in  the  spiny  dogfish  of  long  left  and  right  lobes  and  a  small  median  lobe  in 
which  is  located  the  long  greenish  gall  bladder.  In  the  smooth  dogfish,  the  liver 
is  subdivided  into  right  and  left  lobes,  the  median  lobe  being  absent;  the  gall 
bladder  is  imbedded  in  the  anterior  part  of  the  left  lobe  and  is  visible  as  a  thin 
place  in  the  liver.  In  the  skate  the  liver  is  composed  of  right,  median,  and  left 
lobes  of  equal  length  and  size,  and  the  gall  bladder  is  situated  in  the  angles  between 
the  right  and  median  lobes.  Dorsal  to  the  liver  on  the  left  side  is  the  large 
J -shaped  stomach,  often  distended  with  food.  (In  some  specimens  the  stomach 
is  everted  into  the  mouth  cavity  and  should  be  pulled  back  into  the  pleuroperito- 
neal cavity  by  exerting  a  gentle  traction  on  it.)  The  greater  part  of  the  stomach 
consists  of  a  large  straight  tube  extending  from  the  anterior  end  of  the  pleuro- 
peritoneal cavity  to  a  point  somewhat  posterior  to  the  ends  of  the  liver  lobes. 
It  then  makes  a  sharp  bend,  decreasing  in  diameter  considerably,  and  extends 
anteriorly,  terminating  in  a  constriction,  the  pylorus.  Along  the  posterior  margin 
of  the  bend  of  the  stomach  (or  in  the  skate  on  the  dorsal  side  of  the  bend)  is  a 
dark-colored  organ,  the  spleen,  a  part  of  the  lymphatic  system.  From  the 
pylorus  the  short  intestine  extends  to  the  anus.  The  first  part  of  the  intestine 
beyond  the  pylorus  is  called  the  duodenum.  It  extends  for  a  short  distance  to 
the  right  and  then  curves  posteriorly.  The  bile  duct,  a  long  stout  duct,  is 
easily  seen  descending  from  the  gall  bladder  to  enter  the  duodenum  shortly 
caudad  of  the  bend.  The  bile  duct  accompanied  by  some  blood  vessels  runs 
in  a  strip  of  mesentery.  It  passes  to  the  dorsal  side  of  the  duodenal  wall  and 
runs  for  a  short  distance  caudad  imbedded  in  the  wall  before  it  penetrates  into 
the  cavity  of  the  duodenum.  In  the  curve  of  the  duodenum  reposes  the  ventral 
lobe  of  a  white  gland,  the  pancreas.  The  dorsal  lobe  of  the  pancreas,  which  is 
long  and  slender  in  the  spiny  dogfish,  reaching  to  the  spleen,  should  be  located  by 
raising  the  stomach  and  duodenum  and  looking  dorsal  to  them.  The  duct  of  the 
pancreas  is  somewhat  difficult  to  find  in  the  dogfishes,  less  difficult  in  the  skate; 
it  lies  imbedded  in  the  tissue  of  the  pancreas  near  the  posterior  margin  of  the 
ventral  lobe  and  may  be  exposed  by  picking  away  the  pancreas  tissue  in  this 
region.  Beyond  the  duodenum  the  intestine  widens  considerably,  and  its 
surface  is  marked  by  parallel  rings.  These  rings  are  the  lines  of  attachment  of 
a  spiral  fold,  the  spiral  valve,  which  occupies  the  interior  of  the  intestine.  (A 
portion  of  the  intestine  often  protrudes  through  the  anus  and  should  be  pulled 
back  into  the  coelom  by  grasping  the  portion  in  the  cavity  and  exerting  a  gentle 

1  It  has  already  been  explained  that  the  organs  are  not  in  reality  inside  of  the  coelom. 


THE  COELOM,  DIGESTIVE,  AND  RESPIRATORY  SYSTEMS  165 

pull.)  At  the  posterior  end  of  the  intestine  is  a  small  cylindrical  body,  the 
rectal  gland,  attached  to  the  intestine  by  a  duct.  It  is  possibly  excretory  in 
function.  The  rectal  gland  marks  the  division  between  large  and  small  intestine, 
that  part  of  the  intestine  anterior  to  its  attachment  being  the  small  intestine, 
that  posterior  to  it,  the  large  intestine.  The  latter  is  so  short  as  to  be  almost 
absent  and  opens  at  once  into  a  terminal  chamber,  the  cloaca,  which  opens  to 
the  exterior  through  the  anus. 

Cut  open  the  stomach  and  wash  out  its  contents.  Partly  disintegrated 
fish  and  squids  are  commonly  found  in  the  stomach.  Observe  the  folds  or 
rugae  in  the  walls  of  the  posterior  part  of  the  stomach  and  the  papillae  project- 
ing from  the  walls  of  the  anterior  part.  Cut  open  the  small  intestine  along  one 
side  midway  between  the  large  blood  vessels  which  traverse  its  walls  longitudi- 
nally. Observe  the  spiral  valve  in  its  interior.  It  consists  of  a  fold  of  the  intesti- 
nal wall  spirally  coiled  so  as  to  make  a  series  of  overlapping  cones.  The  purpose  of 
the  spiral  valve  is  to  increase  the  digestive  and  absorptive  surface  of  the  intestine. 

The  reproductive  organs  and  their  ducts  in  part  may  also  be  identified  at 
this  time.  In  the  spiny  dogfish  and  the  skate  the  gonads  are  a  pair  of  soft  bodies 
located  dorsal  to  the  anterior  part  of  the  stomach.  The  lobes  of  the  liver  must 
be  raised  to  see  them.  In  the  smooth  dogfish  they  are  long  slender  bodies  extend- 
ing the  entire  length  of  the  cavity  dorsal  to  the  digestive  tract  and  terminating 
at  the  rectal  gland.  In  mature  females  the  oviducts  are  noticeable  as  stout 
white  tubes,  one  on  each  side,  in  contact  with  the  dorsal  walls  of  the  coelom. 

3.  The  mesenteries. — The  viscera  are  held  in  place  by  delicate  membranes, 
the  mesenteries,  whose  mode  of  origin  was  explained  in  the  introduction  to  this 
section.  In  studying  them  lift  and  spread  each  organ  as  it  is  mentioned.  The 
dorsal  mesentery  extends  from  the  median  dorsal  line  of  the  coelom  to  the  diges- 
tive tract  but  is  not  complete  in  the  animals  under  consideration,  a  gap  being 
present  in  the  region  of  the  small  intestine.  That  part  of  the  dorsal  mesentery 
supporting  the  stomach  is  called  the  mesogaster;  in  the  skate  it  is  limited  to  the 
anterior  part  of  the  stomach.  The  mesogaster  incloses  the  spleen  between  its 
two  walls,  and  that  portion  of  the  mesogaster  from  the  spleen  to  the  stomach 
is  the  gastrosplenic  ligament.  That  portion  of  the  dorsal  mesentery  which  sup- 
ports the  small  intestine  is  called  the  mesentery,  in  the  limited  sense.  This 
is  absent  in  the  skate.  In  the  spiny  dogfish  there  is  a  fusion  between  the  mesen- 
tery and  the  mesogaster  so  that  a  sort  of  pocket  is  formed  dorsal  to  the  bend  of 
the  stomach.  In  the  dorsal  wall  of  this  pocket  is  located  the  greater  part  of  the 
pancreas  which  is  thus  in  the  dorsal  mesentery.  The  dorsal  mesentery  begins 
again  in  the  region  of  the  rectal  gland,  this  portion  of  the  mesentery  being  named 
the  mesorectum. 

The  ventral  mesentery  is  represented  in  these  animals,  as  in  all  vertebrates, 
by  remnants  only.  Such  a  remnant  is  the  gastro-hepato-duodenal  ligament  ex- 
tending from  the  right  side  of  the  stomach  to  the  liver  and  duodenum.  It  is 


166       LABORATORY  MANUAL  FOR  VERTEBRATE  ANATOMY 

also  called  the  lesser  omenlum.  It  may  be  roughly  divided  into  two  portions,  the 
hepatoduodenal  ligament  extending  from  the  liver  to  the  duodenum  and  contain- 
ing the  bile  duct  and  blood  vessels,  and  the  gastrohepatic  ligament  extending 
from  the  stomach  to  the  liver  and  duodenum  and  in  the  dogfishes  occupying  also 
the  angle  formed  by  the  bend  of  the  stomach.  Another  remnant  of  the  ventral 
mesentery  is  the  suspensory  or  falciform  ligament  of  the  liver.  This  will  be 
found  at  the  anterior  end  of  the  liver,  extending  from  the  mid  ventral  surface 
of  the  liver  to  the  midventral  line  of  the  body  wall.  In  mature  females  the 
mouth  of  the  oviduct  will  be  noticed  in  the  falciform  ligament  as  a  funnel-shaped 
aperture.  After  seeing  the  falciform  ligament  the  flap  of  body  wall  left  in  the 
skate  may  be  cut  off. 

Each  gonad  has  a  mesentery  which  is  a  special  fold  arising  from  the  dorsal 
wall  very  near  the  origin  of  the  dorsal  mesentery.  The  mesentery  of  the  ovary 
is  called  the  mesovarium,  of  the  testis,  the  mesorchium.  In  the  case  of  mature 
females,  each  oviduct  has  also  a  mesentery,  the  mesotubarium. 

The  anterior  end  of  the  pleuroperitoneal  cavity  will  be  found  closed  by  a 
partition,  the  transverse  septum,  the  posterior  face  of  which  is  clothed  by  the 
parietal  peritoneum.  The  liver  is  attached  to  the  septum  by  the  strong  coronary 
ligament  which  is,  in  fact,  a  portion  of  the  septum.  In  its  early  development  the 
liver  is  inclosed  in  the  transverse  septum,  and  subsequently,  because  of  increased 
size,  projects  posteriorly  from  the  septum  which  then  narrows  around  the  anterior 
end  of  the  liver  and  forms  the  coronary  ligament  (see  Fig.  48,  p.  195). 

The  pleuroperitoneal  cavity  communicates  with  the  exterior  by  means  of 
the  abdominal  pores.  These  will  be  found  one  on  each  side  of  the  anal  opening 
(in  the  skate  posterior  to  the  anus)  somewhat  concealed  by  a  fold  of  skin.  Probe 
into  them  and  note  that  they  lead  into  the  pleuroperitoneal  cavity.  Their 
purpose  is  obscure. 

The  pleuroperitoneum  is  the  lining  of  the  hypomere  and  the  pleuroperitoneal  cavity  is 
the  cavity  of  the  hypomere.  The  pleuroperitoneal  membrane  is  divided  into  three  regions, 
according  to  its  relations  to  other  structures:  first,  the  parietal  peritoneum,  that  portion  of  the 
membrane  lining  the  inner  surface  of  the  body  wall;  second,  the  visceral  peritoneum,  forming 
the  thin  outer  covering  of  all  of  the  viscera;  and  third,  the  mesenteries  or  ligaments,  portions 
of  the  membrane  extending  from  the  body  wall  to  the  viscera  or  from  one  viscus  to  another. 

Draw  the  contents  of  the  pleuroperitoneal  cavity.  Make  a  diagram  of  an 
imaginary  section  through  the  anterior  end  of  the  pleuroperitoneal  cavity,  show- 
ing gonads,  stomach,  and  liver,  and  the  relations  of  the  pleuroperitoneum  to 
them  and  to  the  body  wall. 

4.  The  pericardial  cavity. — Make  an  incision  through  the  skin  in  the  median 
ventral  line  from  the  pectoral  girdle  up  to  the  lower  jaw.  Leave  the  girdle 
intact.  Gently  strip  off  with  a  forceps  the  layers  of  visceral  muscle  until  you 
have  exposed  a  membrane.  This  membrane  is  the  parietal  pericardium.  Slit 
open  this  membrane  and  see  that  it  incloses  a  cavity,  the  pericardial  cavity,  in 


THE  COELOM,  DIGESTIVE,  AND  RESPIRATORY  SYSTEMS  167 

which  the  heart  is  situated.  To  reveal  this  cavity  more  fully  cut  laterally  along 
the  anterior  face  of  the  girdle  keeping  your  instrument  in  contact  with  the  girdle. 
Portions  of  the  girdle  may  be  sliced  away,  but  the  heart  must  not  be  injured. 
The  pericardial  cavity  is  thus  revealed  as  a  conical  cavity  lined  by  the  parietal 
pericardium  and  containing  the  heart.  By  gently  lifting  up  the  heart  note 
that  it  is  attached  only  at  its  anterior  and  posterior  ends.  At  these  places 
the  pericardial  lining  is  deflected  from  the  walls  of  the  pericardial  cavity  and 
passes  over  the  surface  of  the  heart  as  a  covering  layer,  the  visceral  pericardium, 
which  is  indistinguishably  fused  with  the  heart  wall.  With  the  heart  lifted 
note  that  the  posterior  end  of  the  heart  is  a  fan-shaped  chamber,  the  sinus 
venosus,  whose  walls  are  continuous  with  the  partition  that  forms  the  posterior 
wall  of  the  pericardial  cavity.  This  partition  is  the  transverse  septum,  whose 
posterior  face  we  have  already  seen.  The  septum  is  thus  seen  to  be  a  partition 
whose  anterior  wall  is  composed  of  the  parietal  pericardium  and  whose  posterior 
wall,  of  the  parietal  peritoneum.  The  wings  of  the  sinus  venosus  are  buried  in 
the  transverse  septum ;  they  constitute  large  venous  channels  through  which  the 
venous  blood  is  returned  to  the  heart. 

We  may  now  explain  the  formation  of  the  transverse  septum.  Since  the 
heart  is  situated  in  the  ventral  part  of  the  body  it  is  necessary  in  order  that  the 
blood  from  the  dorsal  body  wall  may  reach  the  heart  that  a  bridge  be  formed 
passing  from  the  dorsal  to  the  ventral  side.  In  early  embryonic  stages  a  bridge 
or  cylinder  of  mesoderm  develops  on  each  side  of  the  posterior  end  of  the  heart 
connecting  the  splanchnic  mesoderm  surrounding  the  sinus  venosus  with  the 
somatic  mesoderm  of  the  dorsal  body  wall.  In  these  bridges  the  main  venous 
channels  pass  from  the  dorsal  body  wall  into  the  sinus  venosus.  Later,  the 
bridges  enlarge  and  finally  fuse  with  each  other  and  with  the  body  wall  laterally, 
forming  a  partition,  the  transverse  septum,  which  thus  cuts  the  heart  off  from 
the  remainder  of  the  coelom.  In  elasmobranchs  the  fusion  is  not  quite  complete, 
leaving  an  opening,  the  pericardia- peritoneal  canal,  dorsal  to  the  sinus  venosus, 
extending  from  the  pericardial  cavity  into  the  pleuroperitoneal  cavity.  This 
opening  will  be  seen  at  a  later  time. 

5.  The  mouth  and  pharyngeal  cavities  and  the  respiratory  system. — Insert 
one  blade  of  a  scissors  into  the  left  corner  of  the  mouth  and  make  a  cut  through 
the  angle  of  the  jaws  back  across  the  ventral  parts  of  the  gill  slits  through  the 
pectoral  girdle  so  that  you  emerge  to  the  left  side  of  the  stomach.  A  flap  is 
thus  formed  which  should  be  turned  over  to  the  right.  A  large  cavity  is  revealed 
which  at  its  posterior  end  converges  into  the  extremely  short  esophagus  which 
passes  at  once  into  the  stomach.  The  esophagus  may  be  slightly  slit  to  aid  in 
opening  the  flap. 

The  anterior  part  of  the  cavity  inclosed  by  the  jaws  and  gill  arches  is  the 
mouth  or  oral  cavity.  It  is  bounded  in  front  by  the  upper  and  lower  jaws,  pro- 
vided with  teeth.  The  upper  and  lower  jaws  are  the  two  halves  of  the  first  or 


i68       LABORATORY  MANUAL  FOR  VERTEBRATE  ANATOMY 

mandibular  gill  arch;  the  section  of  the  arch  should  be  identified  in  the  cut 
surface.  On  the  floor  of  the  mouth  back  of  the  teeth  is  the  tongue;  it  forms  a 
flat,  slight  projection,  which  is  practically  immovable.  It  is  supported  by  the 
second  or  hyoid  gill  arch  which  should  be  felt  within  it  and  identified  in  section 
at  the  cut  surface.  (The  tongue  is  absent  in  the  skate,  owing  to  the  reduction 
of  the  hyoid  arch.) 

The  posterior  and  greater  part  of  the  cavity  under  consideration  is  the  pharynx. 
Its  wall  is  pierced  by  six  internal  gill  slits.  The  first  of  these,  the  spiracle,  is  a 
rounded  opening  in  the  roof  of  the  mouth  immediately  posterior  to  the  mandibular 
arch.  The  remaining  five  gill  slits  are  elongated.  The  internal  gill  slits  communi- 
cate with  large  cavities — the  visceral  pouches — which  in  turn  open  to  the  exterior 
by  way  of  the  external  gill  slits.  The  tissue  between  successive  visceral  pouches  is 
a  visceral  arch.  The  parts  of  a  visceral  arch  should  be  examined  in  section  on  the 
left  side  where  the  arches  have  been  cut  through.  The  center  of  the  section  of 
each  visceral  arch  is  formed  by  theinterbranchial  septumwhich  extends  to  the  outei 
surface  of  the  body,  where  the  spaces  between  successive  septa  form  the  external 
gill  slits.  On  each  face  of  the  interbranchial  septum  is  borne  a  series  of  low,  thin 
folds  or  plates,  the  branchial  or  gill  filaments.  The  set  of  filaments  on  one  face 
of  the  septum  constitutes  a  half-gill  or  demibranch  and  the  demibranchs  on  the 
two  sides  of  a  septum  together  constitute  a  whole  branchia  or  gill.  The  gills 
are  outgrowths  of  the  walls  of  the  visceral  pouches  and  are  covered  with  entoderm. 
By  examining  all  of  the  septa  determine  how  many  demibranchs  are  present 
and  where  they  are  missing.  In  the  inner  end  of  the  section  of  each  visceral 
arch  locate  the  cross-section  of  the  cartilaginous  gill  arch  and  external  to  this, 
lying  in  the  septum,  the  cartilages  of  the  gill  rays.  Just  external  to  the  section 
of  the  gill  arch  is  the  section  of  a  blood  vessel — the  afferent  branchial  vessel— 
which  brings  venous  blood  to  the  gills.  At  each  side  of  the  gill  arch  is  a  section 
of  another  vessel,  which  is  injected  with  a  colored  solution;  these  are  the  efferent 
branchial  vessels,  which  carry  the  aerated  blood  away  from  the  gills.  Note  the 
fine  branches  of  these  vessels  in  the  gill  filaments.  The  gills  are  the  respiratory 
mechanism  of  the  animal  in  which  the  blood  obtains  oxygen  and  gives  off  carbon- 
dioxide.  Water  is  kept  flowing  over  the  gills  by  movements  of  the  gill  arches. 

Draw  the  mouth  cavity  and  pharynx.  Draw  one  visceral  arch  and  all  of 
its  parts  in  cross-section. 

D.      THE   COELOM,  DIGESTIVE,  AND  RESPIRATORY   SYSTEMS   OF   NECTURUS 

Obtain  a  specimen  and  place  in  a  wax-bottomed  dissecting  pan,  fastening 
it  ventral  side  up  by  pins  through  the  legs. 

i.  The  viscera  of  the  pleuroperitoneal  cavity. — Make  a  longitudinal  incision 
through  the  body  wall  a  little  to  the  left  of  the  median  ventral  line  from  the  left 
side  of  the  anus  through  the  pelvic  girdle  to  the  pectoral  girdle.  Spread  apart 
the  two  flaps  of  the  body  wall  and  look  within.  The  large  cavity  is,  as  in  fishes, 


THE  COELOM,  DIGESTIVE,  AND  RESPIRATORY  SYSTEMS  169 

the  pleuro peritoneal  cavity,  lined  by  the  pleuroperitoneum.  The  body  wall 
consists  of  skin,  muscle,  and  peritoneum  as  can  be  seen  on  the  cut  surface. 
In  the  median  ventral  line  on  the  inside  of  the  abdominal  wall  runs  a  large  vein, 
the  ventral  abdominal  vein. 

Examine  the  viscera.  The  liver  is  the  large  greenish  or  brownish  organ 
occupying  the  anterior  half  of  the  pleuroperitoneal  cavity.  Its  margins  are 
divided  into  several  scallop-like  lobes  by  shallow  indentations.  It  is  united  to 
the  median  ventral  line  by  a  mesentery  which  should  not  be  disturbed  at  present. 
On  raising  the  left  side  of  the  liver,  the  elongated  stomach  will  be  seen  dorsal  to 
the  liver.  Along  the  left  side  of  the  stomach  is  situated  the  dark-colored  spleen. 
On  raising  the  spleen  there  will  be  seen  dorsal  to  it  and  lying  along  the  left  side 
of  the  stomach,  the  left  lung,  a  very  long  slender  tubular  structure  which  termi- 
nates some  distance  posterior  to  the  liver.  Follow  the  stomach  posteriorly.  It 
is  a  straight  tube,  somewhat  shorter  than  the  liver,  terminating  at  a  constriction, 
the  pylorus.  From  the  pylorus  the  small  intestine  begins  and  makes  an  abrupt 
right-angled  bend  to  the  right.  In  this  bend  rests  a  white  gland,  the  pancreas, 
which  also  extends  onto  the  dorsal  surface  of  the  liver.  That  part  of  the  small 
intestine  in  contact  with  the  pancreas  is  known  as  the  duodenum.  On  raising 
the  duodenum  and  looking  on  its  dorsal  side  the  pancreas  will  be  seen  to  send 
one  tail  toward  the  spleen  and  another  posteriorly  along  the  small  intestine. 
The  small  intestine  proceeds  posteriorly  somewhat  coiled.  In  the  case  of  females 
it  will  be  found  coiled  on  the  ventral  surface  of  the  large  ovaries,  on  the  surface 
of  which  the  eggs  will  be  noted.  (The  size  of  the  ovaries  varies  with  the  sexual 
state  of  the  animal.)  To  each  side  and  dorsal  to  the  ovaries  is  a  large,  white, 
much-coiled  tube,  the  oviduct.  Trace  the  intestine,  posteriorly  pressing  the 
ovaries  away  from  the  median  line.  The  small  intestine  widens  near  the  anus 
into  the  short  large  intestine.  It  lies  in  female  specimens  between  the  posterior 
terminations  of  the  two  oviducts.  At  the  posterior  end  of  the  pleuroperitoneal 
cavity  ventral  to  the  large  intestine  will  be  found  a  sac,  generally  collapsed  and 
shriveled,  the  urinary  bladder.  Note  the  stalk  by  which  it  is  attached  to  the 
ventral  side  of  the  intestine.  That  part  of  the  intestine  to  which  the  urinary 
bladder  is  attached  (and  into  which  the  ducts  of  the  kidneys  and  gonads  also 
open)  is  the  cloaca.  It  terminates  at  the  anus. 

The  female  gonads  and  ducts  have  already  been  noted.  The  left  male 
gonad  will  be  found  dorsal  to  the  intestine  and  posterior  to  the  spleen.  Dorsal 
to  the  gonad  will  be  seen  the  left  kidney  with  its  duct  attached  to  its  left  margin. 

2.  The  mesenteries.— The  digestive  tract  is  attached  for  most  of  its  length 
to  the  median  dorsal  line  of  the  coelom  by  the  dorsal  mesentery.  This  should 
be  noted  by  pressing  other  organs  away  from  the  median  line.  It  is  missing  in 
the  pyloric  region  of  the  stomach.  That  portion  of  the  dorsal  mesentery  support- 
ing the  stomach  is  the  mesogaster.  The  spleen  is  inclosed  in  the  mesogaster, 
that  portion  of  the  mesogaster  which  extends  from  the  spleen  to  the  stomach 


1 70       LABORATORY  MANUAL  FOR  VERTEBRATE  ANATOMY 

being  designated  the  gastrosplenic  ligament.  The  lung  is  also  attached  to  the 
mesogaster  by  a  short  mesentery.  That  part  of  the  dorsal  mesentery  supporting 
the  small  intestine  is  the  mesentery,  in  the  limited  sense;  that  part  supporting 
the  large  intestine  is  the  mesorectum. 

The  ventral  mesentery  is  present  only  in  the  region  of  the  liver  and  urinary 
bladder.  One  part  of  it  forms  the  long  mesentery  extending  between  the  median 
ventral  line  of  the  body  wall  and  the  median  line  of  the  ventral  face  of  the  liver. 
This  is  the  falciform  ligament  of  the  liver.  It  contains  a  number  of  blood  vessels 
which  pass  from  the  ventral  body  wall  into  the  substance  of  the  liver  (where 
they  join  the  hepatic  portal  vein) .  In  the  free  posterior  margin  of  the  falciform 
ligament  the  ventral  abdominal  vein  crosses  from  the  body  wall  to  the  liver. 
On  raising  the  liver  the  gastrohepatic  ligament  will  be  seen  extending  from  the 
anterior  part  of  the  stomach  to  the  dorsal  face  of  the  liver.  In  the  region  of  the 
pancreas  the  hepatoduodenal  ligament  joins  the  duodenum  and  liver  and  incloses 
the  greater  part  of  the  pancreas.  The  tails  of  the  pancreas,  however,  are  sit- 
uated in  the  mesentery  of  the  small  intestine.  Both  of  the  ligaments  just 
mentioned  are  parts  of  the  ventral  mesentery.  The  last  part  of  this  mesentery 
is  found  extending  from  the  urinary  bladder  to  the  midventral  line  of  the  body 
wall;  this  is  the  median  ligament  of  the  bladder. 

Each  gonad  has  a  mesentery:  mesovarium  in  the  female,  mesorchium  in  the 
male.  The  mesentery  of  the  oviduct  is  the  mesotubarium.  These  should  be 
located  by  lifting  up  the  structures  in  question. 

The  falciform  ligament  should  now  be  severed  without,  however,  cutting 
through  the  ventral  abdominal  vein.  The  numerous  lobes  of  the  liver  appearing 
as  scallops  of  the  margin  may  now  be  seen  more  clearly.  On  raising  the  right 
side  of  the  liver,  the  right  lung  may  be  identified  dorsal  to  it.  Is  it  of  the  same 
length  as  the  left  lung  ?  The  small  gall  bladder  will  be  seen  on  the  dorsal  surface 
of  the  right  side  of  the  liver.  Its  duct  surrounded  by  pancreas  tissue  may  be 
readily  traced  to  the  duodenum.  The  pancreas  is  said  to  open  into  the  duode- 
num by  a  number  of  fine  ducts. 

The  anterior  end  of  the  pleuroperitoneal  cavity  is  closed  by  a  membrane, 
the  transverse  septum.  The  liver  is  attached  to  this  by  the  coronary  ligament, 
which  is  continuous  posteriorly  with  the  falciform  ligament.  The  mode  of  forma- 
tion of  the  septum  and  the  coronary  ligament  was  described  in  connection  with 
the  dogfish. 

3.  The  pericardial  cavity. — Make  a  median  ventral  incision  through  the 
skin  from  the  level  of  the  fore  limbs  forward  to  the  level  of  the  gills.  Remove 
the  underlying  muscles  bit  by  bit  until  you  have  exposed  a  membrane,  the 
parietal  pericardium.  Cut  through  this  membrane.  The  pericardial  cavity  in 
which  the  heart  is  situated  is  thus  exposed.  Widen  the  opening  into  the  cavity 
by  cutting  laterally  along  the  anterior  margin  of  the  pectoral  girdle.  The 
muscles  between  the  pericardial  cavity  and  the  fore  limbs  may  also  be  split. 


THE  COELOM,  DIGESTIVE,  AND  RESPIRATORY  SYSTEMS  171 

The  pericardial  cavity  is  a  conical  cavity  lined  by  the  parietal  pericardium. 
On  gently  raising  the  heart  the  posterior  wall  of  the  cavity  is  seen  to  be  formed 
by  the  transverse  septum.  The  transverse  septum  is  pierced  by  two  veins 
(hepatic  sinuses)  which  extend  forward  and  enter  the  sinus  venosus,  the  most 
dorsal  chamber  of  the  heart. 

4.  The  oral  cavity  and  the  pharynx. — Open  the  mouth  and  cut  through  the 
angle  of  the  jaws  on  each  side  so  that  the  jaws  can  be  spread  widely.     Carry  your 
cuts  back  to  the  gill  arches.    The  cavity  thus  exposed  consists  of  an  anterior 
oral  cavity  and  a  posterior  pharynx. 

The  oral  cavity  is  bounded  externally  by  the  well-developed  lips.  Internal 
to  the  lips  are  the  small  conical  teeth.  There  are  two  rows  of  teeth  on  the  roof 
of  the  mouth,  the  posterior  row  being  the  longer.  External  to  the  last  teeth  of 
the  posterior  row  on  each  side  is  a  slit,  one  of  the  posterior  nares  or  internal 
openings  of  the  nasal  passages.  Probe  into  one  of  the  anterior  nares  and  note 
emergence  of  the  probe  through  the  posterior  naris.  The  floor  of  the  mouth 
cavity  bears  a  single  row  of  teeth  which  on  closing  the  mouth  will  be  found  to  fit 
between  the  two  rows  on  the  roof.  Posterior  to  the  teeth  is  the  tongue  supported 
by  the  strongly  developed  hyoid  arch  which  should  be  felt  within  the  tongue. 

The  walls  of  the  pharynx  are  pierced  by  two  pairs  of  gill  slits.  Probe  through 
them  and  note  emergence  of  the  probe  between  the  external  gills.  Note  the 
cartilaginous  gill  arches  supporting  the  bars  (visceral  arches)  between  and  on 
each  side  of  the  gill  slits.  The  walls  of  the  gill  slits  are  the  visceral  pouches. 
Unlike  the  dogfish  they  bear  no  gills,  the  gills  being  external.  The  pharyngeal 
cavity  narrows  posteriorly  into  a  tube,  the  esophagus.  By  passing  a  probe  into 
the  esophagus  determine  that  it  extends  dorsal  to  the  pericardial  cavity  and  is 
continuous  with  the  stomach. 

5.  The  larynx  and  the  lungs. — In  the  floor  of  the  pharynx  midway  between 
the  second  gills  slits  will  be  found  a  short  slit,  the  glottis.     The  walls  of  the  glottis, 
as  should  be  determined  by  feeling  them  with  a  fine  forceps,  are  stiffened  by  a 
pair  of  delicate  cartilages,  the  arytenoid  cartilages.     These  probably  represent 
reduced  gill  arches.     They  are  the  first  of  the  laryngeal  cartilages  to  appear  in 
the  phylogenetic  series.     The  small  cavity  into  which  the  glottis  leads  and  which 
is  inclosed  between  the  two  arytenoid  cartilages  is  the  larynx.     Cut  across  the 
gill  slits  of  the  left  side  so  that  the  pharyngeal  cavity  can  be  opened  more  widely. 
Slit  the  glottis  posteriorly.     The  larynx  is  thus  seen  to  lead  into  a  narrow  flattened 
passage,  the  trachea.    The  posterior  end  of  this  is  widened  and  receives  two 
openings.     Probe  into  each  with  a  slender  probe  and  note  emergence  of  the  probe 
into  a  lung.     The  trachea  is  thus  seen  to  communicate  with  the  lungs.     The  air 
passage    in    primitive   air-breathing   vertebrates   takes    the   following  course: 
anterior  nares,  nasal  cavities,  posterior  nares,  oral  cavity,  pharyngeal  cavity, 
glottis,  larynx,  trachea,  lungs.     The  lungs  have  already  been  noted.     Slit  open 
one  of  them  and  note  the  smooth  interior,  not  provided  with  air  sacs. 


172  LABORATORY  MANUAL  FOR  VERTEBRATE  ANATOMY 

E.      THE   COELOM,   DIGESTIVE,   AND  RESPIRATORY   SYSTEMS   OF  THE  TURTLE 

Obtain  a  specimen  and  place  in  a  dissecting  pan.  Specimens  which  have 
not  been  injected  should  be  employed.  Remove  the  plastron.  This  is  done  by 
sawing  through  the  bridges  on  each  side,  lifting  up  the  plastron  and  separating 
it  with  a  scalpel  from  the  surrounding  skin  and  underlying  membrane. 

i.  The  divisions  and  relations  of  the  coelom. — The  removal  of  the  plastron 
exposes  a  membrane,  the  parietal  peritoneum,  which  covers  and  conceals  the 
viscera.  Note  that  the  muscle  layer  which  is  normally  present  between  the  skin 
and  the  peritoneum  is  completely  lacking  in  the  ventral  body  wall  of  the  turtle, 
owing  to  the  presence  of  the  plastron.  The  ventral  body  wall  in  turtles  there- 
fore consists  of  but  two  layers,  the  skin  with  its  contained  exoskeleton,  and  the 
peritoneum.  Owing  to  this  circumstance  the  parietal  peritoneum  can  be  easily 
separated  from  the  inside  of  the  body  wall,  a  procedure  which  is  difficult  or 
impossible  in  other  vertebrates.  Note,  however,  the  usual  muscles  in  connec- 
tion with  the  girdles  and  limbs. 

In  the  median  line  in  the  anterior  part  of  the  parietal  peritoneum  shortly 
posterior  to  the  pectoral  girdle  is  situated  a  triangular  membranous  sac,  the 
pericardial  sac,  which  incloses  the  heart.  It  will  be  noticed  that  the  heart  is 
much  more  posterior  in  position  than  is  the  case  in  the  fishes  and  Necturus;  in 
fact,  there  has  occurred  a  posterior  descent  of  the  heart  (and  of  other  viscera  as 
well).  The  membranous  sac  covering  the  heart  is,  as  in  the  dogfish,  the  parietal 
pericardium.  Here  it  takes  the  form  of  an  isolated  sac,  the  pericardial  sac, 
while  in  fishes  and  Necturus  it  formed  the  lining  of  a  chamber  surrounded  by 
the  body  wall.  The  space  between  the  pericardial  sac  and  the  heart  is  the 
pericardial  cavity,  a  portion  of  the  coelom.  The  ventral  face  of  the  pericardial 
sac  rests  in  the  natural  position  against  the  internal  surface  of  the  plastron, 
while  its  dorsal  face  is  fused,  as  we  shall  see,  to  the  parietal  peritoneum.  Cut 
into  the  ventral  wall  of  the  pericardial  sac,  thus  exposing  the  pericardial  cavity 
and  the  contained  heart. 

Two  conspicuous  veins,  the  ventral  abdominal  veins,  run  longitudinally  in  the 
parietal  peritoneum  between  the  pericardial  sac  and  the  pelvic  girdle.  Cut 
through  the  peritoneum  halfway  between  the  heart  and  pelvic  girdle  by  a  trans- 
verse cut  which  severs  both  of  the  abdominal  veins.  The  large  cavity  thus 
exposed  is  the  pleuro peritoneal  cavity,  whose  walls  are  lined  by  the  parietal 
peritoneum. 

The  coelom  of  the  turtle,  like  that  of  the  fishes  and  Necturus,  consists  of  two  parts,  a  small 
pericardial  cavity  and  a  much  larger  pleuroperitoneal  cavity.  We  note,  however,  that  whereas 
in  the  lower  forms  the  pericardial  cavity  is  anterior  to  the  pleuroperitoneal  cavity  and  separated 
from  the  latter  by  the  transverse  septum,  in  the  turtle  the  pericardial  cavity  is  ventral  to  the 
pleuroperitoneal  cavity,  and  the  transverse  septum  seems  to  have  disappeared.  We  may 
explain  this  change  as  follows.  (See  also  Fig.  45, p.  160.)  In  its  posterior  descent  the  heart  must 
necessarily  carry  with  it  the  transverse  septum  and  the  parietal  pericardium.  The  latter  in 
order  to  move  posteriorly  must  separate  from  the  body  wall  to  which  it  is  attached  in  lower 


THE  COELOM,  DIGESTIVE,  AND  RESPIRATORY  SYSTEMS  173 

vertebrates.  It  does  this  and  so  becomes  an  independent  sac,  the  pericardia!  sac.  This 
process  of  the  splitting  of  the  pericardial  sac  from  the  body  wall  is  aided  by  the  invasion  forward 
of  the  pleuroperitoneal  cavity.  The  heart  contained  in  the  pericardial  sac  descends  posteriorly, 
the  pleuroperitoneal  cavity  at  the  same  time  advancing  anteriorly.  The  pericardial  sac  may 
be  thought  of  as  sliding  posteriorly  ventral  to  the  ventral  wall  of  the  pleuroperitoneal  cavity. 
The  pericardial  sac  thus  comes  to  lie  ventral  to  the  anterior  part  of  the  pleuroperitoneal  cavity. 
The  posterior  wall  of  the  pericardial  sac  is  still  the  anterior  face  of  the  transverse  septum,  the 
posterior  face  of  the  latter  as  in  lower  forms  being  placed  between  the  pericardial  sac  and  the 
liver.  The  transverse  septum  thus  in  the  turtle  forms  part  of  the  partition  between  the  peri- 
cardial and  the  pleuroperitoneal  cavities,  the  remainder  of  the  partition  being  composed  of  the 
rest  of  the  parietal  pericardium,  which  is  now  the  pericardial  sac.  These  matters  will  be  better 
understood  by  reference  to  Figure  45^!  and  C. 

2.  The  viscera  and  their  mesenteries. — With  the  bone  scissors  cut  away 
the  margins  of  the  carapace  on  each  side  between  fore  and  hind  limbs  so  as  to 
gain  easy  access  to  the  pleuroperitoneal  cavity.  Masses  of  fat,  greenish-yellow 
material,  will  be  found  in  various  places  and  may  be  removed.  Lift  up  the  edges 
of  the  cut  already  made  in  the  peritoneum,  widening  this  if  necessary,  and  look 
inside.  Identify  in  the  anterior  part  of  the  pleuroperitoneal  cavity  the  large 
brown  liver  lying  on  each  side  of  the  heart.  Posterior  to  the  liver  are  the  coils 
of  the  intestine.  In  female  specimens  the  ovaries  containing  eggs  of  various 
sizes  are  conspicuous  objects  in  the  lateral  and  posterior  part  of  the  pleuroperito- 
neal cavity.  Running  alongside  each  ovary  is  the  coiled  oviduct.  Just  in  front 
of  the  pelvic  girdle  is  the  large  bilobed  urinary  bladder. 

The  liver  consists  of  right  and  left  lobes  whose  lateral  margins  curve  dorsally 
to  fit  the  curves  of  the  carapace.  The  pericardial  sac  rests  in  a  depression 
between  the  two  lobes.  The  latter  are  united  by  a  narrow  bridge  passing  dorsal 
to  the  heart.  Posterior  to  the  heart  the  liver  is  united  to  the  parietal  peritoneum 
by  very  short  mesenteries  corresponding  to  the  falciform  ligament  of  other  verte- 
brates. In  these  mesenteries  the  ventral  abdominal  veins  leave  the  peritoneum 
and  pass  into  the  liver.  Trace  the  parietal  peritoneum  anteriorly  from  this 
region.  It  passes  along  the  dorsal  face  of  the  pericardial  sac,  to  which  it  is 
inseparably  fused.  This  compound  membrane  between  the  heart  and  liver  is 
the  transverse  septum,  which  has  assumed  an  oblique  position,  owing  to  the 
descent  of  the  heart  (Fig.  45).  The  ventral  (original  anterior)  face  of  the  septum 
is  as  before,  part  of  the  wall  of  the  pericardial  cavity;  the  dorsal  (original  pos- 
terior) face  forms  part  of  the  parietal  peritoneum.  The  liver  is  as  usual  attached 
to  the  transverse  septum  by  the  coronary  ligament.  Continue  to  trace  the 
parietal  peritoneum  to  the  anterior  end  of  the  pleuroperitoneal  cavity.  On 
the  posterior  face  of  the  pectoral  girdle  it  turns  dorsally  and  passes  to  the  carapace 
of  which  it  forms  the  inner  lining.  Similarly  trace  the  parietal  peritoneum  pos- 
teriorly by  lifting  the  posterior  cut  edge  of  the  membrane.  It  curves  dorsally, 
following  along  the  anterior  surface  of  the  pelvic  girdle,  and  passes  to  the  inner 
surface  of  the  carapace. 


174       LABORATORY  MANUAL  FOR  VERTEBRATE  ANATOMY 

Press  both  lobes  of  the  liver  forward  against  the  pectoral  girdle  and  look  on 
the  dorsal  surface  of  the  liver.  The  elongated  stomach  will  be  found  curving 
dorsal  to  the  lateral  border  of  the  left  liver  lobe.  On  following  the  stomach 
anteriorly,  the  narrow  esophagus  will  be  found  entering  the  stomach.  The 
stomach  passes  along  the  dorsal  surface  of  the  left  lobe  of  the  liver  to  the  middle 
of  which  it  is  attached  along  its  entire  length  by  the  short  gastrohepatic  ligament. 
About  opposite  the  bridge  connecting  the  two  lobes  of  the  liver,  the  stomach 
passes  insensibly  into  the  small  intestine,  the  first  part  of  which  is  the  duodenum. 
The  duodenum  is  united  to  the  middle  of  the  dorsal  surface  of  the  right  lobe 
of  the  liver  by  the  hepatoduodenal  ligament.  In  this  ligament  is  situated  a  long 
white  gland,  the  pancreas.  About  one-quarter  of  an  inch  back  of  the  right  end 
of  the  pancreas,  a  pancreatic  duct  passes  from  the  pancreas  into  the  duodenum 
and  may  be  revealed  by  picking  away  the  substance  of  the  pancreas  at 
this  point.  On  the  dorsal  surface  of  the  right  lobe  of  the  liver  near  its  lat- 
eral border  is  the  large  gall  bladder,  which  is  connected  to  the  duodenum  by  a 
short  but  stout  bile  duct.  Beyond  the  entrance  of  the  bile  duct  the  small 
intestine  turns  sharply  posteriorly  and  is  then  thrown  into  a  number  of  coils. 
In  the  case  of  female  specimens  it  will  generally  be  necessary  to  remove  one  of 
the  large  egg-bearing  ovaries  at  this  point  before  the  intestine  can  be  conveniently 
traced  farther.  By  lifting  the  coils  of  the  small  intestine  note  the  dorsal  mesentery 
which  attaches  it  to  the  median  dorsal  line  of  the  coelom;  this  part  of  the  dorsal 
mesentery  is  the  mesentery  proper.  Follow  the  dorsal  mesentery  forward  and 
note  the  portions  of  it  which  support  the  duodenum  and  the  stomach,  named 
mesoduodenum  and  mesogaster,  respectively.  The  mesoduodenum  is  fused  to  the 
hepatoduodenal  ligament  so  that  the  two  appear  as  one,  but  the  mesogaster  is 
distinct  from  the  gastrohepatic  ligament.  Trace  the  small  intestine  posteriorly, 
noting  the  coiling  of  the  mesentery  corresponding  to  the  coils  of  the  intestine. 
Find  on  the  right  side  the  entrance  of  the  small  intestine  into  the  large  intestine 
or  colon.  At  the  junction  of  the  small  and  large  intestine  is  a  slight  projection, 
the  caecum.  The  colon  generally  crosses  the  pleuroperitoneal  cavity  transversely 
and  then  turns  posteriorly  and  runs  straight  caudad  to  the  cloaca.  Note  the 
mesocolon  supporting  the  colon.  In  the  transverse  part  of  the  colon  it  is  fused 
to  the  mesogaster.  In  the  mesocolon  on  the  dorsal  side  of  the  colon  shortly 
beyond  the  caecum  is  a  rounded  red  body,  the  spleen.  Trace  the  colon  to  the 
place  where  it  disappears  dorsal  to  the  pelvic  girdle.  At  this  point  ventral  to 
the  colon  will  be  found  the  large,  thin-walled,  bilobed  urinary  bladder.  It  is 
generally  greatly  distended  with  urine  but  in  some  specimens  may  be  contracted 
to  a  small  mass.  The  bladder  has  no  ligaments,  the  peritoneum  leaving  the 
body  wall  around  the  stalk  of  the  bladder  and  passing  over  its  surface  to  form 
its  visceral  investment. 

Cut  away  the  pelvic  girdle  by  making  a  cut  through  each  side  of  it  with  the 
bone  scissors  and  removing  a  median  piece.  The  large  intestine  will  be  traced  into 


THE  COELOM,  DIGESTIVE,  AND  RESPIRATORY  SYSTEMS  175 

a  tube,  the  cloaca,  which  proceeds  dorsal  to  the  girdle  to  the  anus.  At  the  point 
of  entrance  of  the  large  intestine  into  the  cloaca  the  urinary  bladder  will  be 
found  attached  to  its  ventral  surface  by  a  stalk.  On  each  side  of  the  stalk  of 
the  bladder,  in  females,  a  large  white  oviduct  will  also  be  seen  entering  the 
cloaca. 

The  female  reproductive  system  may  be  noted  at  this  time;  that  of  the  male 
is  so  inconspicuous  that  it  will  not  be  described  at  this  point.  The  ovaries  are 
a  pair  of  large  saclike  bodies  containing  in  their  walls  eggs  of  various  sizes. 
Each  ovary  is  attached  by  its  mesentery,  the  mesovarium,  to  the  dorsal  body 
wall.  Lateral  to  each  ovary  runs  the  oviduct,  a  large  coiled  white  tube.  It  is 
supported  by  the  mesotubarium. 

3.  The  respiratory  system. — Pry  open  the  jaws  of  the  turtle  and  cut  through 
the  angles  of  the  jaws,  cutting  nearer  the  lower  than  the  upper  jaw.  The  anterior 
portion  of  the  cavity  thus  revealed  is  the  oral  or  mouth  cavity;  the  posterior 
portion,  the  pharynx.  The  oral  cavity  is  bounded  by  the  jaws  which  have  no 
teeth  but  are  clothed  with  horny  beaks  of  epidermal  origin.  These  beaks  extend 
as  plates  into  the  mouth  cavity.  In  the  roof  of  the  mouth  cavity  posterior  to 
the  plate  is  a  pair  of  elongated  openings,  the  posterior  nares.  Probe  them  and 
determine  that  they  connect  with  the  anterior  nares  by  passages  which  run 
through  the  nasal  cavities.  The  floor  of  the  mouth  cavity  is  occupied  by  the 
fleshy  pointed  tongue. 

In  the  pharynx  note  that  neither  gills  nor  gill  slits  are  present,  although,  as 
we  shall  see  shortly,  the  gill  arches  are  represented.  Behind  the  base  of  the 
tongue  is  an  elevation,  the  laryngeal  prominence,  in  the  center  of  which  is  an 
elongated  slit,  the  glottis.  Feel  the  pair  of  small  arytenoid  cartilages,  one  on  each 
side  of  the  glottis;  they  are  derived  from  one  of  the  gill  arches.  On  each  side 
of  the  roof  of  the  pharynx  posterior  to  the  muscles  which  connect  the  skull  and 
lower  jaw  is  the  opening  of  the  auditory  or  Eustachian  tube,  a  canal  which  leads 
from  the  pharynx  to  the  cavity  of  the  middle  ear.  (The  opening  may  have  been 
destroyed  in  cutting  the  jaws  apart.)  The  auditory  tube  and  also  the  cavity  of 
the  middle  ear  are  outgrowths  from  the  first  visceral  pouch.  Posteriorly  the 
pharynx  narrows  into  the  esophagus. 

Cut  through  the  skin  in  the  median  ventral  line  of  the  neck  and  peel  away 
the  skin  from  neck  and  throat.  Separate  the  muscles  in  the  median  line  of  the 
neck  and  find  a  tube  stiffened  by  rings  of  cartilage.  This  is  the  trachea  or  wind- 
pipe. Trace  it  forward  until  it  disappears  into  the  pharynx.  In  front  of  this 
place  note"  the  hard  body  of  the  hyoid,  and  by  cleaning  away  muscles  find  also 
two  pairs  of  horns  of  the  hyoid  extending  posteriorly.  The  hyoid  and  its  horns 
are  derivatives  of  the  second,  third,  and  fourth  gill  arches.  Open  the  mouth  and 
make  a  cut  around  the  laryngeal  prominence,  freeing  it  from  its  position  on 
the  dorsal  surface  of  the  body  of  the  hyoid.  The  structure  thus  freed  is  the 
larynx,  an  expanded  chamber  at  the  anterior  end  of  the  trachea.  Find  in  the 


176       LABORATORY  MANUAL  FOR  VERTEBRATE  ANATOMY 

lateral  walls  of  the  larynx  the  two  arytenoid  cartilages,  small  cartilages  support- 
ing the  two  triangular  flaps  which  inclose  the  glottis  between  them.  Posterior 
to  the  glottis  is  a  ring-shaped  cartilage,  the  cricoid,  which  is  much  wider  on  the 
ventral  than  on  the  dorsal  side.  The  arytenoids  probably  are  remnants  of  the 
fifth  gill  arches  while  the  cricoid  is  the  enlarged  first  cartilage  of  the  series  of 
rings  in  the  trachea. 

Now  trace  the  trachea  posteriorly.  Note  the  esophagus,  a  soft  tube,  lying 
dorsal  to  or  to  one  side  of  the  trachea.  Find  just  anterior  to  the  heart  the  point 
where  the  trachea  bifurcates  into  the  two  bronchi  which  proceed  to  the  lungs. 
Raise  the  right  and  left  lobes  of  the  liver  and  the  stomach  and  find  dorsal  to 
them  against  the  carapace  a  large  spongy  organ,  the  lung,  on  each  side.  Trace 
a  bronchus  into  each  lung;  it  is  accompanied  by  a  pulmonary,  artery  and  a 
pulmonary  vein.  Study  the  relation  of  the  lung  to  the  pleuroperitoneal  cavity. 
Note  that  the  lung  is  in  contact  with  the  inner  surface  of  the  carapace,  and  that 
the  parietal  peritoneum  passes  over  the  ventral  surface  of  the  lung  leav- 
ing the  lung  outside  of  the  membrane.  Such  a  relation  to  the  peritoneum 
is  spoken  of  as  retro  peritoneal.  The  posterior  end  of  the  lung,  however, 
projects  into  the  pleuroperitoneal  cavity  and  is  clothed  with  the  peritoneum. 
Cut  open  the  lung  and  observe  its  extremely  spongy  texture;  cords  of  connective 
tissue  divide  the  interior  into  air  spaces  or  alveoli. 

The  path  followed  by  the  air  in  respiration  is:  external  nares,  nasal  cavities, 
internal  nares,  mouth  cavity,  pharyngeal  cavity,  glottis,  larynx,  trachea,  bronchi, 
and  lungs.  In  the  pharyngeal  cavity  the  paths  of  food  and  air  cross. 

Make  drawings  to  show  the  parts  of  the  digestive  and  respiratory  systems. 
Make  a  diagram  of  a  cross-section  through  the  body  at  the  level  of  the  heart  to 
show  the  pericardial  and  pleuroperitoneal  cavities  and  membranes  and  their 
relation  to  the  viscera. 

F.      THE   COELOM,   DIGESTIVE,   AND   RESPIRATORY  SYSTEMS   OF   THE   PIGEON 

Obtain  a  specimen  and  place  in  a  dissecting  pan.  The  feathers  must  be 
removed.  It  is  desirable  that  the  air  sacs  should  have  been  inflated  through  the 
trachea. 

i.  The  oral  cavity  and  the  pharynx. — Open  the  mouth  widely  by  cutting 
through  the  angles  of  the  jaws.  An  anterior  oral  cavity  and  a  posterior  pharynx 
are  thus  revealed. 

a)  Oral  cavity:  Roof  and  floor  of  the  oral  cavity  are  bounded  laterally  by 
horny  beaks  of  epidermal  origin  which  incase  the  jaws.  Teeth  are  absent,  as  in 
all  living  birds.  The  roof  of  the  mouth  cavity  bears  a  pair  of  elongated  palatal 
folds  with  free  fimbriated  margins.  These  palatal  folds  correspond  to  the  hard 
palate  of  mammals,  but  differ  in  that  they  do  not  meet  in  the  median  line, 
leaving  here  a  deep  palatal  fissure.  The  hard  palate  of  many  birds  is  therefore 
a  split  palate  and  is  normally  in  the  condition  which  in  mammals  is  the  result 


THE  COELOM,  DIGESTIVE,  AND  RESPIRATORY  SYSTEMS  177 

of  imperfect  development.  In  the  roof  of  the  mouth  cavity,  dorsal  to  the  palatal 
folds  and  concealed  by  them,  are  the  posterior  nares.  Locate  them  by  bending 
aside  or  cutting  away  the  palatal  folds.  Probe  into  the  anterior  nares  and  note 
that  the  probe  emerges  through  the  posterior  nares.  The  floor  of  the  mouth 
cavity  is  occupied  by  the  pointed  tongue,  whose  posterior  free  border  is  fimbriated 
and  terminates  in  a  point  on  each  side.  The  tongue  of  birds  is  not  very  muscular. 
Numerous  glands  open  into  the  oral  cavity  in  birds  but  are  too  small  to  study 
in  gross  dissection. 

b)  Pharynx:  Note  that,  as  in  all  adult  vertebrates  above  urodeles,  gill  slits 
are  absent  from  the  lateral  walls  of  the  pharynx.  In  the  roof  of  the  pharyngeal 
cavity  just  posterior  to  the  caudal  ends  of  the  palatal  folds  is  a  median  aperture, 
the  opening  of  the  paired  auditory  tubes.  Each  auditory  tube  extends  from  this 
opening  to  the  cavity  of  the  middle  ear;  tube  and  cavity  represent  in  part  an 
evagination  from  the  first  visceral  pouch.  In  birds  unlike  other  vertebrates  the 
two  auditory  tubes  unite  to  one  at  the  point  of  communication  with  the  pharynx. 
Posterior  to  this  opening  the  roof  of  the  pharynx  bears  a  pair  of  folds  with  fim- 
briated borders,  which  hang  down  like  a  curtain  into  the  pharyngeal  cavity. 
These  folds  constitute  the  soft  palate.  In  the  floor  of  the  pharynx,  immediately 
posterior  to  the  caudal  end  of  the  tongue,  is  a  hardened  elevation,  the  laryngeal 
prominence,  bearing  in  its  center  an  elongated  opening,  the  glottis.  The  margins 
of  the  glottis  are  also  fimbriated,  and  immediately  posterior  to  the  glottis  on 
each  side  is  a  fringed  fold.  In  the  walls  of  the  glottis  the  supporting  laryngeal 
cartilages  are  readily  felt. 

Make  a  drawing  of  the  oral  and  pharyngeal  cavities. 

2.  The  hyoid  apparatus,  the  larynx,  the  trachea,  and  the  esophagus. — 
Make  a  median  ventral  longitudinal  incision  in  the  skin  of  the  neck  from  the 
throat  to  the  anterior  end  of  the  sternum.  Deflect  the  skin  on  each  side  of  the 
incision.  The  trachea  or  windpipe,  a  tube  with  walls  stiffened  by  rings  of  carti- 
lage, is  immediately  exposed.  Dorsal  to  it  or  to  one  side  of  it  is  the  soft 
esophagus. 

Trace  the  trachea  forward  to  the  glottis,  cleaning  away  the  muscles  which 
cover  its  anterior  end.  At  the  same  time  a  cut  may  be  made  to  the  sides  of  the 
tongue  so  that  the  tongue  may  be  pulled  down  ventrally  from  the  mouth  cavity. 
The  hyoid  apparatus  may  now  be  studied.  It  consists  of  remnants  of  the  hyoid 
(second)  and  third  gill  arches.  It  is  composed  of  three  median  elements,  arranged 
in  a  longitudinal  series,  and  two  pairs  of  horns  or  cornua.  The  most  anterioi 
of  the  three  median  pieces  is  the  entoglossal  cartilage.  It  is  situated  inside  of 
the  tongue  and  may  be  revealed  by  dissecting  off  the  covering  membrane  of  the 
tongue.  It  represents  the  two  fused  ceratohyals.  From  its  posterior  end  pro- 
jects posteriorly  on  each  side  a  small  cartilage  which  occupies  the  caudal  point 
of  the  tongue  already  noted.  These  two  cartilages  constitute  the  anterior 
horns  of  the  hyoid  and  consist  of  the  free  ends  of  the  two  ceratohyals  whose 


178       LABORATORY  MANUAL  FOR  VERTEBRATE  ANATOMY 

anterior  portions  fused  to  form  the  entoglossal  cartilage.  Posterior  to  the  entoglos- 
sal  cartilage  is  a  median  bony  piece,  the  basihyal.  Posterior  to  this  is  the  basi- 
branchial  of  the  third  gill  arch.  From  the  point  of  junction  of  basihyal  and 
basibranchial  projects  on  each  side  the  long  posterior  horn  of  the  hyoid,  consisting 
of  portions  of  the  third  gill  arch.  On  following  the  posterior  horns  they  will 
be  found  to  extend  toward  the  ears  and  to  be  divided  into  a  proximal  longer 
portion,  the  ceratobranchial,  and  a  distal  shorter  rod,  the  epibranchial. 

The  cartilages  of  the  larynx  may  next  be  identified.  Cut  around  the  laryngeal 
prominence  freeing  it  so  that  it  can  be  drawn  ventrally.  Also  free  the  hyoid 
apparatus  from  the  ventral  surface  of  the  larynx.  The  larynx  is  the  expanded 
chamber  thus  revealed  at  the  top  of  the  trachea  and  opening  into  the  pharyngeal 
cavity  by  way  of  the  glottis.  By  dissecting  in  the  margins  of  the  glottis  on 
each  side,  expose  a  slender,  curved,  partially  ossified  arytenoid  cartilage.  On 
the  ventral  side  of  the  larynx  note  the  enlarged  triangular  cricoid  cartilage. 
Follow  this  around  to  the  dorsal  side  where  it  terminates  by  much  narrowed 
ends.  Between  the  two  dorsal  ends  of  the  cricoid  cartilage  is  another  median 
cartilage,  the  procricoid,  which  is  in  contact  with  the  posterior  ends  of  the  aryte- 
noids.  The  arytenoids  are  derived  from  certain  of  the  gill  arches  while  the 
cricoid  and  procricoid  cartilages  are  the  most  anterior  modified  rings  of 
the  trachea.  Although  the  larynx  of  birds  is  morphologically  the  same  as  the 
larynx  of  other  vertebrates  from  which  sounds  issue,  in  birds  the  voice  is  not 
produced  in  the  larynx  but  in  another  part  of  the  trachea  which  will  be 
seen  later. 

Examine  the  cartilages  of  the  trachea.  They  are  broad,  hard,  and  bony  ven- 
trally, but  narrower,  softer,  and  cartilaginous  in  composition  dorsally.  There 
is  consequently  a  somewhat  soft  strip  along  the  dorsal  side  of  the  trachea  which 
lies  against  the  cervical  vertebrae. 

Trace  the  esophagus  posteriorly.  Shortly  in  front  of  the  sternum  it  widens 
into  an  enormous  bilobed  sac,  the  crop.  Birds  swallow  their  food  whole;  the 
food  collects  in  the  crop  which  is  capable  of  great  distension  and  is  passed  on  into 
the  stomach  in  small  quantities.  The  crop  should  be  carefully  loosened  on  all 
sides.  Not  all  birds  possess  a  crop. 

3.  The  anterior  air  sacs  and  the  pectoral  muscles. — The  respiratory  system 
of  birds  is  the  most  remarkable  among  vertebrates.  It  consists,  not  only  of 
the  lungs,  but  also  of  a  number  of  air  sacs  located  among  the  viscera  and  of  air 
spaces  in  the  bones.  These  air  sacs  and  air  spaces  communicate  with  the  lungs 
by  means  of  branches  of  the  bronchi.  This  system  not  only  aids  in  decreasing 
the  specific  gravity  of  the  bird  but  also  insures  a  more  complete  exposure  of  the 
lung  tissue  to  the  air;  for  the  residual  air  is  retained  in  the  air  sacs  and  not  in  the 
lungs  as  in  other  vertebrates,  and  the  air  in  the  lungs  is  consequently  completely 
renewed  at  each  inspiration.  Owing  to  the  delicacy  of  the  air  sacs  the  student 


THE  COELOM,  DIGESTIVE,  AND  RESPIRATORY  SYSTEMS  179 

may  not  be  able  to  locate  all  of  those  mentioned  below,  but  some  of  them  will 
be  seen.  They  are  best  studied  in  freshly  killed  specimens  in  which  they  have 
been  inflated  through  the  trachea. 

Dorsal  to  the  crop  in  the  angle  formed  by  the  two  halves  of  the  furcula  or 
wishbone  is  situated  the  inter  clavicular  air  sac.  Its  delicate  ventral  wall  is  in 
contact  with  the  dorsal  wall  of  the  crop.  It  consists  of  two  lobes,  one  on  each 
side  of  the  median  line;  in  the  embryo  these  lobes  are  separate.  Puncture  the 
interclavicular  air  sac  and  find,  dorsal  to  it  on  each  side,  another  sac,  the  cervical 
air  sac. 

Extend  the  median  ventral  incision  in  the  skin  to  the  anus.  Separate  the 
skin  from  the  underlying  muscles  on  each  side  of  chest  and  abdomen.  The 
great  pectoral  muscles  are  revealed  immediately  internal  to  the  skin  and  occupy- 
ing the  angle  between  the  keel  and  the  body  of  the  sternum.  The  pectoralis 
major  is  the  great  muscle  covering  the  entire  sternum  and  extending  to  the 
humerus.  It  takes  origin  from  the  keel  of  the  sternum,  the  surface  of  the  body 
of  the  sternum,  and  the  furcula  which  will  be  found  imbedded  in  its  anterior 
border;  its  fibers  converge  toward  the  humerus,  and  passing  over  the  shoulder 
are  inserted  on  the  outer  and  dorsal  surface  of  the  humerus.  The  muscle  should 
be  followed  to  its  insertion.  Action,  depresses  .the  wing.  Now  carefully  cut 
through  the  pectoralis  major  slightly  to  the  right  of  the  keel  of  the  sternum 
and  along  the  posterior  margin  of  the  furcula.  The  muscle  can  then  be  deflected 
and  separates  easily  from  the  underlying  pectoralis  minor.  The  large  pectoral 
arteries  and  veins  will  probably  be  noticed  emerging  between  the  pectoral  muscles 
which  they  supply.  The  pectoralis  minor  originates  from  the  body  of  the  sternum 
and  converges  toward  the  humerus.  On  following  the  muscle  laterally  there 
will  be  found  between  it  and  the  pectoralis  major  another  air  sac,  the  axillary 
sac.  Cut  into  the  axillary  sac.  The  anterior  wall  of  this  sac  is  in  contact  with 
the  coracoid  bone,  and  laterally  on  looking  into  the  sac  the  tuberosities  of  the 
humerus  will  be  seen.  A  large  opening  into  the  humerus,  the  pneumatic  foramen, 
is  readily  noticed ;  on  probing  this  it  will  be  found  to  lead  into  the  interior  of  the 
humerus.  It  is  the  entrance  to  the  air  space  of  the  humerus  which  communi- 
cates with  the  axillary  air  sac.  The  axillary  air  sac  communicates  in  front  with 
the  interclavicular  air  sac.  The  pectoralis  minor  may  now  be  followed  to  its 
insertion.  It  converges  to  a  tendon  which  passes  ventral  to  the  posterior  end 
of  the  cervical  air  sac  and  beneath  the  shoulder  to  the  dorsal  side  of  the  humerus 
on  which  it  is  inserted.  To  see  the  insertion  turn  the  bird  dorsal  side  up  and 
dissect  away  the  superficial  muscles  of  the  dorsal  side  of  the  shoulder.  The  ten- 
don of  the  pectoralis  muscle,  owing  to  its  mode  of  insertion,  has  a  pulley-like 
action  which  enables  the  muscle  to  raise  the  wing.  Whereas  in  mammals  all 
of  the  pectoral  muscles  act  together  to  adduct  the  fore  limb,  in  birds  the  actions 
of  the  pectoralis  major  and  minor  are  opposed  to  each  other,  the  one  depressing, 


i8o       LABORATORY  MANUAL  FOR  VERTEBRATE  ANATOMY 

the  other  raising  the  limb.  This  arrangement  eliminates  all  powerful  muscles 
from  the  back  and  enables  all  of  the  wing  muscles  to  take  their  origin  from  the 
firm  and  strong  sternum. 

4.  The  divisions  of  the  coelom  and  the  posterior  air  sacs. — Cut  through  the 
ventral  abdominal  wall  to  the  right  of  the  median  line.  Beneath  the  skin  are 
the  thin  layers  of  abdominal  muscles  corresponding  to  those  of  mammals,  and 
internal  to  this  the  parietal  peritoneum  generally  impregnated  with  streaks 
of  fat.  Cut  through  this  and  extend  the  incision  anteriorly  cutting  through 
the  sternum  slightly  to  the  right  of  the  keel,  keeping  the  scissors  in  contact  with 
the  bone  so  as  to  avoid  injuring  internal  parts.  Spread  apart  the  cut  edges  and 
look  within. 

The  small  cavity  posterior  to  the  sternum  is  the  peritoneal  cavity.  Note 
in  it  the  liver  dorsal  to  the  posterior  end  of  the  sternum,  the  closely  coiled  intes- 
tine, and  to  the  left  the  large  firm  gizzard.  From  the  gizzard  a  mesentery  extends 
to  the  ventral  body  wall  to  the  left  of  the  median  line.  This  may  be  designated 
the  ventral  ligament  of  the  gizzard.1  It  is  continuous  anteriorly  with  the  falci- 
form ligament  of  the  liver  which  extends  from  the  median  ventral  region  of  the 
liver  to  the  mid  ventral  line  of  the  body  wall  and  inner  surface  of  the  sternum. 
The  falciform  ligament  and  ventral  ligament  of  the  gizzard  together  constitute 
a  partition  which  divides  the  peritoneal  cavity  into  a  large  right  portion  and 
smaller  left  portion.  This  division  is  not  found  in  other  vertebrates.  In  the 
partition  courses  a  small  vein2  extending  from  the  mesenteries  in  question  to  the 
liver. 

Deflect  the  pectoralis  major  muscle  on  the  left  side  of  the  sternum  and  make 
a  cut  through  the  left  side  of  the  sternum  slightly  to  the  left  of  the  keel.  Remove 
and  discard  the  median  piece  of  sternum  containing  the  keel. 

Immediately  dorsal  to  the  sternum  is  situated  the  delicate  pericardial  sac 
containing  the  heart.  The  ventral  wall  of  the  pericardial  sac  will  probably  have 
been  opened  in  cutting  through  the  sternum.  The  heart,  as  in  the  turtle,  has 
descended  posteriorly,  and  a  pericardial  sac  has  been  formed  of  the  anterior 
face  of  the  transverse  septum  and  the  parietal  pericardium  as  described  in  con- 
nection with  the  turtle.  The  space  between  the  pericardial  sac  and  the  heart 
is,  as  before,  the  pericardial  cavity,  a  portion  of  the  coelom.  The  pericardial 
sac  is  in  contact  on  its  ventral  surface  with  the  inner  surface  of  the  sternum,  and 
anteriorly  and  laterally  is  also  in  contact  with  the  inner  surface  of  the  body  wall. 
Hence,  only  the  posterior  part  of  the  pericardial  sac  is  freed  from  the  body  wall. 

1  This  is  commonly  called  the  greater  omentum  in  texts  and  manuals,  but  since  it  is  not  at  all 
homologous  with  the  structure  so  named  in  mammals,  it  is  desirable  that  the  name  be  dropped.     The 
ligament  of  the  gizzard  is  a  mesentery  peculiar  to  birds,  and  arises  as  a  secondary  outgrowth  from  the 
serosa  of  the  gizzard  to  the  ventral  body  wall.     It  is  probably  due  to  the  need  for  additional  support  for 
the  heavy  gizzard. 

2  This  vein  is  named  in  manuals  the  ventral  abdominal  vein  but  it  does  not  appear  to  be  homologous 
with  the  vein  of  that  name  in  other  vertebrates. 


THE  COELOM,  DIGESTIVE,  AND  RESPIRATORY  SYSTEMS  181 

From  the  points  where  the  pericardial  sac  meets  the  lateral  body  wall  a  mem- 
branous partition  extends  obliquely  posteriorly  on  each  side.  This  partition  is 
called  the  oblique  septum.  It  contains  a  large  air  sac.  It  stretches  across  from 
the  lateral  body  wall  to  that  part  of  the  pericardial  sac  which  is  derived  from  the 
transverse  septum,  and  thus  divides  the  pleuroperitoneal  cavity  into  anterior  and 
posterior  portions.  That  part  of  the  original  pleuroperitoneal  cavity  left  anterior 
to  the  oblique  septum  consists  of  the  two  pleural  cavities,  one  on  each  side  of 
the  pericardial  cavity.  That  part  of  the  pleuroperitoneal  cavity  posterior  to  the 
oblique  septum  is  the  peritoneal  cavity,  already  mentioned.  The  oblique  septum 
is  produced  by  a  pair  of  mesenterial  folds  which  arise  one  on  each  side  of  the 
esophagus  and  grow  ventrally,  eventually  fusing  with  the  transverse  septum 
and  with  each  other  (Fig.  45!)  and  E,  p  160). 

Inside  of  the  oblique  septum  inclosed  between  its  anterior  and  posterior  walls 
is  a  large  air  sac,  the  posterior  intermediate  air  sac.  Immediately  anterior  to 
this,  lying  to  each  side  of  the  heart,  is  the  small  anterior  intermediate  air  sac. 

In  the  peritoneal  cavity  cut  through  the  falciform  ligament  and  ligament  of 
the  gizzard  at  their  line  of  attachment  to  the  ventral  body  wall.  On  either  side 
of  the  viscera  and  slightly  dorsal  to  them  find  the  large  abdominal  air  sac. 

From  the  foregoing  account  it  is  seen  that  the  coelom  of  birds  is  divided 
into  four  compartments,  the  pericardial  cavity,  the  two  pleural  cavities,  and  the 
peritoneal  cavity. 

5.  The  peritoneal  cavity  and  its  contents. — This  cavity  has  already  been 
mentioned.  As  in  other  vertebrates  it  is  lined  by  the  parietal  peritoneum 
which  is  deflected  at  certain  points  to  form  mesenteries  and  which  continues  over 
the  surface  of  the  viscera  as  the  visceral  peritoneum. 

The  viscera  of  the  peritoneal  cavity  may  now  be  studied  in  more  detail.  At 
the  anterior  end  is  the  large  liver,  consisting  of  right  and  left  lobes,  the  former 
the  larger.  The  pericardial  sac  rests  between  the  two  lobes  of  the  liver.  The 
liver  is  attached  to  the  pericardial  sac  (that  portion  of  it  derived  from  the  trans- 
verse septum)  by  the  coronary  ligament.  The  falciform  ligament  of  the  liver 
was  already  noted  and  severed.  To  the  left  and  slightly  covered  by  the  left  lobe 
of  the  liver  is  the  gizzard.  On  raising  the  left  lobe  of  the  liver  the  gastrohepatic 
ligament  will  be  noted  passing  between  the  gizzard  and  the  liver.  The  mesogaster 
connects  the  gizzard  with  the  dorsal  body  wall.  The  ventral  ligament  of  the 
gizzard  was  already  noted  and  cut.  On  breaking  through  the  gastrohepatic 
ligament  the  soft  proventriculus  will  be  found  extending  anteriorly  from  the 
gizzard  dorsal  to  the  liver.  Proventriculus  and  gizzard  together  correspond  to 
the  stomach  of  other  vertebrates;  they  are  specialized  regions  of  the  stomach 
correlated  with  the  absence  of  teeth.  From  the  stomach,  at  the  place  where 
proventriculus  and  gizzard  join,  the  small  intestine  arises.  The  first  portion  of 
this,  the  duodenum,  makes  a  long  U-shaped  loop  posteriorly.  The  beginning  of 
the  duodenum  is  attached  to  the  right  lobe  of  the  liver  by  the  hepatoduodenal 


182       LABORATORY  MANUAL  FOR  VERTEBRATE  ANATOMY 

ligament.  Between  the  two  sides  of  the  duodenal  loop  stretches  the  mesoduo- 
denum,  a  portion  of  the  mesentery  of  the  intestine.  In  this  is  situated  the  pan- 
creas, lying  between  the  two  limbs  of  the  loop.  From  a  deep  depression  in  the 
dorsal  surface  of  the  right  lobe  of  the  liver,  the  two  bile  ducts  (there  is  no  gall 
bladder)  emerge  and  pass  into  the  duodenum.  The  left  bile  duct  is  the  shorter 
and  stouter  of  the  two  and  enters  the  left  limb  of  the  duodenum  about  half  an 
inch  beyond  the  gizzard.  The  more  slender  right  bile  duct  passes  to  the  right 
limb  of  the  duodenal  loop.  There  are  three  pancreatic  ducts,  all  of  which  pass 
from  the  right  side  of  the  pancreas  into  the  right  limb  of  the  duodenal  loop.  One 
of  these  arises  from  the  anterior  part  of  the  pancreas  and  passes  obliquely  forward, 
entering  the  duodenum  near  the  anterior  termination  of  the  right  limb  of  the 
loop.  The  other  two  ducts  emerge  from  the  middle  of  the  pancreas  and  pass 
across  to  the  right  limb  of  the  duodenum.  The  ducts  are  generally  easily  seen 
by  spreading  out  the  mesentery. 

Trace  the  small  intestine  posteriorly  from  the  duodenum.  It  is  much  coiled 
and  supported  by  the  mesentery, which,  owing  to  the  small  space  into  which  the 
intestine  is  packed,  is  fused  in  many  places.  Near  its  termination  the  small 
intestine  turns  toward  the  median  line,  widens  slightly,  and  then  runs  straight 
caudad  in  the  median  line.  At  about  the  middle  of  the  peritoneal  cavity  it 
passes  without  enlargement  into  the  large  intestine.  The  point  of  junction  of 
large  and  small  intestine  is  marked  by  a  pair  of  small  lateral  diverticula,  the 
caeca.  The  large  intestine  is  so  short  as  to  constitute  little  more  than  a  rectum 
which  soon  passes  into  the  cloaca.  Owing  to  the  absence  of  pubic  and  ischial 
symphyses  in  birds,  the  cloaca  in  birds  does  not  pass  through  the  ring  of  the 
pelvic  girdle  but  may  be  traced  directly  to  the  anus.  There  is  no  urinary  blad- 
der. In  female  specimens  the  single  left  oviduct  will  probably  be  noted  entering 
the  left  side  of  the  cloaca.  The  single  ovary  (left  one)  is  situated  in  the  anterior 
part  of  the  peritoneal  cavity,  dorsal  to  the  gizzard. 

Make  a  drawing  of  the  digestive  tract. 

The  gizzard  and  proventriculus  may  now  be  freed  from  the  adjacent  air  sacs 
and  the  mesenteries.  On  turning  the  gizzard  far  forward  there  will  be  found 
between  the  proventriculus  and  the  anterior  end  of  the  right  limb  of  the  duodenal 
loop  a  rounded  red  body,  the  spleen.  The  gizzard  may  now  be  cut  open  along 
its  posterior  margin.  The  interior  contains  small  stones  and  probably  partially 
digested  food.  Note  the  extremely  thick  muscular  walls, and  the  hard  horny 
lining  of  the  gizzard.  Cut  from  the  gizzard  into  the  proventriculus  and  note  the 
soft  glandular  walls  of  the  latter.  The  gizzard  grinds  up  the  food  into  small 
pieces,  thus  taking  the  place  of  teeth,  and  the  proventriculus  digests  the  food  by 
means  of  the  digestive  fluid  secreted  by  the  glands  in  its  walls. 

6.  The  pleural  cavities  and  their  contents. — The  posterior  intermediate  air 
sac  situated  in  the  oblique  septum  may  now  be  punctured  if  this  has  not  already 
been  done.  The  two  walls  of  the  septum  are  now  more  clearly  observable.  The 


THE  COELOM,  DIGESTIVE,  AND  RESPIRATORY  SYSTEMS  183 

anterior  intermediate  air  sac  may  also  be  punctured.  Against  the  dorsal  wall 
of  the  pleural  cavity  on  each  side  will  be  found  a  reddish,  spongy  flattened  organ, 
the  lung.  The  openings  of  some  of  the  air  sacs  into  the  lungs  will  probably  be 
noted  on  some  specimens.  On  cutting  into  the  lung  the  organ  will  be  found  to 
be  solid,  not  hollow,  as  in  the  preceding  animals. 

The  cavity  in  which  each  lung  is  contained  is,  as  already  explained,  a  pleural 
cavity.  It  is  lined  by  a  coleomic  membrane,  the  pleura.  As  the  lungs  are  flat- 
tened against  the  dorsal  wall  of  the  pleural  cavity,  the  pleura  passes  over  their 
ventral  faces,  leaving  them  outside,  so  to  speak.  The  pleura,  furthermore, 
passes  over  the  surface  of  the  pericardial  sac  and  lines  the  inner  surface  of  the 
body  wall. 

7.  The  syrinx. — Examine  the  posterior  part  of  the  trachea.  Two  slender 
muscles,  the  sternotracheal  muscles,  diverge  from  their  insertion  on  the  ventral 
surface  of  the  trachea  to  their  origin  on  the  sternum.  These  muscles  should  be 
severed.  The  trachea  disappears  dorsal  to  the  heart  and  the  great  blood  vessels 
which  enter  and  leave  the  heart.  These  blood  vessels  must  not  be  injured. 
Loosen  the  trachea  and  pull  it  forward.  The  bifurcation  of  the  trachea  into  the 
two  bronchi  can  then  be  seen  dorsal  to  the  heart.  Cut  across  the  bronchi  with 
a  fine  scissors  and  draw  the  trachea  forward.  At  the  point  where  the  trachea 
forks  into  the  two  bronchi  an  expanded  chamber,  the  syrinx,  is  present.  The 
voice  of  birds  issues  from  the  syrinx,  not  from  the  larynx.  Along  each  side  of  the 
trachea  extending  from  the  point  of  insertion  of  the  sternotracheal  muscles  to 
the  lateral  walls  of  the  syrinx  is  a  muscle,  the  intrinsic  syringeal  muscle.  The 
walls  of  the  syrinx  are  supported  by  the  last  tracheal  rings  and  the  first  bronchial 
half-rings.  The  last  two  tracheal  rings  are  widely  separated  from  each  other 
but  are  connected  in  the  median  ventral  line  by  median  processes.  Make  a  slit 
in  the  ventral  wall  of  the  syrinx  and  spread  apart  the  cut  edges.  The  cavity  of 
the  syrinx  is  named  the  tympanum.  In  the  dorsal  wall  of  the  tympanum  a 
slight  vertical  fold  is  present  in  the  median  dorsal  line,  extending  forward  from 
the  level  of  the  bifurcation  of  the  trachea.  This  fold  is  called  the  semilunar 
membrane,  and  its  vibrations  are  said  to  produce  the  voice.  There  are  also  large 
thickenings  in  the  lateral  walls  of  the  tympanum  which  may  have  some  function 
in  the  production  of  the  voice.  The  sternotracheal  and  syringeal  muscles  doubt- 
less aid  by  changing  the  size  and  shape  of  the  tympanum. 

G.      THE   COELOM,   DIGESTIVE,   AND   RESPIRATORY   SYSTEMS   OF   A  MAMMAL 

The  following  directions  apply  to  both  the  rabbit  and  the  cat.  Whenever 
the  differences  between  the  two  animals  warrant,  a  separate  description  of  each 
will  be  given;  otherwise  they  will  be  described  together. 

i.  The  mouth  cavity  and  the  pharynx. — 

a)  The  salivary  glands:  The  salivary  glands  are  masses  of  gland  tissue  which 
are  outgrowths  of  the  lining  of  the  mouth  cavitv;  the  stalk  of  the  outgrowth 


184       LABORATORY  MANUAL  FOR  VERTEBRATE  ANATOMY 

remains  as  the  salivary  duct.  The  glands  are  situated  among  the  muscles  of  the 
head  and  throat.  They  should  be  located  according  to  the  following  descriptions 
and  their  ducts  followed  as  far  as  practicable.  There  are  four  pairs  of  salivary 
glands  in  the  rabbit,  five  in  the  cat.  The  dissection  should  be  carried  out  on 
the  same  side  of  the  head  as  that  on  which  the  muscles  were  dissected. 

The  parotid  gland  is  located  ventrad  and  craniad  of  the  base  of  the  pinna 
of  the  ear,  just  under  the  skin.  Remove  the  skin  from  this  region  and  find  the 
pinkish  gland  spread  out  under  the  skin  anterior  and  ventral  to  the  ear.  Its 
duct  passes  across  the  external  surface  of  the  masseter  muscle  and  penetrates 
the  upper  lip.  The  submaxillary  gland  has  already  been  noted  as  a  roundish  mass 
at  the  angle  of  the  jaw  near  the  posterior  margin  of  the  masseter.  Loosen  it 
and  find  the  duct  springing  from  the  internal  surface.  In  the  cat  the  beginning 
of  this  duct  is  surrounded  by  the  elongated  sublingual  gland.  Trace  the  sub- 
maxillary  duct  forward;  it  is  accompanied  in  the  cat  by  the  sublingual  duct. 
The  duct  will  be  found  to  pass  internal  to  the  digastric  muscle.  This  muscle 
should  be  severed.  The  duct  (or  two  ducts  in  the  cat)  will  then  be  seen  to  pass 
internal  to  the  mylohyoid  muscle.  This  in  turn  should  be  cut  and  the  duct 
traced  forward.  In  the  rabbit  the  small  flattened  sublingual  gland  will  soon  be 
noted  lying  in  the  path  of  the  submaxillary  duct.  The  submaxillary  duct  (accom- 
panied by  the  sublingual  duct  in  the  cat),  situated  just  external  to  the  lining 
of  the  mouth  cavity,  runs  forward  nearly  to  the  symphysis  of  the  mandible  and 
then  penetrates  the  lining.  In  the  rabbit  the  sublingual  gland  opens  into  the 
mouth  cavity  by  several  short  ducts  which  are  impractical  to  find.  The  molar 
gland,  present  in  the  cat  only,  is  situated  between  the  skin  and  the  external  sur- 
face of  the  mandible,  just  in  front  of  the  masseter  muscle.  It  will  be  found  by 
deflecting  the  skin  at  this  place.  It  opens  onto  the  inside  of  the  cheek  by 
several  small  ducts,  impractical  to  locate.  The  infraorbital  gland  in  both  cat 
and  rabbit  lies  in  the  floor  of  the  orbit  and  will  be  seen  later  when  the  eye  is 
dissected. 

b)  The  mouth  cavity:  Cut  through  the  skin  at  the  corners  of  the  mouth  and 
see  that  the  skin  is  well  cleared  away  over  the  angles  of  the  jaws.  Cut  through 
the  masseter  and  other  muscles  attached  to  the  lower  jaw  at  the  angle  of  the  jaws. 
It  should  then  be  possible  to  pull  the  lower  jaw  down.  Pry  open  the  mouth, 
grasp  the  lower  jaw,  and  exert  a  strong  traction.  The  jaw  will  generally  yield, 
but  if  it  does  not,  the  ramus  of  the  mandible  may  be  cut  through  with  the  bone 
scissors.  The  anterior  part  of  the  cavity  thus  revealed  is  the  mouth  or  oral  cavity. 
It  is  bounded  by  the  lips  and  cheeks.  That  part  of  the  oral  cavity  lying  between 
the  teeth  and  lips  is  called  the  vestibule  of  the  mouth.  The  teeth  were  described 
in  connection  with  the  skull. 

The  anterior  portion  of  the  roof  of  the  oral  cavity  is  occupied  by  the  hard 
palate,  the  posterior  part  by  the  soft  palate  which  is  very  long  in  the  rabbit. 
The  difference  between  the  hard  and  soft  palate  should  be  determined  by  feeling. 


THE  COELOM,  DIGESTIVE,  AND  RESPIRATORY  SYSTEMS  185 

The  hard  palate  is  supported  by  the  premaxillary,  maxillary,  and  palatine  bones, 
as  should  be  recalled  from  the  study  of  the  skull.  The  soft  palate  lacks 
bony  support.  The  mucous  membrane  of  the  hard  palate  is  thrown  into  a 
number  of  roughened  transverse  ridges.  At  the  anterior  end  of  the  hard  palate 
just  behind  the  incisor  teeth  will  be  found  a  pair  of  openings,  the  openings  of  the 
naso palatine  ducts  which  connect  the  mouth  and  nasal  cavities  by  way  of  the 
incisive  foramina  of  the  maxillary  bones.  The  opening  of  the  duct  of  the  parotid 
gland  may  be  sought  for  on  the  inside  of  the  cheek  opposite  the  second  upper 
premolar  tooth  in  the  rabbit,  opposite  the  last  cusp  of  the  third  upper  premolar 
of  the  cat,  in  which  animal  it  is  situated  on  a  slight  ridge:  the  openings  are 
difficult  to  identify  with  certainty  and  not  much  time  should  be  spent  in  looking 
for  them. 

The  floor  of  the  oral  cavity  is  occupied  by  the  tongue,  a  fleshy  muscular 
organ,  more  mobile  in  mammals  than  in  most  other  vertebrates.  The  anterior 
margin  of  the  attachment  of  the  tongue  to  the  floor  of  the  mouth  has  the  form  of 
a  vertical  fold,  ihefrenulum.  Halfway  between  the  lower  incisors  and  the  frenu- 
lum  will  be  found  in  the  rabbit  the  two  small  slitlike  openings  of  the  ducts  of  the 
submaxillary  glands,  the  two  being  about  an  eighth  of  an  inch  apart.  In  the  cat 
a  fold  runs  forward  from  the  frenulum  on  each  side  just  within  the  teeth,  and 
terminates  anteriorly  in  a  well-marked  flattened  papilla  which  bears  the  openings 
of  the  ducts  of  the  submaxillary  and  sublingual  glands. 

Cut  through  the  floor  of  the  mouth  on  each  side,  keeping  the  scalpel  next 
to  the  mandible.  The  tongue  can  now  be  pulled  down  and  out  between  the  two 
halves  of  the  lower  jaw.  The  cuts  may  be  continued  on  each  side  at  the  base  of 
the  tongue  back  to  the  level  of  the  submaxillary  glands  so  that  the  tongue  can 
be  pulled  well  down.  The  surface  of  the  tongue  may  now  be  examined  in  detail. 
In  the  rabbit  the  tongue  is  divisible  into  two  portions,  an  anterior  softer  portion, 
covered  with  minute  pointed  elevations,  the  fungiform  papillae;  and  a  posterior, 
elevated,  smoother,  and  harder  portion.  At  the  posterior  end  of  the  latter  on 
each  side  is  situated  a  vallate  papilla,  consisting  of  a  round  elevation  set  into  a 
pit.  In  front  of  each  vallate  papilla  on  the  side  of  the  tongue  is  an  oval  area  of 
considerable  size  marked  by  numerous  fine  parallel  ridges,  the  foliate  papilla. 
In  the  cat  the  anterior  part  of  the  tongue  is  covered  with  the  filiform  papillae, 
many  of  which  are  hard  and  spinelike,  pointed  posteriorly;  the  remainder  of  the 
tongue  is  provided  with  fungiform  papillae;  among  the  fungiform  papillae  are 
four  to  six  vallate  papillae  arranged  in  a  V-shaped  row,  each  consisting  of  a 
round  elevation  set  into  a  pit.  At  the  sides  of  the  vallate  papillae  are  some  very 
large  fungiform  papillae.  The  papillae  are  provided  with  microscopic  taste  buds. 

c)  The  pharynx:  The  pharynx  is  that  portion  of  the  cavity  lying  posterior 
and  dorsal  to  the  soft  palate.  Pull  the  tongue  well  forward  and  examine  the 
soft  palate.  It  descends  like  a  curtain  across  the  posterior  end  of  the  oral  cavity. 
Find  its  free  posterior  margin,  arching  above  the  base  of  the  tongue.  (The 


1 86  LABORATORY  MANUAL  FOR  VERTEBRATE  ANATOMY 

margin  may  be  concealed  by  a  leaf-shaped  structure,  the  epiglottis,  which  projects 
from  the  base  of  the  tongue.  If  so,  the  epiglottis  should  be  pressed  out  of  the 
way.)  The  opening  formed  by  the  free  border  of  the  palate  is  known  as  the 
isthmus  of  the  fauces.  This  opening  leads  into  the  cavity  of  the  pharynx.  Shortly 
anterior  to  the  free  border  of  the  soft  palate  on  each  side  is  a  pit,  the  tonsillar  fossa, 
which  contains  a  small  mass  of  lymphoid  tissue,  the  palatine  tonsil.  The  tonsillar 
fossa  is  bounded  in  front  and  behind  by  low  folds,  an  anterior  glossopalatine 
arch  and  a  posterior  pharyngo palatine  arch.  Now  slit  the  soft  palate  forward 
along  its  median  line.  A  cavity,  the  nasopharynx,  a  part  of  the  pharynx,  is 
revealed  dorsal  to  the  soft  palate.  At  the  anterior  end  of  the  nasopharynx 
are  the  two  posterior  nares  or  choanae,  the  internal  ends  of  the  nasal  passages. 
Posterior  to  them  on  the  lateral  wall  of  the  nasopharynx  will  be  noted  a  pair  of 
oblique  slits;  they  are  the  openings  of  the  auditory  or  Eustachian  tubes,  canals 
which  connect  the  pharynx  with  the  cavity  of  the  middle  ear. 

The  pharynx  narrows  posteriorly  into  the  esophagus.  Anteriorly  to  the 
entrance  into  the  esophagus  is  situated  the  entrance  into  the  respiratory  tract. 
This  entrance  is  guarded  by  a  projecting  process,  the  epiglottis,  which  if  not 
already  identified  will  be  seen  on  pulling  the  tongue  well  forward.  In  the 
pharynx  the  paths  for  food  and  air  are  crossed  (as  is  the  case  in  all  of  the  air- 
breathing  vertebrates).  It  will  be  noted,  however,  that  owing  to  the  formation 
of  the  palate  and  the  consequent  posterior  migration  of  the  posterior  nares, 
the  air  no  longer  enters  the  oral  cavity  as  is  the  case  in  Amphibia  and  most  rep- 
tiles but  proceeds  directly  into  the  pharynx. 

2.  The  hyoid  apparatus,  the  larynx,  the  trachea,  and  the  esophagus.— 
Press  the  tongue  dorsally  against  the  lower  jaw  and  find  on  its  external  surface 
at  its  base  a  bone,  the  body  of  the  hyoid.  This  is  a  stout  bone  in  the  rabbit,  a 
narrow  bar  in  the  cat.  Clear  away  muscles  from  its  surface  so  as  to  reveal  it 
and  the  two  horns  or  cornua  which  extend  from  its  sides.  In  the  rabbit  the 
horns  are  short  processes  which  are  connected  by  slender  tendinous  muscles 
with  the  jugular  process  of  the  occipital  bone.  In  the  cat  the  anterior  horn  is 
long  and  slender  and  consists  of  a  chain  of  four  bony  pieces,  the  last  of  which 
articulates  with  the  tympanic  bulla;  the  posterior  horn  is  short  and  is  united 
to  the  larynx.  The  hyoid  and  its  horns  are  derived  in  mammals  from  the 
second  and  third  gill  arches.  The  hyoid  supports  the  base  of  the  tongue  and 
serves  for  the  origin  and  insertion  of  muscles. 

In  the  median  ventral  line  posterior  to  the  body  of  the  hyoid  is  a  chamber 
with  cartilaginous  walls,  the  larynx  or  voice  box,  which  constitutes  the  projec- 
tion in  the  throat  popularly  known  as  Adam's  apple.  By  making  a  cut  through 
the  base  of  the  tongue  and  gently  severing  the  muscle  attachments,  the  larynx 
may  be  freed  and  lifted  forward.  At  the  top  of  the  larynx  is  a  large  opening, 
the  glottis,  from  whose  ventral  margin  the  epiglottis  projects.  Dorsal  to  the 
glottis  and  bound  with  it  by  muscles  is  another  opening,  generally  collapsed  and 


THE  COELOM,  DIGESTIVE,  AND  RESPIRATORY  SYSTEMS  187 

concealed  from  view  by  portions  of  the  larynx.  This  opening  should  be  located 
by  probing;  the  probe  will  be  found  to  enter  a  soft  tube  which  proceeds  poste- 
riorly dorsal  to  the  larynx.  This  tube  is  the  esophagus. 

The  structure  of  the  larynx  should  now  be  examined  in  detail.  The  ventral 
wall  of  the  larynx  is  supported  by  a  large  shield-shaped  cartilage,  the  thyroid 
cartilage.  A  short  distance  posterior  to  this  is  the  cricoid  cartilage,  which 
forms  a  ring  around  the  larynx.  The  dorsal  rim  of  the  glottis  between  the  glottis 
and  the  opening  to  the  esophagus  is  supported  by  a  pair  of  projecting  cartilages, 
the  arytenoids.  On  looking  into  the  glottis  a  pair  of  folds,  the  vocal  cords,  will 
be  seen  extending  from  the  arytenoid  cartilages  to  the  thyroid  cartilage.  They 
nearly  occlude  the  opening.  In  the  cat,  in  addition  to  these  true  vocal  cords, 
there  is  a  pair  of  false  vocal  cords,  situated  lateral  to  the  former  and  extending 
from  the  tips  of  the  arytenoid  cartilages  to  the  base  of  the  epiglottis.  It  will 
be  noted  that  the  vocal  cords  are  not  cords  but  folds  of  the  lateral  wall  of  the 
larynx.  On  dissecting  away  the  esophagus  from  the  dorsal  side  of  the  larynx 
the  dorsal  side  of  the  cricoid  cartilage  will  be  exposed.  It  is  much  broader  than 
the  ventral  side.  By  cleaning  away  the  mucous  membrane  covering  it,  the  two 
arytenoid  cartilages  which  rest  on  the  anterior  extremity  of  the  dorsal  part  of 
the  cricoid  will  be  exposed. 

From  the  larynx  the  trachea  or  windpipe  proceeds  posteriorly.  Its  walls 
are  stiffened  by  cartilaginous  rings,  which  are  incomplete  dorsally,  leaving  a 
soft  strip  in  the  dorsal  wall  of  the  trachea  into  which  the  esophagus  fits.  On 
each  side  of  the  trachea  lying  against  the  trachea  and  internal  to  the  muscles  is 
a  flattened  elongated  body,  one  of  the  lobes  of  the  thyroid  gland.  The  anterior 
end  of  each  lobe  is  at  a  level  with  the  cricoid  cartilage.  The  caudal  ends  of  the 
two  lobes  are  connected  by  a  median  portion,  the  isthmus,  which  crosses  the 
ventral  side  of  the  trachea.  The  trachea  is  not  to  be  traced  farther  posteriorly 
at  this  time. 

3.  The  pleural  and  pericardial  cavities. — The  trunk  of  mammals  is  divided 
into  an  anterior  thoracic  region  and  a  posterior  abdominal  region.  Each  of  these 
regions  contains  cavities  which  are  portions  of  the  coelom.  The  thoracic  region 
has  three  coelomic  cavities,  the  two  pleural  cavities,  laterally  located,  and  the 
median  pericardial  cavity,  situated  between  the  two  pleural  cavities. 

With  the  bone  scissors  make  a  cut  through  the  ribs  one-half  inch  to  the  left 
of  the  sternum,  extending  the  cut  the  length  of  the  sternum.  At  each  end  of 
this,  cut  laterally  and  dorsally  between  two  adjacent  ribs  at  right  angles  to  the 
first  cut.  In  this  way  a  flap  is  formed  in  the  chest  wall.  Open  the  flap  and  bend 
it  dorsally  so  that  you  can  look  within.  The  cavity  thus  revealed  is  the  left 
pleural  cavity  or  pleural  sac,  as  it  is  often  called ;  a  similar  sac  exists  on  the  right 
side.  The  pleural  sac  contains  the  soft  spongy  lung.  In  the  median  region 
under  the  sternum  lies  the  large  heart.  Note  the  delicate  partition  which 
stretches  from  the  heart  to  the  ventral  median  line.  This  partition  is  called  the 


i88       LABORATORY  MANUAL  FOR  VERTEBRATE  ANATOMY 

mediastinal  septum.  It  consists  of  the  two  medial  walls  of  the  right  and  left 
pleural  sacs  in  contact  with  each  other.  At  the  level  of  the  heart  the  two  walls 
separate  so  that  the  heart  and  its  pericardial  sac  are  inclosed  between  them. 
This  space  between  the  two  walls  of  the  mediastinal  septum  is  called  the  medias- 
tinum. The  posterior  wall  of  the  pleural  sac  is  formed  by  a  muscular  dome- 
shaped  partition,  the  diaphragm.  The  pleural  sac  is  lined  by  a  smooth  moist 
membrane,  the  pleura.  The  pleura  is  divided  into  parietal  and  visceral  parts. 
The  parietal  pleura  lines  the  inside  of  the  pleural  cavity,  covers  the  anterior  face 
of  the  diaphragm,  and  together  with  the  medial  wall  of  the  other  pleural  sac  forms 
the  mediastinal  septum.  The  visceral  pleura  is  that  part  of  the  pleura  which 
passes  over  the  surface  of  the  lung  to  which  it  is  indistinguishably  fused. 
Examine  the  left  lung.  It  is  a  soft  spongy  organ  divided  into  three  lobes,  a 
smaller  anterior,  and  larger  middle,  and  posterior  lobes.  The  anterior  lobe  is 
quite  small  in  the  rabbit.  The  large  posterior  lobe  fits  very  neatly  on  the  convex 
surface  of  the  diaphragm.  Cut  into  the  lung;  it  appears  solid  but  is  really  com- 
posed of  innumerable  minute  air-cells. 

Now  carefully  cut  through  the  mediastinal  septum  ventral  to  the  heart  and 
look  into  the  right  pleural  cavity.  The  diaphragm  may  be  slit  along  its  left  side 
so  as  to  facilitate  the  spreading  apart  of  the  thoracic  walls.  The  right  pleural 
cavity  is  similar  to  the  left  cavity.  It  contains  the  right  lung.  The  right  lung 
is  somewhat  larger  than  the  left  lung.  It  is  divided  into  anterior,  middle,  and 
posterior  lobes.  The  large  posterior  lobe  is  subdivided  into  two  lobules,  a  medial 
and  a  lateral.  The  medial  lobule  projects  into  a  pocket  formed  by  a  special 
dorsally  directed  fold  of  the  mediastinal  septum.  This  fold,  the  caval  fold,  has 
the  function  of  supporting  a  large  vein,  the  postcaval  vein,  which  ascends  from 
the  liver  to  the  heart  and  will  be  found  inclosed  in  the  free  dorsal  margin  of  the 
caval  fold. 

Examine  the  heart  and  the  pericardial  sac.  The  pericardial  sac  or  parietal 
pericardium  is  a  sac  of  thin  tissue  inclosing  the  heart  but  not  attached  to  it  except 
at  the  anterior  end  where  the  great  vessels  enter  and  leave  the  heart.  The  heart 
is  freely  movable  inside  of  the  pericardial  sac.  The  narrow  space  between  the 
pericardial  sac  and  the  heart  is  the  pericardial  cavity,  a  portion  of  the  coelom. 
Cut  through  the  pericardial  sac  so  as  to  expose  the  heart.  The  surface  of  the 
heart  is  invested  by  a  thin  membrane,  the  visceral  pericardium,  inseparably 
adherent  to  the  heart  wall.  The  visceral  pericardium  is  continuous  with  the 
pericardial  sac  at  the  anterior  end  where  the  blood  vessels  enter  and  leave  the 
heart.  As  the  heart  with  its  pericardial  sac  is  situated  in  the  mediastinum,  it  is 
evident  that  there  are  three  coelomic  layers  surrounding  the  heart:  the  visceral 
pericardium  closely  adherent  to  the  heart  wall,  the  parietal  pericardium  or 
pericardial  sac  separated  from  the  heart  by  the  pericardial  cavity,  and  the  parietal 
pleura  of  the  mediastinal  septum,  which  is  closely  fused  to  the  pericardial  sac 
(Fig.4o£,  p.  197). 


THE  COELOM,  DIGESTIVE,  AND  RESPIRATORY  SYSTEMS  189 

In  the  mediastinum  in  the  median  line  ventral  to  the  anterior  part  of  the  heart 
and  extending  forward  will  be  found  a  mass  of  gland  tissue,  the  thymus.  It  is 
larger  the  younger  the  specimen.  In  searching  for  it  do  not  injure  the  large 
blood  vessels  occurring  in  this  region.  The  thymus  is  possibly  one  of  the  glands 
of  internal  secretion  and  is  derived  from  the  entodermal  lining  of  certain  of 
the  visceral  pouches  of  the  embryo. 

Now  press  the  heart  and  the  left  lung  over  to  the  right.  The  lung  will  be 
found  attached  by  a  narrow  region,  the  radix  or  root  of  the  lung.  An  artery,  a 
vein,  and  a  bronchus  or  air  tube  pass  to  the  lung  and  veins  from  the  lung  in  the 
root,  but  these  structures  are  better  investigated  at  a  later  tune.  In  the  cat, 
furthermore,  the  lung  is  attached  along  most  of  its  length  to  the  dorsal  thoracic 
wall  by  the  pulmonary  ligament,  a  fold  of  the  pleura.  Note  that  dorsal  to  the  root 
of  the  lung  the  pleura  continues  onto  the  dorsal  and  lateral  surfaces  of  the 
pleural  cavity  and  that  certain  structures  can  be  seen  that  lie  internal  to  the 
pleura.  These  structures  lie  between  the  dorsal  portions  of  the  two  walls  of 
the  mediastinal  septum  and  consequently  are  situated  in  the  mediastinum. 
The  most  conspicuous  of  these  structures  lying  in  the  mediastinum  is  the  dorsal 
aorta,  a  very  large  vessel  injected  with  a  colored  solution  which  arches  away  from 
the  heart  to  the  left  and  descends  toward  the  diaphragm.  About  one-half  an 
inch  ventral  to  the  aorta  is  another  tube,  the  esophagus,  also  lying  in  the  medias- 
tinum. Trace  it  posteriorly  to  the  place  where  it  penetrates  the  diaphragm. 

The  diaphragm  is  a  curved  sheet  forming  the  posterior  wall  of  the  thoracic 
cavity  and  completely  separating  it  from  the  abdominal  cavity.  The  center  of 
the  diaphragm  is  seen  to  consist  of  connective  tissue  forming  a  circular  tendon, 
the  central  tendon  of  the  diaphragm.  The  remainder  of  the  diaphragm  is  muscu- 
lar. The  diaphragm  takes  origin  from  the  ribs,  sternum,  and  vertebrae,  and 
is  inserted  on  the  central  tendon.  It  is  an  important  respiratory  muscle.  When 
contracted,  it  flattens,  thus  lengthening  the  pleural  cavities  posteriorly  and  caus- 
ing air  to  rush  into  the  lungs.  The  diaphragm  is  pierced  at  several  points  to 
allow  important  structures  to  pass  through;  the  chief  ones  which  penetrate  the 
diaphragm  were  already  noted,  i.e.,  the  esophagus,  the  aorta,  and  thepostcaval 
vein.  The  diaphragm  is  a  structure  peculiar  to  mammals.  It  is  formed  in  part 
of  the  transverse  septum  and  in  part  of  other  coelomic  membranes;  it  then 
becomes  invaded  by  muscle  buds  from  the  adjacent  cervical  myotomes. 

Make  a  diagram  of  a  cross-section  through  the  thorax  showing  the  pleural 
and  pericardial  cavities  and  the  relation  of  their  linings  to  the  thoracic  wall,  lungs, 
and  heart. 

4.  The  peritoneal  cavity  and  its  contents. — Make  a  longitudinal  slit  through 
the  abdominal  wall,  a  little  to  the  left  of  the  median  ventral  line  from  the  inguinal 
region  up  to  the  diaphragm.  Widen  the  opening  by  a  transverse  slit  in  the 
middle  of  the  left  abdominal  wall.  A  large  cavity,  the  abdominal  or  peritoneal 


ioo       LABORATORY  MANUAL  FOR  VERTEBRATE  ANATOMY 

cavity,  is  exposed.  Its  anterior  wall  is  formed  by  the  concavely  arched  diaphragm 
which  completely  separates  the  peritoneal  from  the  pleural  cavities.  Posterior 
to  the  diaphragm  and  shaped  so  as  to  fit  the  concave  surface  of  the  diaphragm 
is  the  large,  lobed  liver,  generally  grayish  brown  in  preserved  specimens.  Pos- 
terior to  the  liver  the  peritoneal  cavity  is  filled  by  the  coils  of  the  intestine.  In 
the  cat  the  intestine  is  covered  ventrally  by  a  thin  membrane  impregnated  with 
streaks  of  fat,  the  greater  omentum.  This  membrane  is  present  also  in  the  rabbit, 
but  is  very  much  smaller  and  less  conspicuous.  At  the  posterior  end  of  the  per- 
itoneal cavity  may  be  noted  the  pear-shaped  urinary  bladder,  generally  distended 
with  fluid.  On  raising  the  liver  and  looking  dorsally  and  to  the  left  of  it  will  be 
found  the  stomach  with  the  spleen  attached  to  its  left  border.  On  the  dorsal  wall 
of  the  peritoneal  cavity  at  about  the  level  of  the  posterior  ends  of  the  liver  lobes 
are  the  kidneys,  round  organs;  to  see  them,  gently  lift  the  coils  of  the  intestine. 
In  female  specimens,  especially  those  which  are  pregnant,  the  horns  of  the  uterus 
will  be  noted  as  a  tube  on  each  side  in  the  posterior  part  of  the  peritoneal  cavity. 

The  peritoneal  cavity  is  lined  by  a  membrane,  the  peritoneum.  As  in  all 
coelomate  animals,  that  portion  of  the  membrane  on  the  inside  of  the  body  wall 
is  the  parietal  peritoneum.  In  both  dorsal  and  ventral  regions  the  peritoneum 
is  deflected  from  the  body  wall  and  passes  over  the  surface  of  the  viscera,  forming 
a  covering  layer,  the  visceral  peritoneum  or  serosa,  for  all  of  the  viscera.  In 
passing  to  and  from  the  body  wall  to  the  viscera,  the  peritoneum  forms  double- 
walled  membranes,  the  mesenteries  or  ligaments.  The  dorsal  mesentery  is  present 
intact  in  mammals,  and  the  ventral  mesentery  persists  in  the  region  of  the  liver 
and  urinary  bladder  as  in  other  vertebrates. 

Examine  the  stomach  first,  by  raising  the  liver  and  pressing  it  craniad.  The 
exposure  of  the  stomach  is  facilitated  by  slitting  the  diaphragm  on  the  left  side. 
The  stomach  is  a  large  and  rounded  organ  in  the  rabbit,  smaller  and  more  elongated 
in  the  cat.  Find  where  the  esophagus  emerges  from  the  diaphragm  and  enters 
the  anterior  surface  of  the  stomach.  The  area  of  junction  of  the  stomach  and 
esophagus  is  called  the  cardia;  and  the  region  of  the  stomach  adjacent  to  the  junc- 
tion, the  cardiac  end  of  the  stomach.  The  shorter,  slightly  concave  anterior 
surface  of  the  stomach  from  the  cardia  to  the  pylorus  is  the  lesser  curvature;  the 
larger  convex  posterior  surface,  the  greater  curvature.  The  saclike  bulge  of 
the  stomach  to  the  left  of  the  cardia  is  known  as  the  fundus;  the  remainder 
of  the  stomach,  the  body.  At  the  right  the  stomach  passes  into  the  small 
intestine,  the  point  of  junction,  known  as  the  pylorus,  being  marked  by  a  con- 
striction, beyond  which  the  small  intestine  makes  an  abrupt  bend.  Along  the  left 
side  of  the  stomach  lies  the  spleen,  a  rather  large  organ  in  the  cat,  but  smaller  in 
the  rabbit. 

The  relations  of  the  stomach  to  the  peritoneum  are  somewhat  complicated. 
Raise  the  fundus  and  note  the  mesogaster  extending  from  the  dorsal  wall  to  the 
stomach.  Only  a  small  portion  of  the  mesogaster  passes  directly  to  the  stomach; 


THE  COELOM,  DIGESTIVE,  AND  RESPIRATORY  SYSTEMS  191 

the  greater  part  of  it  first  descends  posteriorly,  forming  a  bag,  the  greater  omentum. 
This  is  a  very  large  and  extensive  sheet  in  the  cat,  covering  the  intestine  ventrally 
as  noted  above.  In  the  rabbit  it  is  a  short  membrane  dependent  from  the  greater 
curvature  of  the  stomach.  The  greater  omentum  is  to  be  thought  of  as  formed 
in  the  following  way.  Suppose  one  should  grasp  the  mesogaster  and  pull  it  pos- 
teriorly, drawing  it  into  a  sac.  Such  a  sac  would  have  two  walls,  each  double, 
i.e.,  composed  of  the  two  layers  of  the  mesogaster;  the  sac  would  contain  a 
cavity  which  is  known  as  the  lesser  peritoneal  sac  and  would  open  anteriorly  (this 
opening  will  be  seen  later)  (see  Fig.  50,  p  198.).  By  manipulating  the  omentum 
determine  that  it  consists  of  two  separate  walls.  Having  formed  the  greater 
omentum  the  mesogaster  returns  to  the  stomach  wall  and  passes  onto  the  stomach 
along  the  greater  curvature.  The  spleen  is  inclosed  in  the  ventral  wall  of  the 
great  omentum  just  before  the  latter  passes  to  the  stomach.  The  portion  of  the 
great  omentum  between  the  spleen  and  the  stomach  is  called  the  gastrosplenic 
(or  gastrolienal)  ligament.  Posterior  to  the  spleen,  near  the  left  kidney,  a  second- 
ary fusion,  the  gastrocolic  ligament,  has  formed  between  the  mesogaster  and  the 
mesentery  of  the  intestine  (see  Fig.  50). 

The  greater  omentum  owes  its  origin  in  part  to  the  rotation  of  the  stomach. 
The  line  of  attachment  of  the  omentum  to  the  greater  curvature  is  the  original 
dorsal  surface  of  the  stomach,  while  the  lesser  curvature  is  the  original  ventral 
surface.  The  mesogaster  passes  over  the  stomach,  forming  the  visceral  peri- 
toneum of  the  stomach,  and  inclosing  the  stomach  between  its  walls,  and  at 
the  lesser  curvature  is  continued  by  a  strong  ligament,  the  lesser  omentum,  which 
passes  to  the  liver. 

The  liver  may  be  studied  next.  It  presents  a  convex  anterior  surface,  fitting 
against  the  posterior  surface  of  the  diaphragm,  and  a  concave  posterior  surface, 
fitting  over  the  stomach  and  first  part  of  the  small  intestine.  The  liver  is  divided 
into  right  and  left  lobes,  each  of  which  is  subdivided  into  two  lobes,  a  median  and  a 
lateral.  The  left  lateral  and  right  median  lobes  are  larger  than  the  others.  In 
the  cat  the  right  lateral  lobe  is  deeply  cleft  into  two  lobules.  The  large  elongated 
gall  bladder  is  imbedded  in  the  right  median  lobe,  on  its  dorsal  surface  in  the 
rabbit,  in  a  cleft  in  this  lobe  in  the  cat.  On  raising  the  liver  and  looking  between 
the  liver  and  the  stomach  another  small  lobe,  the  caudate  lobe,  will  be  seen.  It  is 
situated  between  the  two  layers  of  the  lesser  omentum.  The  lesser  omentum, 
or  gastro-hepato-duodenal  ligament,  is  the  ligament  passing  from  the  lesser  curva- 
ture of  the  stomach  to  the  posterior  surface  of  the  liver.  It  is  divisible  into  two 
portions:  the  gastrohepatic  ligament  from  the  lesser  curvature  to  the  liver,  and 
the  hepatoduodenal  ligament  from  the  liver  to  the  first  part  of  the  small  intestine. 
That  portion  of  the  gastrohepatic  ligament  which  contains  the  caudate  lobe  of 
the  liver  forms  a  sac  which  continues  anteriorly  the  cavity  of  the  greater  omen- 
tum. In  the  hepatoduodenal  ligament  runs  the  bile  duct  which  should  be  traced 
from  the  gall  bladder  by  gently  dissecting  in  the  ligament.  Note  the  cystic  duct 


IQ2       LABORATORY  MANUAL  FOR  VERTEBRATE  ANATOMY 

from  the  gall  bladder  and  the  hepatic  ducts  from  the  lobes  of  the  liver,  that  from 
the  right  lateral  lobe  being  especially  large.  The  cystic  and  hepatic  ducts  unite 
to  form  the  common  bile  duct  which  passes  to  the  intestine  in  the  hepatoduodenal 
ligament.  It  should  be  traced  to  the  duodenum  by  cleaning  away  the  connective 
tissue  around  it.  Note  to  the  right  and  dorsal  to  the  bile  duct,  lying  also  in 
the  hepatoduodenal  ligament,  the  large  hepatic  portal  vein.  This  must  not  be 
injured.  Immediately  dorsal  to  this  vein,  posterior  to  its  branch  into  the  right 
lateral  lobe  of  the  liver,  the  hepatoduodenal  ligament  has  a  free  border  which 
forms  the  ventral  rim  of  an  opening  or  slit  of  some  size,  the  foramen  epiploicum 
or  entrance  into  the  cavity  of  the  omen  turn.  It  can  be  identified  with  certainty 
by  making  a  slit  into  the  cavity  of  the  omen  turn  and  probing  through  the  slit 
toward  the  right,  toward  the  spot  just  described. 

The  lesser  omentum  extends  to  the  middle  of  the  posterior  face  of  the  liver, 
where  it  becomes  the  serosa  of  the  liver;  here  its  two  walls  part  and  inclosing  the 
liver  between  them  pass  to  the  anterior  face  of  the  liver,  where  they  again  unite 
to  form  ligaments.  The  falciform  ligament  extends  from  between  the  two  median 
lobes  of  the  liver  to  the  median  ventral  line;  it  is  a  thin  sheet  with  a  concave 
posterior  border.  Anteriorly  and  dorsally  it  is  continuous  with  the  coronary 
ligament,  a  stout  ligament  which  attaches  the  liver  to  the  central  tendon  of  the 
diaphragm.  The  coronary  ligament  is  circular  in  form,  and  its  ring  of  attachment 
to  the  liver  bounds  a  small  space  on  the  anterior  face  of  the  liver  which  is  free  from 
serosa. 

Now  trace  the  intestine  from  the  pylorus.  Its  first  portion,  the  duodenum, 
is  bound  to  the  liver  by  the  hepatoduodenal  ligament.  The  duodenum  curves 
abruptly  caudad.  In  the  rabbit  it  is  very  long  and  forms  a  loop.  The  part  of 
this  loop  which  descends  posteriorly  is  named  the  descending  limb;  the  short 
turn  at  the  most  posterior  part  of  the  loop  is  the  transverse  limb;  and  the  part 
which  ascends  anteriorly  toward  the  stomach  is  called  the  ascending  limb.  The 
duodenum  of  the  cat  descends  caudad  for  about  two  inches  and  then  turns  to  the 
left.  The  duodenum  is  supported  by  a  part  of  the  dorsal  mesentery,  the  mesoduo- 
denum.  It  is  also  attached  to  the  right  kidney  by  a  mesenterial  fold,  the  duode- 
norenal  ligament.  Located  in  the  mesoduodenum  is  the  pancreas.  It  will  be 
seen  by  spreading  the  mesentery.  In  the  rabbit  the  pancreas  consists  of  streaks 
of  gland  tissue  scattered  in  the  mesentery  and  situated  chiefly  along  the  courses 
of  blood  vessels.  In  the  cat  the  pancreas  is  a  definite  compact  white  gland  which 
extends  to  the  left  into  the  dorsal  wall  of  the  greater  omentum,  dorsal  to  the 
greater  curvature  of  the  stomach.  In  the  rabbit  the  pancreatic  duct  enters  the  duo- 
denum about  an  inch  or  an  inch  and  one-half  anterior  to  the  beginning  of  the 
ascending  limb  of  the  duodenum.  This  location  of  the  pancreatic  duct  is  unusual 
in  mammals.  In  the  cat  there  are  two  pancreatic  ducts.  The  principal  one 
joins  the  common  bile  duct  at  the  point  where  the  latter  enters  the  duodenum. 
On  picking  away  the  substance  of  the  pancreas  at  this  point  the  duct  is  readily 


THE  COELOM,  DIGESTIVE,  AND  RESPIRATORY  SYSTEMS  193 

located.  The  common  slightly  swollen  chamber  where  bile  and  pancreatic  ducts 
unite  is  known  as  the  ampulla  of  Vater.  The  second  or  accessory  pancreatic  duct 
in  the  cat  enters  the  duodenum  about  three-quarters  of  an  inch  caudad  of  the 
principal  duct  but  is  not  easy  to  find. 

From  the  duodenum  trace  the  coils  of  the  remainder  of  the  small  intestine. 
It  is  supported  by  a  part  of  the  dorsal  mesentery,  the  mesentery  proper.  The 
first  portion  of  the  small  intestine  beyond  the  duodenum  is  called  the  jejunum, 
and  the  remainder,  the  ileum,  but  there  is  no  definite  boundary  between  the  two. 
Note  the  coils  of  the  mesentery  accompanying  the  intestine  and  the  frequent 
fusions  which  occur  (especially  in  the  rabbit)  between  these  coils.  In  the  cat 
it  will  be  necessary  to  withdraw  the  greater  omentum  from  the  coils  of  the 
intestine;  the  omentum  may  then  be  cut  across  near  the  spleen  and  discarded. 
Follow  the  small  intestine  to  its  enlargement  into  the  large  intestine.  In  doing 
this  it  may  be  necessary  to  tear  slightly  the  fusions  of  the  mesentery,  but  the 
structures  should  be  kept  as  intact  as  practicable. 

Rabbit:  At  the  point  of  juncture  of  the  large  and  small  intestine  there  is  an 
enlargement,  the  sacculus  rotundus.  From  the  sacculus  rotundus  extends  an 
enormous  blind  sac,  about  a  foot  and  a  half  in  length.  The  first  foot  of  this  is 
very  large  and  is  known  as  the  caecum;  the  last  five  or  six  inches  is  reduced  in 
diameter  and  constitutes  the  vermiform  appendix.  Both  caecum  and  appendix 
are  very  much  longer  in  the  rabbit  than  in  most  mammals,  owing  probably 
to  the  habit  of  the  animal  of  ingesting  large  quantities  of  vegetable  food.  The 
wall  of  the  caecum  is  marked  by  a  spiral  line  which  denotes  the  position  of  an 
internal  spiral  fold  of  the  lining  membrane  of  the  caecum.  From  the  sacculus 
rotundus  trace  the  large  intestine  after  it  has  given  off  the  caecum.  The  large 
intestine  beyond  the  caecum  is  named  the  colon.  The  colon  is  supported  by  a 
part  of  the  dorsal  mesentery  called  the  mesocolon.  The  first  part  of  the  colon,  the 
ascending  colon,  is  rather  long  and  pursues  a  winding  course.  At  first  its  wall 
bears  three  longitudinal  muscular  bands,  the  bands  or  taeniae  of  the  colon.  Be- 
tween the  taeniae  the  wall  of  the  colon  is  greatly  puckered,  forming  little  saccula- 
tions,  the  haustra.  Beyond  the  ascending  colon,  the  colon  runs  for  a  short 
distance  transversely  across  the  peritoneal  cavity  from  right  to  left  and  is  then 
named  the  transverse  colon.  At  the  left  it  turns  abruptly  posteriorly  as  the  de- 
scending colon.  At  this  turn  the  mesocolon  is  fused  to  the  mesogaster.  The 
descending  colon  passes  straight  posteriorly  and  disappears  dorsal  to  the  uro- 
genital  organs. 

Cat:  The  junction  of  small  and  large  intestine  is  marked  by  a  slight  pro- 
jection, the  caecum,  a  vermiform  appendix  being  practically  absent.  The  large 
intestine  or  colon  passes  forward  as  the  ascending  colon;  then  turns  and  extends 
across  the  peritoneal  cavity  from  right  to  left  as  the  transverse  colon ;  and  turns 
abruptly  at  the  left  and  proceeds  straight  posteriorly  as  the  descending  colon. 
The  mesentery  of  the  colon  is  named  the  mesocolon.  At  the  left  where  the 


194       LABORATORY  MANUAL  FOR  VERTEBRATE  ANATOMY 

transverse  colon  turns  caudad  the  mesocolon  is  fused  secondarily  to  the 
mesogaster. 

The  urinary  bladder  is  a  sac  occupying  the  posterior  end  of  the  peritoneal 
cavity,  immediately  internal  to  the  body  wall  and  ventral  to  the  large  intestine. 
From  the  ventral  surface  of  the  bladder  a  mesentery,  the  median  ligament  of  the 
bladder  (median  umbilical  fold  of  human  anatomy)  extends  to  the  median  ventral 
line  and  here  continues  forward  for  some  distance.  Near  the  exit  of  the  bladder 
from  the  peritoneal  cavity  there  is  on  each  side  a  slightly  developed  ligament, 
the  lateral  ligament  of  the  bladder. 

The  terminal  portion  of  the  descending  colon  is  the  rectum.  Both  the  rectum 
and  the  duct  of  the  bladder  pass  to  the  exterior  through  the  ring  formed  by  the 
pelvic  girdle  and  vertebral  column.  They  will  be  followed  at  a  later  time  in 
connection  with  the  urogenital  system.  For  the  present  it  may  be  stated  that 
the  rectum  and  urogenital  ducts  are  completely  separated  from  each  other.  A 
cloaca  is  therefore  absent. 

The  small  bodies  which  may  have  been  noted  in  the  mesentery,  usually  buried 
in  fat,  are  lymph  glands,  parts  of  the  lymphatic  system.  Small  portions  of 
lymphatic  tissue  called  lymph  nodules  are  also  abundantly  present  in  the  wall 
of  the  intestine.  Aggregations  of  lymph  nodules  are  known  as  Peyer's  patches. 
Peyer's  patches  are  thickened  oval  spots  on  the  surface  of  the  intestine,  of 
slightly  different  color  from  the  rest  of  the  intestinal  wall.  Look  for  them. 
They  occur  in  the  rabbit  along  the  entire  small  intestine  on  the  side  opposite 
that  attached  to  the  mesentery;  there  is  a  larger  patch  at  the  place  of  junction 
of  sacculus  rotundus  and  caecum.  The  walls  of  the  sacculus  rotundus,  caecum, 
and  vermiform  appendix  are  composed  almost  entirely  of  lymph  nodules.  In 
the  cat,  Peyer's  patches  are  readily  found  as  oval  light-colored  spots  along  the 
colon. 

Slit  open  various  parts  of  the  digestive  tract  along  the  side  opposite  the  attach- 
ment of  the  mesentery.  Wash  out  the  interior.  In  the  cat  note  the  marked 
ridges  or  rugae  in  the  wall  of  the  stomach;  these  are  very  slight  in  the  rabbit. 
Cut  through  the  pylorus  and  note  the  thickened  ridge  or  pyloric  valve  at 
this  place.  In  the  wall  of  the  small  intestine  observe  the  velvety  appearance 
due  to  the  villi  (finger-like  projections  of  the  mucous  membrane).  Find  also 
the  depressions  marking  the  positions  of  the  lymph  nodules.  In  the  rabbit  slit 
open  the  sacculus  rotundus,  caecum,  and  appendix,  and  note  the  spotted  appear- 
ance of  the  interior  owing  to  the  lymph  nodules  composing  the  walls.  In  the 
interior  of  the  caecum  is  a  spiral  ridge.  Cut  through  the  junction  of  large  and 
small  intestine  and  note  in  both  animals  an  elevation,  the  ileocolic  valve,  projecting 
into  the  ileum. 

H.   THE  COMPARATIVE  ANATOMY  OF  THE  COELOM  AND  THE  MESENTERIES 

Since  the  development  and  comparative  anatomy  of  the  coelom  and  mesenteries  are 
extremely  difficult  and  complicated  subjects,  it  seems  advisable  at  the  risk  of  repetition  to 


THE  COELOM,  DIGESTIVE,  AND  RESPIRATORY  SYSTEMS 


195 


make  some  concluding  general  remarks  concerning  them.  Only  a  simplified  account  can  be 
given  here. 

The  coelom  of  fishes  and  urodeles  is  divided  into  two  compartments,  a  small  anterior 
pericardial  cavity  and  a  much  larger  posterior  pleuroperitoneal  cavity.  The  former  is  situated 
entirely  anterior  to  the  latter  and  far  anterior  in  the  body,  at  the  level  of  the  pharynx.  The 
partition  between  the  two  coelomic  cavities  in  these  groups  is  the  transverse  septum.  This 
septum  is  formed,  as  previously  explained,  for  the  purpose  of  conveying  the  great  veins  from 
the  dorsal  body  wall  into  the  posterior  end  of  the  heart.  It  arises  first  as  a  column  of  mesen- 
chyme  on  each  side,  conveying  a  vein  into  the  heart;  later  these  columns  enlarge  and  fuse  to 
form  a  partition.  The  transverse  septum  is  a  rather  thick  membrane  with  anterior  and 
posterior  walls,  which  inclose  between  them  the  great  veins  and  the  posterior  end  of  the  heart. 
The  septum  in  fishes  and  urodeles  lies  as  the  name  implies  in  a  plane  transverse  to  the  body 
axis  (Fig.  45^!  and  B,  p.  160). 

The  lining  of  the  pericardial  cavity  is  named  the  parietal  pericardium.  In  fishes  and 
urodeles  it  also  constitutes  the  lining  of  the  body  wall  in  the  region  under  consideration.  The 


serosa  of  liver 


intestine 

peritoneal 
cavity 

cloaca 


peritoneal 
cavity 


cloaca 


FIG.  48. — Diagrams  to  show  the  separation  of  the  liver  from  the  transverse  septum  and  formation 
of  the  coronary  ligament.  A,  liver  inclosed  in  the  transverse  septum.  B,  beginning  of  constriction  of 
the  liver  from  the  septum  at  x.  C,  completion  of  the  constriction,  leaving  the  liver  suspended  from 
the  septum  by  the  coronary  ligament.  The  falciform  ligament  is  also  formed  at  the  same  time  by  the 
same  constriction.  (B  and  C  after  McMurrich's  Development  of  the  Human  Body,  copyright  by  P.  Blakis- 
ton's  Son  and  Company.) 

parietal  pericardium  is  continuous  with  the  anterior  face  of  the  transverse  septum,  which,  in 
fact,  forms  the  posterior  part  of  the  parietal  pericardium  and  the  posterior  wall  of  the  pericardial 
cavity.  The  lining  of  the  pleuroperitoneal  cavity  is  the  pleuroperitoneum.  This  is  continuous 
anteriorly  with  the  posterior  face  of  the  transverse  septum,  which  constitutes  the  anterior  part 
of  the  pleuroperitoneum  and  forms  the  anterior  wall  of  the  pleuroperitoneal  cavity  (see  Fig. 
45^4  and  B). 

The  relations  of  the  transverse  septum  to  the  liver  are  somewhat  complicated  and  require 
explanation.  The  liver  is  a  diverticulum  from  the  small  intestine.  It  happens  that  at  the 
point  where  the  liver  diverticulum  grows  out  ventrally  from  the  intestine,  the  transverse 
septum  is  situated.  Consequently,  the  liver  is  compelled  to  grow  out  into  the  septum.  It 
lies  at  first  within  the  mesenchyme  of  the  septum,  between  the  two  walls  of  the  septum.  Owing 
to  the  fact  as  already  explained  that  the  great  veins  enter  the  heart  by  way  of  the  septum,  the 
liver  also  acquires  important  relations  to  these  veins,  as  will  be  discussed  in  the  section  on  the 
circulatory  system.  The  liver  rapidly  increases  in  size  so  that  it  can  no  longer  be  contained 
within  the  limits  of  the  septum.  It  consequently  bulges  posteriorly,  carrying  with  it  the 


196       LABORATORY  MANUAL  FOR  VERTEBRATE  ANATOMY 

posterior  wall  of  the  septum,  which  thus  becomes  the  peritoneal  covering  or  serosa  of  the  liver. 
Later,  the  region  where  the  liver  bulges  from  the  septum  narrows  down  on  dorsal  and  lateral 
sides,  leaving  anterior  and  ventral  connections  between  the  liver  and  the  septum  (Fig.  48). 
The  anterior  connection  is  named  the  coronary  ligament  and  is,  as  the  name  indicates,  a  circular 
ligament,  by  means  of  which  the  liver  is  permanently  in  all  vertebrates  suspended  from  the 
transverse  septum  or  its  derivative.  The  ventral  partition  formed  by  the  constriction  of  the 
liver  from  the  septum  is  the  falciform  ligament  of  the  liver,  which  is  also  by  virtue  of  its  loca- 
tion a  part  of  the  ventral  mesentery  (Fig.  44$,  p.  158).  We  thus  see  that  the  transverse  septum 
has  the  following  parts:  the  posterior  wall  of  the  pericardial  cavity,  the  anterior  wall  of  the 
pleuroperitoneal  cavity,  the  falciform  and  coronary  ligaments  of  the  liver,  the  serosa  of  the 
liver,  and  the  mesoderm  tissue  of  the  liver. 

In  Anura  and  reptiles  the  coelom  is  divided  in  the  same  way  as  in  fishes  and  urodeles  into 
two  compartments,  the  pericardial  and  pleuroperitoneal  cavities.  In  these  groups,  however, 
the  pericardial  cavity  is  no  longer  situated  anterior  to  the  pleuroperitoneal  cavity  but  has 
descended  posteriorly  until  it  lies  ventral  to  the  anterior  part  of  the  latter  cavity.  This  descent 
occurs  during  embryonic  development.  In  this  descent  the  pericardial  cavity  necessarily 
carries  with  it  the  transverse  septum,  since  this  septum  formed  the  posterior  wall  of  the  cavity. 
Furthermore,  since  the  parietal  pericardium  or  wall  of  the  pericardial  cavity  in  the  lower 
vertebrates  lined  the  inside  of  the  body  wall,  it  is  necessary  that  this  membrane  become  split, 
at  least  in  part,  from  the  body  wall  before  the  descent  can  take  place.  This  happens  mainly 
by  the  invasion  of  the  pleuroperitoneal  cavity  forward.  As  a  result  of  these  processes,  the 
parietal  pericardium  is  separated  from  the  body  wall,  at  least  in  part,  and  forms  a  delicate 
sac  about  the  heart,  the  pericardial  sac.  The  separation  of  the  pericardial  sac  from  the  body 
wall  is  complete  in  the  Anura  but  in  the  turtle  is  incomplete  ventrally  so  that  the  ventral  wall 
of  the  pericardial  cavity  is  still  in  contact  ventrally  with  the  body  wall  (see  Fig.  456",  p.  160). 

In  the  groups  under  consideration  the  relations  of  the  transverse  septum  remain  the  same 
as  before,  but  its  position  is  now  oblique,  indeed  almost  frontal,  while  formerly  it  was  trans- 
verse (Fig.  45Q.  It  still  presents  anterior  and  posterior  walls  (which  are  now  nearly  ventral 
and  dorsal  in  position)  and  still  incloses  the  great  veins  and  the  posterior  end  of  the  heart 
between  its  walls.  Its  anterior  (ventral)  face  forms  part  of  the  pericardial  sac,  while  its 
posterior  (dorsal)  face  forms  part  of  the  lining  of  the  pleuroperitoneal  cavity  as  previously.  To 
this  latter  portion  the  liver  is  attached  as  before  by  the  coronary  and  falciform  ligaments. 

The  posterior  descent  of  the  pericardial  cavity  brings  a  portion  of  the  pleuroperitoneal 
cavity  dorsal  to  the  pericardia!  cavity.  In  this  portion  of  the  pleuroperitoneal  cavity  which 
lies  above  the  heart,  the  lungs  grow  out. 

In  birds  the  relations  of  pericardial  cavity  and  sac  and  of  the  transverse  septum  are  the 
same  as  just  described.  The  pericardial  sac  is,  however,  only  partially  separated  from  the 
body  wall.  The  remainder  of  the  coelom,  corresponding  to  the  pleuroperitoneal  cavity  of 
fishes,  amphibians,  and  reptiles  is  in  birds  further  subdivided.  This  is  accomplished  by  means 
of  a  pair  of  mesenterial  folds  which  grow  ventrally  from  the  sides  of  the  esophagus  and  fuse  with 
the  transverse  septum  (Fig.  45!)  and  E,  p.  160).  The  pleuroperitoneal  cavity  is  thus  divided 
into  anterior  and  posterior  compartments.  The  anterior  compartment  consists  of  the  two 
pleural  cavities,  one  on  each  side  of  the  pericardial  cavity  and  each  containing  a  lung.  The 
posterior  compartment  is  named  the  peritoneal  cavity.  The  lining  of  the  pleural  cavities  is 
known  as  the  pleura;  that  of  the  peritoneal  cavity  as  the  peritoneum.  The  partition  between 
pleural  and  peritoneal  cavities  is  called  the  oblique  septum.  Its  median  portion  is  composed 
of  the  transverse  septum;  its  lateral  portions,  of  the  two  folds  already  mentioned  which  are 
known  as  the  pleuroperitoneal  membranes. 

In  mammals  the  coelom  is  divided  as  in  birds  into  four  compartments:  the  pericardial, 
the  two  oleural,  and  the  peritoneal  cavities.  The  pericardial  cavity  has  descended  posteriorly 


THE  COELOM,  DIGESTIVE,  AND  RESPIRATORY  SYSTEMS 


197 


as  described  above,  and  the  pericardial  sac  is  free  on  all  sides  from  the  body  wall.  That  part 
of  the  former  pleuroperitoneal  cavity  which  lies  dorsal  and  lateral  to  the  heart  is  separated  from 
the  posterior  part  of  the  same  cavity  by  a  muscular  partition,  the  diaphragm,  which  extends 
across  the  body  cavity  in  a  transverse  plane.  The  two  pleural  cavities  lie  anterior  to  the 
diaphragm,  one  on  either  side  of  the  pericardial  cavity  which  is  inclosed  between  their  medial 
walls.  The  pericardial  cavity  is  free  on  all  sides,  owing  to  the  extension  of  the  two  pleural 
cavities  ventrally  (Fig.  49).  The  two  pleural  cavities,  at  first  dorsal  in  position  in  mammals, 
grow  ventrally  and  split  the  pericardium  from  the  ventral  body  wall.  They  push  in  between 
the  pericardial  sac  and  the  ventral  body  wall  and  their  medial  walls  meet  ventral  to  the  peri- 
cardial sac  to  form  the  mediastinal  septum  seen  in  the  dissection.  The  cavity  posterior  to 
the  diaphragm  is  the  peritoneal  cavity. 

The  diaphragm  corresponds  in  large  part  to  the  oblique  septum  of  birds  but  contains  addi- 
tional components.  It  is  a  structure  peculiar  to  mammals.  It  is  formed  in  part  by  mesenterial 
folds,  the  pleuroperitoneal  membranes,  which  grow  down  from  the  dorsal  body  wall  and  fuse 


pleuro-pericardial 
membrane 


neural  tube 
notochord 


parietal 
pleura 


body  wall 


parietal 
pericardium 

FIG.  49. — Diagrams  to  show  the  separation  of  the  pericardial  sac  in  mammals.  A,  early  stage  in 
which  the  parietal  pericardium  forms  the  lining  of  the  body  wall;  the  pericardial  and  pleural  cavities 
are  separated  by  the  pleuropericardial  membrane,  which  is  the  anterior  continuation  of  the  transverse 
septum.  B,  later  stage,  showing  how  the  ventral  extension  of  the  two  pleural  cavities  splits  the  parietal 
pericardium  from  the  body  wall,  and  gives  rise  to  the  mediastinal  septum;  the  parietal  pericardium 
then  becomes  the  pericardial  sac.  (B  from  Prentiss  and  Arey's  Textbook  of  Embryology,  courtesy  of 
the  W.  B.  Saunders  Company.) 

with  the  posterior  wall  of  the  transverse  septum,  much  as  in  birds  (Fig.  45!)  and  E,  p.  160).  To 
these  components  there  are  also  added  some  portions  of  the  dorsal  body  wall  and  a  portion 
of  the  dorsal  mesentery.  Finally  the  diaphragm  is  penetrated  by  buds  from  adjacent  myo- 
tomes  and  thus  becomes  in  large  part  muscular. 

The  derivatives  and  relations  of  the  transverse  septum  in  mammals  are  similar  to  those 
previously  described.  Its  anterior  face  forms  part  of  the  pericardial  sac.  The  great  veins 
have,  however,  gradually  been  drawn  out  of  it  (as  also  in  birds)  and  now  all  of  the  veins  enter 
the  anterior  end  of  the  heart.  The  posterior  face  of  the  septum  forms  part  of  the  diaphragm, 
that  part  to  which  the  liver  is  attached  by  the  coronary  ligament.  Coronary  and  falciform 
ligaments  of  the  liver  are  present  as  previously  and  originate  in  the  same  way. 

The  coelomic  linings,  pericardium,  pleuroperitoneum,  or  pleura  and  peritoneum,  present 
the  same  relations  in  all  of  the  vertebrates.  Each  has  a  parietal  portion  lining  the  body  wall, 
a  visceral  portion  covering  the  viscera,  and  mesenteries  or  ligaments  connecting  the  parietal 
and  visceral  portions. 


i98 


LABORATORY  MANUAL  FOR  VERTEBRATE  ANATOMY 


The  relations  of  the  mesenteries  are  very  similar  in  all  of  the  vertebrates.  The  dorsal 
mesentery  extends,  usually  unbroken,  from  the  dorsal  median  line  to  the  digestive  tract. 
Each  portion  of  it  bears  a  special  name,  according  to  the  part  of  the  digestive  tract  which  it 
supports.  In  mammals  the  mesentery  of  the  stomach  is  drawn  out  posteriorly  into  a  bag,  the 
greater  omentum,  which  apparently  serves  to  protect  the  abdominal  viscera  (Fig.  50).  The 
ventral  mesentery  extends  from  the  digestive  tract  to  the  ventral  body  wall  but  is  reduced  to 
remnants  in  the  adults  of  all  vertebrates.  These  remnants  are  found  in  the  region  of  the 
liver  and  the  bladder  where  they  form  the  gastrohepatic  and  hepatoduodenal  ligaments,  the 
falciform  ligament,  and  the  median  ligament  of  the  bladder.  All  viscera  of  vertebrates  either 
are  retroperitoneal,  i.e.,  situated  external  to  the  coelomic  lining,  or  they  are  situated  between 
the  two  layers  of  a  mesentery.  The  liver,  the  heart,  and  the  urinary  bladder  are  situated  in 
the  ventral  mesentery;  the  pancreas  is  generally  in  the  dorsal  mesentery  but  may  be  in  the 
ventral  mesentery  as  well;  the  digestive  tube  may  be  regarded  as  situated  in  the  dorsal 
mesentery. 


FIG.  50. — Diagrams  to  show  the  formation  of  the  greater  omentum  in  mammals  and  the  fusion 
of  the  mesogaster  and  the  mesocolon.  A,  early  stage,  in  which  the  mesogaster  is  beginning  to  form  a 
bag  at  g.  B,  the  mesogaster  is  drawn  posteriorly  into  a  long  bag  g,  which  is  the  greater  omentum; 
the  mesogaster  and  mesocolon  are  fusing  at  i.  C,  completion  of  the  fusion  of  mesogaster  and  mesocolon 
at  i.  a,  liver;  6,  serosa  of  the  liver;  c,  lesser  omentum  or  gastro-hepato-duodenal  ligament;  d,  stomach; 
e,  lesser  peritoneal  sac  or  cavity  of  the  greater  omentum;  /,  mesocolon;  g,  portion  of  the  mesogaster 
which  forms  the  greater  omentum;  h,  intestine;  i,  fusion  of  the  mesogaster  and  mesocolon.  (From 
Prentiss  and  Arey's  Textbook  of  Embryology,  after  Hertwig,  courtesy  of  the  W.  B.  Saunders  Company.) 

I.      SUMMARY 

1.  The  parts  of  the  digestive  tract  are  much  the  same  hi  all  of  the  vertebrates.    There  has 
been  little  evolution  of  this  system.    In  general,  the  intestine  is  longer  and  more  coiled  in  the 
higher  forms. 

2.  The  mouth  cavity  is  provided  with  glands  in  land  vertebrates. 

3.  The  nasal  cavities  are  blind  sacs  in  fishes,  but  beginning  with  Amphibia  they  com- 
municate with  the  mouth  and  pharyngeal  cavities  for  respiratory  purposes.    At  first  these 
respiratory  passages  through  the  nose  open  into  the  anterior  end  of  the  mouth  cavity  but  later 
a  hard  palate,  a  shelf  of  bone,  is  developed  ventral  to  the  nasal  passages.    By  this  means  the 
internal  nares  are  moved  posteriorly.    In  birds  and  mammals  the  nasal  passages  are  extended 
still  farther  posteriorly  by  the  development  of  the  soft  palate;  the  air  then  passes  directly 
into  the  pharyngeal  cavity  without  first  passing  the  mouth  cavity. 

4.  The  wall  of  the  pharynx  is  pierced  by  gill  slits  provided  with  gills  in  fishes  and  without 
gills  in  urodeles.    Above  the  urodeles  the  gill  slits  vanish,  with  the  exception  of  the  first  which 
contributes  to  the  external  auditory  meatus.    The  first  visceral  pouch  evaginates  to  form  the 


THE  COELOM,  DIGESTIVE,  AND  RESPIRATORY  SYSTEMS  199 

auditory  tube  and  tympanic  cavity.  The  entodermal  lining  of  the  visceral  pouches  persists 
as  certain  glandlike  bodies,  such  as  the  tonsils,  the  thymus,  and  the  epithelial  bodies.  The 
thyroid  gland  is  an  evagination  from  the  floor  of  the  pharynx  between  the  second  visceral 
pouches. 

5.  The  pharynx  in  fishes  leads  into  a  single  tube,  the  esophagus.    Above  fishes  it  leads 
into  two  tubes,  the  esophagus  and  the  trachea.    At  the  caudal  end  of  the  trachea  are  the 
paired  lungs. 

6.  The  upper  end  of  the  trachea  is  expanded  into  a  chamber,  the  larynx,  the  walls  of  which 
are  supported  by  the  modified  gill  arches. 

7.  Both  esophagus  and   trachea  become  more  and  more  elongated  in  higher  forms, 
owing  to  a  descent  of  the  viscera  posteriorly.    This  descent  leaves  a  space  in  the  anterior 
dorsal  part  of  the  body  cavity  into  which  the  lungs  grow  out  from  the  lower  end  of  the  trachea. 

8.  Lungs  are  present  in  all  vertebrates  above  fishes  (they  have  secondarily  disappeared 
in  some  urodeles).    They  are  probably  homologous  with  the  swim  bladder  of  fishes.    Their 
walls  become  more  and  more  complicated  and  divided  into  air  spaces,  the  higher  one  ascends 
in  the  phylogentic  series.    In  birds  the  lungs  connect  with  air  sacs  in  the  viscera  and  air  spaces 
in  the  bones. 

9.  The  intestine  of  vertebrates  is  provided  with  two  glands,  the  liver  and  the  pancreas, 
which  are  attached  by  ducts  to  the  duodenum  (the  bile  duct  is  generally  and  the  pancreatic 
duct  sometimes  lacking  in  cyclos tomes). 

10.  The  intestine  terminates  in  all  vertebrates  except  placental  mammals  (cyclostomes  and 
teleostomes)  in  a  chamber,  the  cloaca,  into  which  the  urinary  and  genital  ducts  also  open. 
In  mammals  (and  cyclostomes  and  teleostomes)  the  cloaca  is  lacking  and  the  urinary  and  genital 
ducts  open  separately  from  the  intestine. 

11.  In  most  vertebrates  there  is  a  saclike  outgrowth,  the  urinary  bladder,  from  the 
ventral  wall  of  the  cloaca. 

12.  In  all  vertebrates  except  birds  and  mammals  the  coelom  is  divided  into  two  compart- 
ments, the  pericardial  cavity  containing  only  the  heart  and  the  pleuroperitoneal  cavity  contain- 
ing the  other  viscera.    The  two  cavities  are  separated  by  a  partition,  the  transverse  septum. 

13.  In  cyclostomes,  fishes,  and  urodeles  the  pericardial  cavity  is  anterior  to  the  pleuro- 
peritoneal cavity.     From  Anura  on  the  pericardial  cavity  is  ventral  to  the  anterior  part  of 
the  pleuroperitoneal  cavity.    This  change  is  due  to  a  posterior  descent  of  the  heart  and  peri- 
cardial cavity  carrying  the  transverse  septum  with  them.    As  a  consequence  of  the  descent  the 
wall  of  the  pericardial  cavity  together  with  the  transverse  septum  forms  a  sac,  the  pericardial 
sac,  around  the  heart.    That  portion  of  the  pleuroperitoneal  cavity  dorsal  to  the  heart  is 
destined  to  form  the  pleural  cavities. 

14.  In  birds  and  mammals  the  pleuroperitoneal  cavity  is  divided  into  anterior  and  pos- 
terior portions  by  the  formation  of  a  partition  which  descends  from  the  dorsal  body  wall  and 
unites  with  the  transverse  septum.    This  partition  is  called  the  oblique  septum  in  birds  and 
the  diaphragm  in  mammals.    The  diaphragm  is  more  complex  in  origin  than  the  oblique  septum 
and  unlike  the  latter  contains  a  considerable  amount  of  striated  muscle.    Anterior  to  the 
oblique  septum  or  diaphragm  are  the  two  pleural  cavities,  each  of  which  contains  a  lung; 
posterior  to  these  partitions  is  the  peritoneal  cavity,  containing  the  digestive  and  reproductive 
systems.    Thus,  in  birds  and  mammals  the  coelom  is  divided  into  four  compartments — the 
pericardial,  the  two  pleural,  and  the  peritoneal  cavities. 

15.  A  dorsal  mesentery  supports  the  digestive  tract  in  all  vertebrates  and  remains  practi- 
cally complete  throughout.    The  ventral  mesentery  of  the  digestive  tract  is  absent  in  the  adult 
except  in  the  regions  of  the  liver  and  the  bladder.    There  are  special  mesenteries  for  the  gonads 
and  their  ducts.    In  mammals  the  mesentery  of  the  stomach  develops  a  special  posterior  pro- 
longation called  the  greater  omen  turn. 


XL     THE  COMPARATIVE  ANATOMY  OF  THE  CIRCULATORY 

SYSTEM 

A.      GENERAL  CONSIDERATIONS 

i.  The  parts  of  the  circulatory  system. — The  circulatory  system  of  vertebrates  com- 
prises two  systems  of  branching  tubes  inclosing  circulating  fluids,  the  blood-vascular  system 
and  the  lymphatic  system.  The  former  is  the  larger  and  more  conspicuous  of  the  two  and  is 
the  one  referred  to  when  the  expression  circulatory  system  is  employed  without  qualification. 

The  blood- vascular  system  is  a  closed  system,  that  is,  it  consists  of  a  set  of  branching  tubes, 
the  blood  vessels,  which  are  continuous  with  each  other,  unconnected  with  other  systems  (except 
the  lymphatic  system),  and  in  which  the  inclosed  fluid  travels  in  a  circuit.  The  parts  of  the 
blood-vascular  system  are  the  heart,  the  arteries,  the  veins,  and  the  capillaries.  The  heart  is  a 
contractile  muscular  organ  situated  in  the  median  ventral  region  in  the  anterior  part  of  the 
body.  It  has  essentially  the  form  of  an  s-shaped  tube  subdivided  into  chambers  which  will 
be  named  in  connection  with  the  dissection.  The  arteries  are  the  vessels  which  leave  the  heart 
and  in  which  the  contained  fluid  flows  away  from  the  heart.  The  veins  are  vessels  in  which  the 
contained  fluid  flows  toward  the  heart.  The  student  should  particularly  note  that  arteries  and 
veins  cannot  be  defined  on  the  basis  of  the  kind  of  blood  which  they  contain.  The  veins  of 
the  vertebrate  body  fall  into  three  classes:  the  systemic  veins,  which  flow  directly  into  the 
right  side  of  the  heart;  the  pulmonary  veins,  which  flow  from  the  lungs  into  the  left  side  of 
the  heart ;  and  the  portal  veins  or  portal  systems,  in  which  the  blood  does  not  return  directly 
to  the  heart  but  passes  into  a  system  of  capillaries  from  which  it  is  re-collected  into  systemic 
veins.  The  capillaries  are  the  minute  microscopic  vessels  which  connect  the  ends  of  the  arteries 
with  the  beginnings  of  the  veins  and  through  which  the  circulation  is  completed.  All  the 
tissues  of  the  body  are  permeated  with  networks  of  capillaries  through  the  walls  of  which  the 
gaseous  and  other  exchange  between  the  blood  and  the  body  cells  takes  place. 

The  blood-vascular  system  incloses  a  fluid,  the  blood,  which  in  vertebrates  is  colored  red. 
The  blood  consists  of  a  colorless  fluid,  the  plasma,  in  which  float  microscopic  cells,  the  cor- 
puscles. The  latter  are  of  two  general  kinds,  red  and  white,  the  former  giving  the  red  color  to 
the  blood.  Study  of  the  blood  lies  outside  of  the  limits  of  this  course. 

The  lymphatic  system  is  an  open  system,  that  is,  it  consists  not  only  of  branching  tubes, 
the  lymph  vessels,  but  of  large  spaces,  the  lymph  sinuses;  and  it  is  further  in  communication 
with  the  coelomic  spaces  of  the  body.  Lymph  sinuses  occur  beneath  the  skin  (the  student  may 
recall  the  large  subcutaneous  lymph  sinuses  in  the  frog),  between  the  muscles,  in  the  mesen- 
teries, in  the  walls  of  the  digestive  tract,  around  the  central  nervous  system,  etc.  From  these 
sinuses  the  fluid  passes  into  more  or  less  definite  lymph  vessels,  which  eventually  open  into 
the  veins  of  the  blood-vascular  system.  In  the  lower  vertebrates  contractile  lymph  hearts  are 
placed  in  the  course  of  the  lymphatic  vessels,  to  aid  the  flow,  but  these  are  absent  in  mammals. 
The  lymphatic  system  further  differs  from  the  blood-vascular  system  in  that  nodules  of  tissue, 
the  lymph  glands,  are  placed  in  the  path  of  the  lymph  vessels.  The  lymph  glands  consist  of 
a  network  of  connective  tissue  hi  which  are  imbedded  masses  of  cells,  known  as  lymphocytes. 
Lymphocytes  are  a  variety  of  white  blood  corpuscles.  The  function  of  the  lymph  glands 
appears  to  be  to  destroy  foreign  particles,  bacteria,  etc.,  and  to  add  white  blood  corpuscles  to 
the  circulation.  The  tonsils,  the  thymus,  and  the  spleen  belong  to  the  category  of  lymph 
glands. 

200 


COMPARATIVE  ANATOMY  OF  THE  CIRCULATORY  SYSTEM  201 

The  lymphatic  system  contains  a  colorless  fluid,  the  lymph,  whose  composition  is  similar 
to  the  plasma  of  the  blood.  The  lymph  contains  white  blood  corpuscles  but  red  ones  are 
absent.  The  lymph  fills  aU  of  the  spaces  of  the  body  and  bathes  all  of  the  cells,  and  aU  exchange 
between  the  tissues  and  the  blood  must  occur  by  way  of  the  lymph. 

Owing  to  the  delicate  nature  of  the  lymph  vessels  and  the  general  diffuse  character  of  the 
lymphatic  system,  very  little  of  this  system  can  be  made  out  in  an  ordinary  dissection.  Our 
study  of  the  circulatory  system  will  therefore  be  confined  almost  entirely  to  the  blood-vascular 
system. 

2.  The  origin  of  the  blood  and  of  the  blood  vessels. — The  blood  vessels  and  the  blood 
arise  from  the  mesoderm,  from  mesenchyme  cells.  In  the  mesenchyme  little  patches  of  cells 
form;  the  central  cells  of  these  patches  become  modified  into  blood  corpuscles;  the  peripheral 


FIG.  51. — Drawings  of  cross-sections  through  four  successive  stages  of  development  of  the  chick 
embryo,  between  25  and  29  hours  of  incubation,  to  show  the  formation  of  the  heart.  A,  early  stage 
showing  the  open  intestine  at  c,  the  vitelline  veins  d  in  the  splanchnic  mesoderm/,  and  the  thickening  I  in 
the  splanchnic  mesoderm  covering  the  veins.  B,  next  stage  in  which  the  splanchnopleure  has  fused 
together  at  m,  closing  the  intestine  c  and  the  bringing  the  two  vitelline  veins  d  closer  together.  C,  later, 
showing  the  two  vitelline  veins  d  in  contact.  D,  completion  of  the  heart  by  the  fusion  of  the  vitelline 
veins;  the  thickened  mesoderm  /  becomes  the  muscular  wall  of  the  heart,  a,  neural  tube;  b,  notochord; 
c,  anterior  part  of  the  digestive  tract;  d,  vitelline  vein;  e,  ectoderm;  /,  somatic  and  splanchnic  layers  of 
the  mesoderm;  g,  entoderm;  h,  dorsal  mesentery  of  the  heart  or  dorsal  mesocardium;  i,  ventral  mesen- 
tery of  the  heart,  or  ventral  mesocardium  (it  is  disappearing  in  Z>) ;  j,  lining  of  the  heart  formed  by  the 
union  of  the  vitelline  veins;  k,  pericardial  cavity;  /,  muscle  layer  of  the  heart  formed  by  the  thickening 
of  the  splanchnic  mesoderm;  m,  point  of  fusion  of  the  splanchnopleure  of  the  two  sides.  (After  Patten's 
Early  Embryology  of  the  Chick,  copyright  by  P.  Blakiston's  Son  and  Company.) 


cells  arrange  themselves  to  form  tubes,  the  blood  vessels.  The  blood  vessels  which  arise  in 
the  somatic  mesoderm  are  somatic  vessels;  those  in  the  splanchnic  mesoderm  are  visceral  or 
splanchnic  vessels. 

3.  The  origin  of  the  heart.— The  heart  arises  in  the  ventral  mesentery  in  the  anterior 
part  of  the  embryo.  In  cases  where  the  embryo  is  closed  below  from  the  beginning,  a  tubular 
cavity  appears  in  the  ventral  mesentery  and  in  the  walls  of  this  cavity  the  heart  tissue  differ- 
entiates. In  the  majority  of  vertebrates,  with  meroblastic  development,  the  embryo  is  at 
first  open  below  on  the  yolk  sac  (see  Fig.  8C  and  D,  p.  40).  In  the  splanchnic  mesoderm  of  the 


2O2 


LABORATORY  MANUAL  FOR  VERTEBRATE  ANATOMY 


hypomere  there  arises  on  each  side  a  tubular  cavity,  of  the  same  nature  as  the  blood  vessels. 
As  the  hypomere  closes  below  in  the  median  ventral  line  the  two  cavities  are  brought  together 
and  fuse  to  form  the  heart  (Fig.  51).  By  either  method  of  formation  the  heart  necessarily 
lies  in  the  ventral  mesentery  of  the  gut  and  is  therefore  provided  with  dorsal  and  ventral 
mesenteries,  known  as  the  dorsal  and  ventral  mesocardia  (see  Fig.  44^4).  These,  however,  very 
rapidly  disappear,  the  ventral  mesocardium  vanishing  as  soon  as  formed.  The  heart  then 
swings  free  in  the  coelom,  attached  only  at  its  two  ends.  The  portion  of  the  coelom  in  which 
the  heart  lies  is  at  first  continuous  with  the  general  cavity  of  the  hypomeres,  but  is  soon  cut 
off  from  the  rest  of  the  coelom  by  the  formation  of  the  transverse  septum  and  is  thereupon 
named  the  pericardial  cavity. 


internal 
carotid 

first  aortic 
arch 


internal 
carotid 


dorsal 
aorta 


vitelline 
artery 


caudal 
artery 


FIG.  52. — Diagrams  of  early  vertebrate  embryos  to  show  the  development  of  the  main  blood 
vessels.  A,  earliest  stage,  in  which  the  circulatory  system  consists  of  the  vitelline  veins,  heart,  ventral 
aorta,  first  aortic  arch,  dorsal  aortae,  and  vitelline  arteries.  B,  later  stage  showing  the  development  of 
successive  aortic  arches  following  the  first  one.  The  method  of  formation  of  the  aortic  arches  by  buds 
from  the  ventral  and  dorsal  aortae  is  illustrated  by  the  last  aortic  arch  in  B.  (From  Wilder's  History  of 
the  Human  Body,  courtesy  of  Henry  Holt  and  Company.) 

4.  The  chief  embryonic  blood  vessels. — The  earliest  vessels  to  form  in  the  vertebrate 
embryo  are  the  vitelline  veins.  These  veins  develop  on  the  surface  of  the  yolk  sac  (or  in  the 
absence  of  a  yolk  sac,  along  the  intestine)  in  the  splanchnic  mesoderm  of  the  hypomere.  They 
pass  to  the  embryo  in  the  mesentery  of  the  gut  and  enter  the  posterior  end  of  the  heart.  At  the 
anterior  end  of  the  heart  a  blood  vessel,  the  ventral  aorta,  arises  and  connects  with  the  heart. 
The  ventral  aorta  extends  forward  to  the  anterior  end  of  the  pharynx.  Here  it  divides  in  two, 
and  the  two  branches  turn  dorsally,  one  on  either  side  of  the  pharynx.  This  pair  of  branches 
encircling  the  pharynx  is  the  first  pair  of  aortic  arches.  Each  vessel  lies  in  the  center  of  the 
first  or  mandibular  visceral  arch.  On  reaching  the  dorsal  side  of  the  pharynx  the  two  vessels 
turn  posteriorly  and  as  the  dorsal  aortae  proceed  backward,  situated  in  the  median  dorsal  region 
of  the  body  wall.  Each  dorsal  aorta  on  reaching  the  region  of  the  yolk  sac  sends  out  a  vitelline 
artery  over  the  surface  of  the  yolk  sac.  The  early  embryonic  circulation  is  thus  completed 
(Fig. 


COMPARATIVE  ANATOMY  OF  THE  CIRCULATORY  SYSTEM  203 

There  subsequently  develop  around  the  pharynx  additional  aortic  arches  connecting  the 
ventral  and  dorsal  aortae.  In  typical  vertebrates  six  such  pairs  of  aortic  arches  appear,  one 
pair  to  each  pair  of  visceral  arches  (Fig.  52^).  The  aortic  arch  runs  in  the  center  of  the  vis- 
ceral arch.  The  aortic  arches  develop  in  order,  the  second  after  the  first,  then  the  third,  and 
so  on.  Posterior  to  the  pharyngeal  region,  the  two  dorsal  aortae  soon  fuse  to  form  a  single 
dorsal  aorta  which  continues  into  the  tail  as  the  caudal  artery.  From  the  aorta,  as  development 
proceeds,  branches  arise  for  the  various  parts  of  the  body.  In  fishes  the  middle  part  of  each 
aortic  arch  later  breaks  up  into  a  system  of  capillaries  which  occupies  the  gill  filaments,  but 
in  non-aquatic  forms  the  arches  remain  unbroken. 

It  will  now  be  perceived  that  provision  must  be  made  for  the  return  of  the  venous  blood 
of  the  embryo  to  the  heart.  For  this  purpose  two  pairs  of  veins  arise  in  the  somatic  mesoderm 
of  the  body  wall.  These  are  a  pair  of  anterior  cardinal  veins  returning  blood  from  the  anterior 
part  of  the  body  and  a  pair  of  posterior  cardinal  veins  returning  blood  from  the  posterior  part 


FIG.  53. — Later  stage  of  the  development  of  the  circulatory  system,  showing  the  chief  veins. 
Veins  blank,  arteries  black,  a,  internal  carotid;  b,  dorsal  aorta;  c,  anterior  cardinal  vein;  d,  the  six 
aortic  arches;  e,  the  six  gill  slits;  /,  the  conus  arteriosus  of  the  heart  continuing  into  the  ventral  aorta; 
g,  main  part  of  the  heart;  h,  sinus  venosus  of  the  heart;  *,  duct  of  Cuvier  or  common  cardinal  vein; 
jy  posterior  cardinal  vein;  k,  vitelline  vein;  /,  yolk  sac;  m,  subintestinal  vein;  «,  vitelline  artery; 
o,  lateral  abdominal  or  umbilical  vein;  p,  allantois  (urinary  bladder);  q,  allantoic  or  umbilical  artery; 
r,  caudal  artery;  s,  caudal  vein.  (Slightly  modified  from  Vialleton's  Intents  de  Morphologic  des 
Vertebres.) 

of  the  body.  In  order  for  these  veins,  which  are  situated  in  the  dorsal  body  wall,  to  reach  the 
heart,  a  bridge  of  mesoderm  is  formed  on  each  side  extending  between  the  somatic  and 
splanchnic  mesoderm.  We  have  already  learned  that  this  pair  of  bridges  is  the  beginning  of 
the  transverse  septum;  and  by  the  union  and  extension  of  the  bridges  the  septum  is  completed. 
At  the  level  of  the  posterior  end  of  the  heart  the  anterior  and  posterior  cardinal  veins  on  each 
side  unite  to  a  common  vessel,  the  duct  of  Cuvier  or  common  cardinal  vein.  The  common 
cardinal  vein  then  passes  to  the  heart  from  each  side  by  way  of  the  transverse  septum  between 
the  two  walls  of  which  this  vein  is  inclosed.  In  addition  to  the  cardinal  veins  there  appears 
in  vertebrate  embryos  a  pair  of  veins  in  the  lateral  or  ventral  abdominal  walls,  known  as  the 
lateral  or  ventral  abdominal  veins,  which  enter  the  heart  along  with  the  common  cardinal  veins. 
They  are  also  called  the  umbilical  or  allantoic  veins  in  the  embryos  of  amniotes.  The  main 
veins  of  the  embryo  at  this  time  are  illustrated  in  Figure  53. 

The  two  vitelline  veins  are  soon  extended  posteriorly  in  the  embryo  by  means  of  a  tribu- 
tary, the  subintestinal  vein,  which  courses  in  the  mesentery  of  the  gut  and  constitutes  the  chief 
vein  of  the  digestive  tract.  It  is  shown  in  Figure  53.  It  continues  into  the  tail  as  the  caudal 
vein,  making  a  loop  around  the  anus.  In  embryos  without  a  yolk  sac  the  subintestinal  and 
vitelline  veins  appear  as  one  continuous  vein. 


204 


LABORATORY  MANUAL  FOR  VERTEBRATE  ANATOMY 


The 'arteries  and  veins  thus  far  described  are  the  chief  longitudinal  trunks  of  the  embryo. 
From  them,  branches,  usually  segmentally  arranged,  extend  to  various  parts  of  the  body. 
The  branches  of  the  main  longitudinal  vessels  are  classified  into  three  kinds:  the  median  visceral 
or  splanchnic  branches,  unpaired  branches  to  and  from  the  digestive  tract;  the  lateral  -visceral 
branches,  paired  branches  to  and  from  the  urogenital  organs;  and  the  parietal  or  somatic 
branches,  paired  vessels  to  and  from  the  body  wall.  This  arrangement  is  most  obvious  in 
the  arteries  (Fig.  54).  It  is  more  or  less  persistent  in  the  adult,  chiefly  in  the  posterior  part 
of  the  body.  The  vessels  to  the  paired  appendages  are  simply  enlarged  somatic  branches. 
5.  The  origin  of  the  portal  systems. — It  has  already  been  explained  that  a  portal  system 
is  a  portion  of  the  venous  system,  the  constituent  veins  of  which  instead  of  entering  the  heart 
pass  into  a  network  of  capillaries  from  which  the  blood  is  re-collected  by  a  systemic  vein.  In 
other  words,  in  a  portal  system,  a  network  of  capillaries  is  interposed  in  the  path  of  a  vein  or 
veins.  There  are  two  portal  systems,  the  hepatic  portal  system,  in  which  the  interposed 
capillaries  are  located  in  the  liver,  and  the  renal  portal  system,  in  which  they  are  in  the  kidney. 

The  origin  of  each  of  these  sys- 
tems may  be  given  briefly. 

a)  The  hepatic  portal  system: 
This  arises  as  follows.  When 
the  transverse  septum  develops 
at  the  posterior  end  of  the 
heart,  the  two  vitelline  veins 
must  naturally  pierce  the  sep- 
tum on  their  way  to  the  heart. 
At  this  region  the  liver  buds 
out  from  the  small  intestine 


vertebra 


dorsal  aorta 


peritoneum 


intestine 
body  wall 


FIG.  54. — Diagram  of  a  cross-section  through  a  vertebrate 
embryo,  to  show  the  segmental  branches  of  the  aorta.  (After 
McMurrich's  Development  of  the  Human  Body,  copyright  by 
P.  Blakiston's  Son  and  Company.) 


and  extends  into  the  transverse 
septum.  As  it  grows,  the  liver 
gradually  uses  up  the  sub- 
stance of  the  transverse  sep- 
tum and  fills  the  available  space  in  the  septum.  The  liver  substance  thus  comes  to 
surround  the  proximal  ends  of  the  two  vitelline  veins  (Fig.  55^!  and  B).  At  first  the  vitelline 
veins  pass  through  the  liver  into  the  heart,  but  soon  they  begin  to  break  up  into  smaller  and 
smaller  vessels  (Fig.  5$C)  in  the  liver  until  the  liver  is  occupied  by  a  network  of  capillaries  (of 
the  kind  known  as  sinusoids)  which  permeate  the  liver  substance.  Thus,  the  circulation  in 
the  vitelline  veins  passes  from  the  yolk  sac  and  digestive  tract  to  the  liver,  passes  through  a 
capillary  network  in  the  liver,  and  from  the  liver  to  the  heart  in  the  remaining  proximal  portions 
of  the  vitelline  veins  (Fig.  ss/?).  The  latter  are  now  called  the  hepatic  veins.  Posterior  to 
the  liver  by  ringlike  unions  between  the  two  vitelline  veins  (Fig.  5$C  and  Z>),  a  single  vessel,  the 
hepatic  portal  vein,  is  formed,  and  caudad  of  these  unions  the  right  vein  disappears,  leaving 
the  left  vein  with  its  tributary,  the  subintestinal  vein,  now  named  the  mesenteric  vein,  to  form 
the  chief  vein  of  the  digestive  tract.  This  arrangement  insures  that  all  of  the  venous  blood 
from  the  digestive  tract  caudad  of  the  cardia  must  pass  through  a  capillary  network  in  the 
liver  before  it  can  reach  the  heart. 

V)  The  renal  portal  system:  This  develops  as  follows.  At  first  the  caudal  vein  opens  into 
the  subintestinal  vein,  forming  a  loop  around  the  anus  as  in  Figure  5$C  and  D.  Later  the 
posterior  cardinal  veins  grow  posteriorly  and  connect  with  the  caudal  vein.  The  union  of 
the  caudal  vein  with  the  subintestinal  vein  is  then  broken  as  in  Figure  55^,  the  latter  vein  then 
becoming  a  tributary  of  the  hepatic  portal  system.  There  next  develops  between  the  kidneys 
a  vein,  at  first  single,  later  paired — the  subcardinal  vein  (Fig.  55^).  This  connects  with  the 
caudal  vein  and  posterior  ends  of  the  posterior  cardinal  veins.  The  blood  flows  from  the  tail 


COMPARATIVE  ANATOMY  OF  THE  CIRCULATORY  SYSTEM 

f — n 


205 


FIG.  55. — Diagrams  to  show  the  development  of  the  veins  in  elasmobranchs.  A,  earliest  stage, 
showing  the  two  vitelline  veins  a  passing  through  the  liver  q  into  the  sinus  venosus  p.  B,  the  sub- 
intestinal  vein  b  has  appeared;  it  connects  with  one  of  the  vitellines,  makes  a  loop  /  around  the  anus, 
and  continues  into  the  tail  as  the  caudal  vein  j.  C,  the  anterior  and  posterior  cardinal  veins,  c  and  /, 
have  appeared  and  connect  with  the  sinus  by  way  of  the  common  cardinal  vein  d;  the  vitelline  veins 
are  breaking  up  in  the  liver  at  r  and  caudal  to  the  liver  are  connected  by  ring-shaped  anastomoses  at  s. 
D,  the  vitelline  veins  have  broken  up  into  a  network  of  capillaries  in  the  liver  at  v;  their  proximal  por- 
tions remain  as  the  hepatic  veins  e;  their  distal  portions  have  formed  the  hepatic  portal  vein  g,  which 
is  continuous  with  the  subintestinal  6.  E,  the  posterior  cardinals  /  have  extended  posteriorly  and  at  u 
have  joined  the  loop  t  formed  by  the  caudal  vein  j  around  the  anus;  the  subintestinal  has  severed  its 
connection  with  the  caudal;  the  lateral  vein  i  and  its  tributary,  the  subclavian  vein  h,  have  appeared. 

F,  a  new  vein,  the  subcardinal  vein  k,  has  appeared  between  the  kidneys  and  connects  with  the  posterior 
parts  of  the  posterior  cardinals  /  by  means  of  cross- vessels  n  and  also  connects  with  the  caudal  vein  j. 

G,  the  posterior  cardinal  veins  have  joined  the  subcardinals  k  at  w,  their  intermediate  portions  m  dis- 
appearing; the  posterior  parts  of  the  posterior  cardinals  persist  as  the  renal  portal  veins  x,  which  flow 
into  a  network  of  capillaries  o  in  the  kidneys;  the  lateral  abdominal  veins  have  grown  posteriorly  and 
developed  iliac  tributaries  /  from  the  pelvic  fins,  a,  vitelline  vein;    b,  subintestinal  vein;   c,  anterior 
cardinal  vein;  d,  duct  of  Cuvier  or  common  cardinal  vein;  e,  hepatic  vein;  /,  posterior  cardinal  vein; 
g,  hepatic  portal  vein;   h,  subclavian  vein;  i,  lateral  abdominal  vein;  j,  caudal  vein;   k,  subcardinal 
vein;  /,  iliac  vein;  m,  obliterated  portion  of  the  posterior  cardinals;  n,  communications  between  sub- 
cardinals  and  renal  portals;  0,  capillary  network  in  kidneys;  p,  sinus  venosus;  q,  liver;  r,  branching 
of  vitelline  veins  in  the  liver;  s,  rings  between  the  two  vitellines;  /,  loop  around  the  anus;  u,  union 
of  posterior  cardinals  with  the  caudal  vein;  v,  capillary  network  between  hepatic  portal  and  hepatic 
veins;  w,  union  of  posterior  cardinals  with  subcardinals;    x,   renal  portal  veins.     (Slightly  modified 
after  Hochstetter  in  Hertwig's  Handbuch  der  vergleichenden  und  experimentellen  Entwickelungslehre 
der  Wirbeltiere.) 


206       LABORATORY  MANUAL  FOR  VERTEBRATE  ANATOMY 

into  the  subcardinal  veins  and  through  the  kidneys  into  the  posterior  cardinal  veins.  There 
next  occurs  a  break  between  the  anterior  and  posterior  parts  of  the  posterior  cardinal  veins. 
The  anterior  parts  form  a  connection  with  the  subcardinal  veins.  The  posterior  parts  retain 
their  connection  with  the  caudal  vein.  There  is  then  a  reversal  of  flow  through  the  kidneys, 
since  the  blood  now  passes  from  the  caudal  vein  into  the  posterior  parts  of  the  posterior  cardinal 
veins,  now  called  the  renal  portal  veins,  through  the  kidneys  into  the  subcardinal  veins,  and 
from  them  into  the  anterior  portions  of  the  posterior  cardinal  veins  (Fig.  556).  At  first  the 
channels  through  the  kidneys  are  direct  connections  between  the  renal  portal  and  subcardinal 
veins,  but  later  they  break  up  into  a  capillary  network.  In  this  way  the  renal  portal  sys- 
tem is  established.  The  blood  from  the  tail  must  pass  through  a  capillary  network  in  the 
kidneys. 

We  have  now  carried  the  circulatory  system  up  to  the  stage  of  development  in  which  it 
occurs  in  adult  elasmobranchs.  The  further  evolution  of  the  circulatory  system  will  best  be 
followed  on  the  specimens. 

6.  The  segmental  character  of  the  circulatory  system. — Like  most  systems  of  vertebrates, 
the  circulatory  system  exhibits  marked  traces  of  an  originally  highly  segmented  condition. 


mouth         __ 1 -+— __  caudal  artery 


caudal  vein 
—     anus 
aortkarch    ventral  aorta      hca^t     vitelline  vein  abdominal  vein 

sUbmtestinal  vein 

FIG.  56. — Diagram  of  the  hypothetical  primitive  vertebrate  circulation  from  which  that  of  present 
vertebrates  was  probably  derived,  emphasizing  the  markedly  segmental  arrangement  of  the  primitive 
vessels.  (After  Kingsley's  Comparative  Anatomy  of  Vertebrates,  copyright  by  P.  Blakiston's  Son  and 
Company.) 

However,  even  in  vertebrate  embryos  the  segmental  arrangement  of  the  blood  vessels  is  some- 
what imperfect.  It  is  helpful  to  an  understanding  of  the  cirulatory  system  to  imagine  that  it 
is  derived  from  a  highly  segmented  condition  such  as  that  present  in  annelids.  In  this  hypo- 
thetical ancestral  state,  as  illustrated  in  Figure  56,  there  are  three  main  longitudinal  vessels: 
a  dorsal  somatic  vessel,  corresponding  to  the  dorsal  aorta;  a  ventral  somatic  vessel,  correspond- 
ing to  the  abdominal  vein;  and  a  splanchnic  vessel,  corresponding  to  the  subintestinal  and 
vitelline  veins,  and  the  ventral  aorta.  In  each  segment  these  longitudinal  vessels  are  connected 
by  loops,  which  are  of  two  kinds,  somatic  and  splanchnic.  In  the  anterior  part  of  the  body 
only  the  splanchnic  connecting  loops  are  present,  represented  by  the  aortic  arches  connecting 
the  dorsal  and  ventral  aortae.  Posterior  to  the  heart  each  segment  is  provided  with  a  somatic 
and  a  splanchnic  loop,  the  former  passing  in  the  body  wall  from  the  aorta  to  the  ventral  abdomi- 
nal vein,  and  the  latter  passing  in  the  intestinal  wall  from  the  aorta  to  the  subintestinal  vein. 
These  transverse  loops  pictured  as  continuous  are  in  reality  interrupted  by  capillaries  in  the 
body  and  intestinal  walls.  In  vertebrates  all  of  the  connecting  loops  are  lost  except  the  aortic 
arches;  but  segmentally  arranged  transverse  vessels  corresponding  to  the  loops  are  plainly 
evident  in  vertebrate  embryos,  and  persist  in  certain  regions  of  the  adult,  as  in  Figure  54. 
For  more  extensive  accounts  of  the  comparative  anatomy  of  the  circulatory  system,  K  or 
VV  should  be  consulted. 


COMPARATIVE  ANATOMY  OF  THE  CIRCULATORY  SYSTEM  207 

B.      THE  CIRCULATORY   SYSTEM   OF  ELASMOBRANCHS 

The  following  account  applies  to  both  species  of  dogfish  and  to  the  skate. 

1.  The  chambers  of  the  heart. — The  heart  when  first  formed  is  a  simple 
straight  tube,  but  it  soon  becomes  bent  upon  itself  in  the  shape  of  the  letter  S, 
and  its  wall  becomes  differentiated  into  a  number  of  chambers.     The  heart  of 
the  elasmobranchs  is  in  this  condition,  consisting  of  four  chambers.    The  peri- 
cardial  cavity  has  already  been  exposed  in  the  preceding  dissection.     Spread  its 
walls  apart.     Identify  the  chambers  of  the  heart  as  follows.     On  raising  the 
heart  a  triangular  chamber  will  be  seen  extending  from  the  heart  to  the  transverse 
septum,  its  two  corners  buried  in  the  septum.    This  is  the  sinus  venosus,  the 
most  posterior  chamber  of  the  heart.     Each  corner  of  the  sinus  venosus  is  con- 
tinuous with  a  large  vein,  the  duct  of  Cuvier  or  common  cardinal  vein,  which  is 
inclosed  in  the  transverse  septum  and  will  be  seen  later.     Anterior  to  the  sinus 
venosus  is  the  atrium  or  auricle,  a  large  thin-walled  chamber  expanded  on  each 
side  of  the  heart  and  appearing  as  if  paired.     Between  the  two  sides  of  the 
auricle  rests  the  ventricle,  a  thick- walled,  heart-shaped  chamber,  the  most  con- 
spicuous portion  of  the  heart  from  ventral  view.    The  pointed  posterior  end  of 
the  ventricle  is  known  as  the  apex,  the  broad  anterior  end,  the  base.    From  the 
base  of  the  ventricle  a  thick- walled  tube  runs  forward  and  penetrates  the  anterior 
wall  of  the  pericardial  cavity.     This  is  the  conus  arteriosus,  the  fourth  and  most 
anterior  chamber  of  the  heart.     The  blood  circulates  through  the  chambers  of 
the  heart  in  the  following  order:  sinus  venosus,  auricle,  ventricle,  conus  arteriosus. 

2.  The  systemic   veins. — Systemic   veins  have    already  been  defined   as 
those  veins  which  enter  the  heart.     All  systemic  veins  in  vertebrates  open  into 
the  sinus  venosus  or  its  equivalent,  that  is,  they  enter  the  phylogenetically  poste- 
rior end  of  the  heart.     Owing  to  differences  between  them,  the  dogfishes  and 
skate  will  be  described  separately. 

In  dissecting  the  veins,  they  are  followed  away  from  the  heart,  and  it  is  often 
convenient  to  speak  of  them  as  if  they  proceeded  from  the  heart  to  body 
structures.  The  student  must,  however,  always  bear  in  mind  the  fact  that 
they  convey  the  blood  from  the  parts  of  the  body  to  the  heart. 

Dogfish:  Insert  one  blade  of  a  fine  scissors  in  the  sinus  wall  and  slit  the 
ventral  wall  of  the  sinus  venosus  open  in  a  crosswise  direction.  The  cavity  of 
the  sinus  is  thus  exposed  and  should  be  washed  out  thoroughly  under  a  stream 
of  running  water.  All  of  the  systemic  veins  open  into  the  cavity  of  the  sinus 
and  the  openings  may  now  be  identified,  with  the  cut  edges  of  the  sinus  wall 
spread  well  apart.  Each  lateral  wing  of  the  sinus  which  lies  buried  in  the  trans- 
verse septum  receives  a  very  large  opening,  the  entrance  of  the  duct  of  Cuvier 
or  common  cardinal  vein.  The  natural  relations  of  this  entrance  are  best  observed 
on  the  intact  right  side.  On  the  left  side  carry  the  slit  in  the  sinus  laterally  to 


208       LABORATORY  MANUAL  FOR  VERTEBRATE  ANATOMY 

meet  the  incision  previously  made  across  the  gill  slits.  The  entrance  of  the 
common  cardinal  vein  into  the  sinus  is  thus  slit  open.  The  following  may  then 
be  noted.  In  the  spiny  dogfish  just  medial  to  the  main  opening  of  the  common 
cardinal  vein  into  the  sinus  are  several  small  apertures,  most  of  which  appear  to 
be  subdivisions  of  the  chief  opening.  The  most  anterior  of  these  small  apertures 
is,  however,  the  opening  of  the  inferior  jugular  vein.  In  the  smooth  dogfish  the 
accessory  openings  are  lacking,  and  the  entrance  of  the  inferior  jugular  vein  is 
situated  just  anterior  to  the  opening  of  the  common  cardinal  vein.  Probe  into 
the  opening  and  note  that  the  vein  comes  from  the  floor  of  the  mouth  and  pha- 
ryngeal  cavities  where  it  runs  alongside  the  ventral  ends  of  the  gill  arches.  By 
turning  back  the  flap,  previously  formed,  of  the  floor  of  the  mouth  and  pharyngeal 
cavities,  the  course  of  the  vein  will  be  more  readily  followed.  Now  look  into 
the  posterior  part  of  the  wall  of  the  sinus,  putting  this  on  a  stretch.  In  the 
median  line  of  the  posterior  wall  will  be  noted  a  white  fold,  and  on  each  side  of 
this  is  an  opening.  Probe  into  both  openings  and  note  that  your  probe  passes 
internal  to  the  coronary  ligament  and  into  the  liver.  Follow  your  probe  into 
the  liver  by  slitting  the  substance  of  the  liver,  and  note  the  cavity  in  the  right 
and  left  lobes  of  the  liver  thus  revealed.  These  cavities  which  extend  nearly  the 
entire  length  of  the  liver  lobes  are  the  hepatic  sinuses.1  The  two  hepatic  sinuses 
are  the  persistent  proximal  parts  of  the  vitelline  veins  of  the  embryo. 

Now  probe  posteriorly  into  one  of  the  common  cardinal  veins.  Raise  the 
viscera  in  the  anterior  part  of  the  pleuroperitoneal  cavity  and  observe  that  your 
probe  has  entered  a  large  bluish  sac  located  in  the  dorsolateral  wall  of  the  pleuro- 
peritoneal cavity.  Follow  this  sac  and  its  fellow  of  the  opposite  side  posteriorly. 
Each  bends  toward  the  median  region,  narrowing  considerably,  and  on  pressing 
the  viscera  to  one  side,  each  may  be  traced  posteriorly  as  a  narrow  tube  lying 
immediately  to  each  side  of  the  attachment  of  the  dorsal  mesentery  to  the 
median  dorsal  line.  These  two  vessels  are  the  posterior  cardinal  sinuses;  they 
are  the  chief  somatic  veins  of  the  trunk.  At  the  level  of  the  anterior  part  of 
the  liver  the  two  posterior  cardinal  sinuses  communicate  with  each  other  by  a 
broad  connection  which  may  be  found  by  probing  into  the  left  vein  and  directing 
the  probe  toward  the  right  one.  In  this  same  region  each  vein  has  on  its  ventral 
surface  an  extensive  communication  with  a  large  blood  sinus,  the  genital  "sinus, 
surrounding  each  gonad.  Each  posterior  cardinal  sinus  also  receives  numerous 
segmentally  arranged  branches  from  the  body  wall  (parietal  veins)  and  from  the 
kidneys  (renal  veins).  The  kidneys  are  the  long,  slender,  flat  organs,  brownish 
in  color,  which  lie  immediately  lateral  to  the  posterior  cardinal  sinuses,  extend- 
ing the  entire  length  of  the  pleuroperitoneal  cavity.  The  parietal  and  renal  veins 
are  readily  identifiable  in  those  specimens  in  which  they  happen  to  be  filled  with 
blood,  but  are  impossible  to  see  when  empty. 

1  Owing  to  the  fact  that  the  veins  of  elasmobranchs  are  not  definite  vessels  but  spaces  in  the  tissues 
without  definite  walls,  they  are  more  correctly  designated  sinuses. 


COMPARATIVE  ANATOMY  OF  THE  CIRCULATORY  SYSTEM  209 

Now  turn  the  animal  dorsal  side  up  and  locate  the  lateral  line.  Make  a 
longitudinal  incision  above  the  gill  slits  on  the  left  side  along  the  lateral  line  in 
the  spiny  dogfish  and  about  one-third  of  the  way  between  the  lateral  line  and  the 
ends  of  the  gill  slits  in  the  smooth  dogfish.  Deepen  the  incision  until  you  break 
into  a  large  cavity  with  a  smooth  lining.  This  cavity  is  the  anterior  cardinal 
sinus.  Probe  it  anteriorly  and  follow  your  probe  by  an  incision.  The  anterior 
cardinal  sinus  can  thus  be  traced  forward  above  the  spiracle  to  the  eye,  where 
it  connects  with  an  orbital  sinus  surrounding  the  eyeball.  At  the  level  of  the 
posterior  end  of  the  eyeball  is  situated  an  opening  in  the  ventral  wall  of  the 
anterior  cardinal  sinus.  On  probing  into  this  it  will  be  found  to  extend  medially 
into  the  skull.  It  is  the  opening  of  the  interorbital  sinus,  which  connects  the 
two  orbital  sinuses.  Locate  the  hyoid  arch.  In  the  floor  of  the  anterior  cardinal 
sinus,  between  the  hyoid  and  third  gill  arches,  is  an  opening.  On  probing  this 
it  will  be  found  to  lead  into  a  vessel  which  extends  ventrally  along  the  outer 
surface  of  the  hyoid  arch.  This  vessel  is  the  hyoidean  sinus  and  it  connects  with 
the  inferior  jugular  vein.  Next  trace  the  anterior  cardinal  sinus  posteriorly. 
It  turns  abruptly  ventrally  and  joins  the  posterior  cardinal  sinus.  On  probing 
into  the  anterior  cardinal  sinus  at  the  turn  the  probe  will  be  found  to  emerge 
into  the  posterior  cardinal  sinus.  The  union  of  the  two  sinuses  forms  the  common 
cardinal  vein  already  described. 

Running  along  the  lateral  walls  of  the  pleuroperitoneal  cavity,  immediately 
external  to  the  pleuroperitoneum,  on  each  side  is  a  conspicuous  vein,  the  lateral 
abdominal  (often  called  simply  lateral)  vein.  Note  the  parietal  branches  which 
it  receives  segmentally  from  between  the  myotomes.  Trace  the  right  vein 
anteriorly  and  find  where  it  enters  the  common  cardinal  vein  just  in  front  of 
the  baglike  expansion  of  the  posterior  cardinal  sinus.  Slit  open  the  lateral  vein 
at  its  point  of  entrance  into  the  common  cardinal  vein  and  find  at  the  same 
place  immediately  posterior  to  the  pectoral  girdle  the  opening  of  the  subclavian 
vein  which  drains  the  pectoral  fin.  The  subclavian  vein  passes  along  the  pos- 
terior surface  of  the  pectoral  girdle  in  contact  with  the  cartilage,  and  may  be  picked 
up  easily  on  the  left  side  where  the  girdle  has  been  cut  across.  The  vein  may  also 
be  found  by  cutting  across  the  base  of  the  fin;  it  will  then  be  seen  in  the  section  of 
the  fin  as  an  opening  dorsal  to  the  fin  rays  in  the  posterior  half  of  the  fin.  Next 
trace  the  lateral  vein  posteriorly.  It  passes  internal  to  the  inner  surface  of  the 
pelvic  girdle.  In  the  spiny  dogfish  it  then  continues  posteriorly  along  the  lateral 
margin  of  the  cloacal  aperture  as  the  doacal  vein.  At  about  the  middle  of  the  base 
of  the  pelvic  fin  the  lateral  vein  receives  an  iliac  vein  from  the  fin.  The  opening 
of  the  iliac  vein  into  the  lateral  vein  will  be  found  by  slitting  open  the  latter 
vein.  The  iliac  vein  is  a  short  vessel  situated  just  under  the  skin  of  the  dorsal 
surface  of  the  fin.  In  the  smooth  dogfish  the  two  lateral  veins  connect  with 
each  other  by  a  cross  vein  which  runs  along  the  internal  surface  of  the  pelvic 
girdle.  Beyond  this  connection  each  vein  continues  as  the  cloacal  vein  along  the 


210       LABORATORY  MANUAL  FOR  VERTEBRATE  ANATOMY 

lateral  margin  of  the  cloacal  aperture,  and  near  the  anterior  part  of  the  base  of 
the  pelvic  fin  receives  the  iliac  vein  from  the  fin.  Trace  this  according  to  the 
directions  for  the  spiny  species. 

In  the  median  ventral  line  are  traces  of  a  ventral  cutaneous  vein  which  seems, 
however,  to  dwindle  away  anteriorly. 

Draw  the  systemic  veins. 

Skate:  The  sinus  venosus  consists  of  a  tube  on  each  side,  the  central  portion 
being  reduced  in  size  and  attached  to  the  transverse  septum  by  a  sheet  of  connec- 
tive tissue  which  may  be  broken.  Each  side  of  the  sinus  is  buried  in  the  trans- 
verse septum.  Follow  out  the  right  side  to  the  point  where  it  disappears  dorsal 
to  the  cartilage  of  the  pectoral  girdle.  Carefully  shave  away  the  cartilage  and 
surrounding  tissues  until  the  sinus  venosus  can  be  followed  laterally.  Insert  one 
blade  of  a  fine  scissors  in  the  ventral  wall  of  the  sinus  and  slit  it  open  in  a  cross- 
wise direction.  The  sinus  is  seen  to  be  continuous  on  each  side  with  a  tube  or 
chamber,  the  common  cardinal  vein  or  duct  of  Cuvier,  which  turns  dorsally.  All 
of  the  systemic  veins  open  into  the  common  cardinal  vein,  and  their  openings 
may  now  be  identified.  The  junction  of  common  cardinal  vein  with  the  sinus 
venosus  is  marked  by  a  slight  fold.  In  the  anterior  wall  of  the  common  cardinal 
vein  concealed  by  this  fold  is  the  small  opening  of  the  inferior  jugular  vein.  This 
opening  is  so  small  that  the  probe  can  probably  not  be  passed  into  it.  The 
inferior  jugular  vein  drains  the  walls  of  the  pericardial  cavity  and  the  floor  of 
the  mouth  and  pharyngeal  cavities.  In  the  posterior  wall  of  the  common  cardinal 
vein,  at  its  junction  with  the  sinus  venosus,  is  the  opening  of  the  hepatic  sinus. 
Probe  posteriorly  into  this.  It  leads  into  the  hepatic  sinus,  a  large  space  situated 
between  the  anterior  margin  of  the  liver  and  the  transverse  septum  and  inclosed 
by  the  coronary  ligament.  In  females  the  hepatic  sinus  lies  dorsal  to  the  mouth 
of  the  oviduct  which  is  inclosed  in  the  falciform  ligament.  In  locating  the  hepatic 
sinus  press  the  liver  caudad  away  from  the  transverse  septum;  the  sinus  forms  a 
bag  between  liver  and  septum  ventral  to  the  esophagus.  Cut  into  .the  hepatic 
sinus.  A  tube,  the  pericardia- peritoneal  canal,  extends  through  the  center  of  its 
cavity.  Look  in  the  posterior  wall  of  the  sinus  for  the  small  openings  of  the 
hepatic  veins  which  drain  the  liver.  The  sinus  opens  on  each  side  into  the 
common  cardinal  vein. 

Now  probe  into  the  main  cavity  of  the  common  cardinal  vein  in  a  dorsal 
and  posterior  direction.  The  probe  enters  the  posterior  cardinal  sinus  which  is 
a  broad,  thin-walled  tube  lying  against  the  dorsal  wall  of  the  pleuroperitoneal 
cavity.  In  females  it  is  on  the  dorsal  side  of  the  oviduct;  in  males,  dorsal  to 
the  testis.  Follow  the  posteror  cardinal  sinus  posteriorly.  It  swerves  toward 
the  median  line  where  it  soon  meets  its  fellow  of  the  opposite  side  to  form  a  single 
large  median  sinus.  This  sinus  communicates  on  its  ventral  surface  with  the 
large  genital  sinus  surrounding  each  gonad.  More  posteriorly  the  posterior  cardi- 
nal sinus  separates  again  into  two  veins  which  proceed  caudad  on  the  medial 


COMPARATIVE  ANATOMY  OF  THE  CIRCULATORY  SYSTEM  211 

side  of  the  kidneys.  The  kidneys  are  rounded  lobes  at  the  posterior  end  of  the 
pleuroperitoneal  cavity  against  the  dorsal  wall.  To  see  them,  remove  the 
pleuroperitoneum  from  the  dorsal  wall  on  each  side  of  the  cloaca.  Do  not 
injure  the  ducts  from  the  kidneys.  After  exposure  of  the  kidneys  the  posterior 
cardinal  veins  will  be  found  on  the  medial  side  of  the  kidneys.  They  connect 
with  each  other  between  the  kidneys. 

Return  to  the  common  cardinal  vein  and  probe  into  it  in  a  dorsal  direction. 
Turn  the  animal  dorsal  side  up  and  locate  the  end  of  your  probe.  Make  an 
incision  into  the  spot  indicated  by  the  probe  and  extend  the  incision  longitudinally 
forward  to  the  eye.  On  carefully  deepening  the  incision  an  elongated  cavity  with 
smooth  walls,  the  anterior  cardinal  sinus,  is  exposed.  It  is  situated  just  medial 
to  the  dorsal  ends  of  the  gill  arches  and  visceral  pouches.  It  may  be  followed 
forward  with  the  aid  of  the  probe.  It  turns  laterally  in  front  of  the  first  visceral 
pouch,  follows  along  the  anterior  border  of  this  pouch,  and  then  turns  anteriorly 
again. 

With  the  dorsal  side  of  the  animal  still  facing  you,  locate  by  feeling  the 
chief  anterior  cartilage  (propterygium)  of  the  pectoral  fin.  It  forms  a  crescentic 
ridge  lateral  to  the  gill  region  nearly  halfway  from  the  mid-dorsal  line  to  the 
margin.  Make  an  incision  along  the  medial  face  of  this  cartilage  on  the  left 
side.  A  vein  will  be  exposed  running  along  the  cartilage  here.  It  is  one  of  the 
brachial  veins.  Follow  it  posteriorly.  It  will  be  found  to  enter  the  common 
cardinal  vein. 

Turn  the  animal  ventral  side  up  again.  Along  the  lateral  wall  of  the  pleuro- 
peritoneal cavity  runs  the  lateral  abdominal  vein.  Note  the  parietal  branches 
which  it  receives  from  the  body  wall  at  each  myoseptum.  Trace  the  lateral 
abdominal  vein  anteriorly.  It  passes  along  the  internal  surface  of  the  cartilages 
of  the  pectoral  fin  and  pectoral  girdle,  and  enters  the  common  cardinal  vein. 
Cut  into  the  vein  where  it  passes  the  cartilages.  Immediately  on  the  posterior 
side  of  the  cartilage  of  the  pectoral  girdle  a  brachial  vein  will  be  found  entering 
the  lateral  vein.  Immediately  posterior  to  this  is  another  cartilage,  and  on  the 
caudal  side  of  that  another  brachial  vein  joins  the  lateral  vein.  A  third  brachial 
vein  was  mentioned  in  the  preceding  paragraph.  Trace  the  lateral  vein  poste- 
riorly. It  originates  in  a  network  of  small  vessels  on  the  sides  of  the  large  intestine 
and  cloaca.  It  passes  on  the  inner  surface  of  the  cartilages  of  the  pelvic  girdle 
and  pelvic  fin.  Slit  the  vein  open  along  the  surface  of  the  cartilages.  Iliac  veins 
will  be  found  emerging  from  between  the  cartilages  and  entering  the  lateral  vein. 
The  largest  of  the  iliac  veins  is  located  along  the  posterior  side  of  the  cartilage 
of  the  pelvic  girdle.  Probe  into  the  iliac  veins  and  note  their  distribution  in  the 
pelvic  fin. 

Draw  the  systemic  veins. 

6.  The  hepatic  portal  system. — A  portal  system  is  a  system  of  veins  which 
ilows  into  a  network  of  capillaries  in  some  organ.  The  hepatic  portal  system 


212       LABORATORY  MANUAL  FOR  VERTEBRATE  ANATOMY 

consists  of  veins  which  collect  the  venous  blood  from  the  digestive  tract  and 
spleen,  and  pour  it  into  a  network  of  capillaries  in  the  liver.  Locate  the  bile 
duct.  Lying  in  the  hepatoduodenal  ligament  alongside  the  bile  duct  is  a  large 
vein,  the  hepatic  portal  vein.  Trace  it  posteriorly  and  identify  the  branches 
which  it  receives  from  the  digestive  tract.1  These  branches  differ  slightly  in  the 
three  forms  under  consideration. 

Spiny  dogfish:  On  tracing  the  hepatic  portal  vein  posteriorly,  it  will  be 
found  to  receive  first  a  very  small  branch,  which  may  be  called  the  duodenal 
branch,  which  runs  along  the  bile  duct  and  collects  from  the  duodenum  and 
anterior  portion  of  the  spiral  valve.  Posterior  to  this  the  hepatic  portal  vein  is 
seen  to  be  formed  by  the  union  of  three  large  branches.  The  left  branch,  the 
gastric  vein,  passes  at  once  to  the  stomach,  where  it  is  formed  by  the  union  of 
the  dorsal  and  ventral  gastric  veins,  which  branch  on  the  dorsal  and  ventral  sur- 
faces of  the  stomach.  The  middle  of  the  three  main  branches  of  the  hepatic 
portal  is  the  lienomesenteric  vein.  It  passes  posteriorly  dorsal  to  the  duodenum 
and  is  imbedded  in  the  substance  of  the  dorsal  lobe  of  the  pancreas,  from  which 
it  receives  branches.  At  the  posterior  end  of  the  pancreas  the  vein  is  seen  to  be 
formed  by  the  union  of  two  branches:  one,  the  posterior  splenic  vein,  from  the 
spleen;  and  the  other  the  posterior  intestinal  vein,  which  comes  from  the  left 
side  of  the  small  intestine.  Note  its  numerous  branches  from  the  intestinal  wall 
along  the  lines  of  attachment  of  the  turns  of  the  spiral  valve.  The  right  branch 
of  the  three  that  form  the  hepatic  portal  vein  is  the  pancreatico-mesenteric.  It 
passes  dorsal  to  the  pylorus  and  is  imbedded  in  the  substance  of  the  ventral  lobe 
of  the  pancreas.  Here  it  receives  the  anterior  splenic  vein  from  the  anterior 
part  of  the  spleen  and  several  veins  from  the  pancreas  and  duodenum.  Its  main 
trunk,  the  anterior  mesenteric  vein,  is  situated  along  the  right  side  of  the  intestine 
from  which  it  receives  branches  hi  the  same  manner  as  the  posterior  intestinal 
vein. 

Smooth  dogfish:  The  branches  are  similar  to  those  of  the  spiny  species. 
The  hepatic  portal  first  receives  the  very  small  duodenal  branch  from  along  the 
bile  duct  and  shortly  beyond  this  point  is  formed  by  the  union  of  three  large  veins : 
the  gastric  vein  on  the  left,  the  lienomesenteric  in  the  middle,  and  the  pancreatico- 
mesenteric  to  the  right.  The  gastric  vein  passes  toward  the  stomach  and  receives 
an  anterior  ventral  gastric  vein.  It  is  situated  in  the  mesentery  between  the  two 
limbs  of  the  stomach  and  in  its  course  receives  branches  from  both  limbs.  The 
lienomesenteric  vein  passes  dorsal  to  the  pylorus  lying  imbedded  in  the  dorsal 
lobe  of  the  pancreas.  At  the  anterior  margin  of  this  lobe  of  the  pancreas  it 
receives  the  anterior  dorsal  gastric  vein  from  the  corresponding  part  of  the 
stomach.  It  receives  numerous  pancreatic  veins  from  the  pancreas.  At  the 

1  These  branches  are  generally  filled  with  blood  and  therefore  easily  traced.  If  they  are  empty, 
they  may  be  readily  injected  through  the  hepatic  portal  vein,  even  in  specimens  which  have  been 
preserved  for  a  long  time. 


COMPARATIVE  ANATOMY  OF  THE  CIRCULATORY  SYSTEM  213 

posterior  margin  of  the  pancreas  it  is  found  to  be  formed  by  the  union  of  two 
veins,  a  left-hand  lienogastric  vein  and  a  right-hand  posterior  mesenteric  vein. 
The  lienogastric  vein  has  a  posterior  dorsal  gastric  vein  from  the  stomach  and  the 
posterior  splenic  vein  from  the  posterior  end  of  the  spleen,  and  adjacent  walls  of 
the  stomach.  The  posterior  mesenteric  vein  runs  along  the  left  side  of  the  small 
intestine,  where  it  receives  branches  on  both  sides  along  the  lines  of  insertion  of 
the  turns  of  the  spiral  valve.  The  pancreatico-mesenteric  vein  passes  dorsal 
to  the  pylorus,  receiving  pancreatic  veins  from  both  lobes  of  the  pancreas  and 
the  anterior  splenic  vein  from  the  anterior  part  of  the  spleen  and  adjacent  stomach 
wall.  At  about  the  region  of  the  pylorus  the  vein  is  formed  by  the  union  of  two 
large  branches  from  the  intestinal  wall.  One  of  these,  the  intraintestinal  vein, 
is  a  short  branch  from  the  anterior  part  of  the  spiral  valve.  The  other  branch, 
the  anterior  mesenteric,  lies  along  the  right  side  of  the  intestine  corresponding  in 
position  and  branches  to  the  posterior  mesenteric. 

Skate:  The  hepatic  portal  vein  is  soon  seen  to  be  formed  by  the  union  of 
three  tributaries:  a  gastric  vein  from  the  left,  a  lienomesenteric  vein  from  the 
middle,  and  a  pancreatico-mesenteric  from  the  right.  Follow  each  of  these.  The 
gastric  vein  passes  to  the  right  margin  of  the  stomach  and  there  receives 
the  dorsal  and  ventral  gastric  veins  from  the  dorsal  and  ventral  surfaces  of  the 
stomach.  The  dorsal  gastric  vein  receives  tributaries  from  the  spleen.  The 
lienomesenteric  vein  receives  a  splenic  branch  from  the  spleen.  Its  main 
tributary,  the  posterior  mesenteric  vein,  runs  along  the  left  side  of  the  intes- 
tine beginning  in  the  tip  of  the  rectal  gland;  in  its  course  along  the  intestine 
it  receives  branches  along  the  lines  of  attachment  of  the  spiral  valve.  It  also 
collects  from  the  pancreas.  The  pancreatico-mesenteric  vein  collects  from  the 
pancreas  and,  as  the  anterior  mesenteric  vein,  from  the  duodenal  region.  It  also 
receives  a  posterior  gastric  vein  from  the  narrow  portion  of  the  stomach  between 
the  pylorus  and  the  bend. 

Trace  the  hepatic  portal  vein  anteriorly  in  all  three  forms.  It  reaches  the 
dorsal  surface  of  the  liver  and  here  divides  into  branches  which  penetrate  the 
substance  of  the  liver.  In  the  liver  the  branches  fork  into  smaller  and  smaller 
veins  and  finally  pass  into  capillaries.  From  these  capillaries  originate  other 
veins  which  empty  into  the  hepatic  sinuses  which  as  already  seen  open  in  the 
sinus  venosus  (dogfish)  or  common  cardinal  vein  (skate).  As  already  explained 
(see  Fig.  55,  p.  205)  the  hepatic  sinuses  are  the  persistent  proximal  portions  of  the 
vitelline  veins  of  the  embryo,  while  the  hepatic  portal  vein  and  its  branches 
develop  from  one  of  the  vitelline  veins  and  the  subintestinal  vein  of  the  embryo. 

Draw  an  outline  of  the  digestive  tract  and  place  on  this  outline  the  hepatic 
portal  vein  and  its  tributaries  from  the  various  parts  of  the  digestive  tract. 

4.  The  renal  portal  system. — In  the  renal  portal  system  the  venous  blood 
passes  into  a  network  of  capillaries  in  the  kidneys.  Cut  across  the  tail  just 
posterior  to  the  anal  opening.  In  the  cross-section  locate  the  caudal  blood 


214       LABORATORY  MANUAL  FOR  VERTEBRATE  ANATOMY 

vessels,  inclosed  in  the  haemal  arch.  The  caudal  artery  is  dorsal,  the  caudal 
vein  immediately  ventral  to  the  artery. 

Dogfish:  Probe  into  the  caudal  vein.  Observe  that  the  probe  can  be  passed 
either  to  the  right  or  left,  showing  that  the  vein  forks  at  the  anus.  The  two 
forks  are  the  renal  portal  veins.  Leaving  your  probe  in  one  of  the  renal  portal 
veins,  locate  the  kidney  in  the  pleuroperitoneal  cavity.  It  is  a  long  brown 
organ  situated  against  the  dorsal  body  wall,  one  on  each  side  of  the  mid-dorsal 
line  external  to  the  pleuroperitoneum.  Slit  the  pleuroperitoneum  along  the 
lateral  border  near  the  posterior  end  of  the  kidney  on  the  side  where  your  probe 
is  inserted  and  gently  lift  the  kidney  away  from  the  body  wall.  A  space  will  be 
found  between  the  kidney  and  the  body  wall;  into  this  space  your  probe  has 
passed.  This  space  is  the  renal  portal  vein  or  sinus.  It  branches  into  the 
kidney  and  also  receives  tributaries  from  the  body  wall. 

Skate:  The  kidney  has  already  been  exposed.  Look  along  the  medial  side 
of  the  posterior  part  of  the  kidney  for  a  vein  coming  from  the  vertebral  column. 
Do  not  injure  any  ducts  on  the  ventral  surface  of  the  kidney.  In  males  the  vein 
in  question  lies  immediately  to  the  dorsal  side  of  the  male  duct,  which  will  be 
seen  passing  along  the  ventral  surface  of  the  kidney  to  the  cloaca.  The  duct 
may  be  lifted  from  the  kidney  surface  and  bent  to  one  side.  The  vein  in  question 
is  the  renal  portal  vein.  At  first  it  lies  along  the  medial  border  of  the  kidney, 
but  soon  turns  onto  the  ventral  surface  of  that  organ,  giving  off  branches  into 
its  substance  and  receiving  tributaries  from  the  body  wall  lateral  to  the  kidney. 
The  renal  portal  veins  are  continuations  of  the  caudal  vein;  the  latter  forks  at 
the  anus  giving  rise  to  the  two  renal  portal  veins.  The  forking  is,  however, 
difficult  to  trace  in  the  skate. 

Reference  to  Figure  55,  page  205,  will  show  that  the  renal  portal  veins  are  the 
posterior  parts  of  the  posterior  cardinal  veins  and  that  the  apparent  posterior 
parts  of  the  posterior  cardinal  veins  of  the  adult  are  in  reality  the  subcardinal 
veins.  Whereas  in  the  embryo  the  blood  flows  from  the  sub  cardinals  into  the 
posterior  cardinals,  in  the  adult  the  direction  of  flow  is  reversed.  The  renal  por- 
tal system  provides  that  the  blood  from  the  tail  must  pass  into  a  capillary 
system  in  the  kidneys  from  which  the  blood  is  re-collected  into  the  subcardinal 
veins.  The  purpose  of  this  arrangement  is  obscure;  it  seems  to  have  been  disad- 
vantageous, for  the  vertebrates  later  shunted  part  of  this  blood  into  another 
system  and  finally  abandoned  the  renal  portal  system  altogether. 

Draw  the  renal  portal  system. 

5.  The  ventral  aorta  and  the  afferent  branchial  vessels. — Turn  once  more 
to  the  pericardial  cavity  of  the  specimen.  The  conus  arteriosus  runs  forward  and 
penetrates  the  anterior  wall  of  the  pericardial  cavity.  Carefully  pick  away 
muscles  and  connective  tissue  from  the  region  extending  from  the  anterior  end 
of  the  pericardial  cavity  to  the  lower  jaw.  In  the  median  ventral  line  will  be 


COMPARATIVE  ANATOMY  OF  THE  CIRCULATORY  SYSTEM  215 

revealed  a  large  vessel,  the  ventral  aorta,  which  continues  torward  from  the  conus 
arteriosus.  By  dissecting  carefully  to  the  left  side  find  the  branches  of  the 
ventral  aorta.  They  are  as  follows: 

Dogfishes:  There  are  three  main  pairs  of  branches,  two  of  which  subdivide 
into  two.  The  most  posterior  pair  of  branches  arises  just  at  the  point  where 
the  conus  arteriosus  passes  into  the  ventral  aorta.  (At  this  point  note  the 
coronary  artery,  paired  in  the  spiny  dogfish,  single  in  the  smooth  species,  passing 
along  the  conus  arteriosus  onto  the  surface  of  the  ventricle  and  to  the  walls  of  the 
pericardial  cavity.  This  artery  should  be  preserved  as  far  as  possible.)  Follow 
the  most  posterior  branch  of  the  ventral  aorta.  It  very  shortly  divides  in  two, 
the  posterior  branch  penetrating  the  interbranchial  septum  of  the  sixth  visceral 
arch,  the  anterior  branch  the  septum  of  the  fifth.  The  middle  brarch  of  the 
ventral  aorta  arises  shortly  in  front  of  the  third  branch  and  passes  without 
division  into  the  interbranchial  septum  of  the  fourth  visceral  arch.  After  giving 
off  this  branch  the  ventral  aorta  proceeds  forward  without  branching  to  a  point 
just  posterior  to  the  lower  jaw.  Here  it  forks  to  form  its  anterior  pair  of  branches. 
Trace  the  left  branch  laterally.  After  some  distance  it  forks,  supplying  the 
second  and  third  visceral  arches.  Trace  any  one  of  the  branches  of  the  ventral 
aorta  out  into  the  interbranchial  septum,  slitting  the  septum.  Note  the  small 
branches  from  the  artery  into  the  gill  filaments.  The  five  pairs  of  branches  of 
the  ventral  aorta  are  named  the  afferent  branchial  arteries.  How  many  gills 
does  each  supply  ? 

Skate:  There  are  two  main  pairs  of  branches  from  the  ventral  aorta.  The 
posterior  pair  arises  where  the  conus  arterious  passes  into  the  ventral  aorta.  (At 
this  point  note  the  coronary  artery,  paired,  passing  to  the  conus  arteriosus  and 
to  the  visceral  muscles.  Preserve  it  as  well  as  possible.)  Follow  out  the  posterior 
branch  of  the  ventral  aorta.  After  some  distance  it  subdivides  into  three  branches 
which  pass  to  the  fourth,  fifth,  and  sixth  visceral  arches,  penetrating  the  inter- 
branchial septa.  Trace  them  into  the  septum  by  slitting  the  septum,  and  note  the 
branches  from  each  to  the  gill  filaments  of  both  demibranchs  of  the  septum.  Fol- 
low the  ventral  aorta  forward  beyond  the  pair  of  posterior  branches.  It  passes 
without  branching  for  some  distance  and  then  forks  into  right  and  left  branches. 
Follow  the  left  branch.  After  a  considerable  distance  it  forks  into  two  vessels, 
which  penetrate  the  interbranchial  septa  of  the  second  and  third  visceral  arches. 
The  five  pairs  of  branches  of  the  ventral  aorta  are  named  the  afferent  branchial 
arteries.  How  many  demibranchs  does  each  supply  ? 

In  front  of  the  anterior  fork  of  the  ventral  aorta  will  be  found  some  soft 
brownish  diffuse  material,  composing  the  thyroid  gland. 

The  afferent  branchial  arteries  are  the  ventral  halves  of  the  aortic  arches 
which  were  previously  described  as  present  in  the  embryo.  We  noted  that 
there  are  six  such  arches  in  vertebrate  embryos.  Since  the  adult  elasmobranch 


2i6       LABORATORY  MANUAL  FOR  VERTEBRATE  ANATOMY 

has  but  five  pairs,  one  pair  must  be  missing.  The  missing  pair  is  the  first,  which 
should  supply  the  first  visceral  arch;  it  has  disappeared  during  development, 
probably  because  this  arch  bears  no  gills. 

Draw  the  ventral  aorta  and  the  afferent  branchial  arteries  showing  their 
relation  to  the  visceral  arches. 

6.  The  efferent  branchial  arteries  and  the  dorsal  aorta. — Open  the  flap 
already  formed  of  the  floor  of  the  mouth  and  pharyngeal  cavities;  turn  it  out- 
ward and  fasten  it  in  that  position.  The  esophagus  may  be  cut  to  facilitate  this 
procedure.  With  a  forceps  strip  off  the  mucous  membrane  from  the  roof  of  the 
mouth  and  pharyngeal  cavities.  In  the  roof  there  will  now  be  seen  four  pairs 
(dogfishes)  or  three  pairs  (skate)  of  large  blood  vessels  extending  from  the  angles 
of  the  gill  slits  obliquely  caudad.  They  are  the  efferent  branchial  arteries  and 
represent  the  dorsal  halves  of  the  aortic  arches.  Clean  away  the  connective 
tissue  from  these  arteries  so  that  they  are  clearly  exposed.  Trace  them  to  the 
left  and  note  that  they  disappear  dorsal  to  the  cartilages  of  the  gill  arches. 
Remove  these  cartilages  carefully.  This  is  best  done  by  cutting  across  them  in 
the  visceral  arch,  grasping  the  cut  end,  and  loosening  the  cartilage  toward  the 
median  dorsal  line.  After  the  cartilages  are  removed  trace  each  of  the  efferent 
branchial  arteries  toward  the  gills.  Note  that  each  is  formed  at  the  dorsal  angle 
of  the  gill  slit  by  the  union  of  two  vessels,  a  smaller  pretrematic  branch  which 
comes  from  the  demibranch  on  the  anterior  face  of  the  visceral  pouch  and  a 
much  larger  post-trematic  branch  which  comes  from  the  demibranch  on  the 
posterior  wall  of  the  visceral  pouch.  Note  the  small  vessel  which  runs  from  each 
gill  filament  into  the  pre-  and  post-trematic  branches.  In  the  skate  the  first 
two  efferent  branchial  arteries  unite  to  one  so  that  there  are  but  three  pairs  of 
main  vessels  in  the  roof  of  the  pharyngeal  cavity. 

Next,  dissect  on  the  right  side,  which  has  been  kept  intact,  in  order  to  see 
the  full  course  of  the  efferent  branchial  arteries.  Remove  the  mucous  membrane 
from  the  floor  of  the  mouth  and  pharyngeal  cavities,  thus  exposing  the  ventral 
portions  of  the  gill  arches.  Remove  these  cartilages  carefully  without  disturbing 
any  of  the  arteries,  and  also  remove  the  cartilage  from  the  full  length  of  the 
visceral  arches.  It  will  now  be  seen  that  the  pre-  and  post-trematic  branches 
are  united  at  their  ventral  ends  so  that  they  form  a  complete  loop  around  each 
gill  cleft.  Note  further  that  the  post-trematic  branch  on  the  anterior  wall  of 
each  visceral  arch  is  connected  with  the  pretrematic  branch  on  the  posterior 
face  of  the  same  arch  by  means  of  cross-branches,  about  three  in  number  in  the 
spiny  dogfish,  one  in  the  smooth  dogfish  and  skate.  Thus  each  efferent  branchial 
artery  collects  from  several  demibranchs. 

From  the  ventral  ends  of  some  of  the  loops  formed  by  the  pre-  and  post- 
trematic  branches  vessels  arise  on  each  side  and  proceed  posteriorly  in  the  floor 
of  the  pharyngeal  cavity,  ventral  to  the  cartilages  of  the  gill  arches,  to  supply 
the  pericardial  cavity,  wall  of  the  heart,  and  visceral  muscles.  These  vessels 


COMPARATIVE  ANATOMY  OF  THE  CIRCULATORY  SYSTEM  217 

constitute  the  coronary  arteries.  In  the  spiny  dogfish,  the  coronary  artery 
arises  on  each  side  from  the  ventral  end  of  the  loop  around  the  third  gill  cleft 
(counting  the  spiracle  as  the  first).  It  gives  off  a  branch  anteriorly  and  then 
proceeds  posteriorly  to  the  anterior  end  of  the  pericardial  chamber.  Here  it 
forks,  giving  one  branch  to  the  conus  arteriosus  and  other  chambers  of  the  heart 
and  the  other  to  the  walls  of  the  pericardial  cavity.  The  latter  branch  after  a 
short  distance  again  passes  to  the  floor  of  the  pharyngeal  cavity  and  is  distributed 
to  the  esophagus.  In  the  smooth  dogfish  the  coronary  artery  is  formed  by  paired 
branches  from  the  ventral  ends  of  the  loops  around  the  third  and  fourth  gill 
clefts.  These  branches  pass  to  the  median  line  and  join  to  form  a  single  vessel 
which  penetrates  the  pericardial  cavity  and  runs  along  the  ventral  surface  of 
the  conus  arteriosus  where  it  branches  to  the  conus  and  ventricle.  From  the 
branches  arising  from  the  ends  of  the  loops  of  the  fourth  gill  clefts  a  vessel  runs 
posteriorly  on  each  side  in  the  dorsal  wall  of  the  pericardial  cavity  and  into  the 
walls  of  the  esophagus.  In  the  skate  the  disposition  of  the  coronary  arteries  is 
somewhat  irregular.  Vessels  arise  from  or  near  the  ventral  ends  of  the  loops 
around  the  fourth  and  fifth  gill  slits.  The  former  passes  forward  along  the  floor 
of  the  mouth  cavity;  the  latter  gives  rise  to  the  anterior  coronary  arteries  which 
run  along  the  conus  arteriosus  to  the  heart  walls.  In  the  posterior  part  of  the 
pericardial  cavity  are  the  posterior  coronary  arteries,  running  along  the  sinus 
venosus.  These  originate  from  the  subclavian  artery  which  will  be  identified 
later. 

Turn  again  to  the  roof  of  the  mouth.  From  the  dorsal  end  of  the  loop 
around  the  second  gill  slit  a  vessel  runs  forward.  It  is  the  common  carotid  artery. 
After  a  short  distance  it  bends  toward  the  median  line.  At  this  bend  it  gives 
off  a  branch,  the  external  carotid  artery,  which  will  be  found  by  gently  shaving 
away  the  cartilage  at  this  point.  Beyond  this  branch  the  main  artery,  now 
known  as  the  internal  carotid  artery,  passes  to  the  median  line  where  it  joins  its 
fellow  of  the  opposite  side;  the  vessel  thus  formed  penetrates  the  cartilage  of 
the  skull.  In  the  dogfishes  slender  vessels  connect  the  first  efferent  branchial 
arteries  with  the  common  carotid  artery;  in  the  spiny  dogfish  these  connecting 
branches  are  paired  all  of  the  way,  while  in  the  smooth  species  the  vessel  arises 
unpaired  from  the  median  line  and  subsequently  forks. 

Clear  away  all  tissue  from  the  pretrematic  branch  of  the  first  efferent 
branchial  artery.  From  about  the  middle  of  the  pretrematic  branch  a  vessel 
arises  and  passing  forward  very  soon  turns  sharply  dorsally  and  disappears. 
This  is  the  hyoidean  artery.  Turn  the  animal  dorsal  side  up  and  remove  the 
skin  around  and  posterior  to  the  spiracle.  Pick  up  the  hyoidean  artery  again 
below  the  spiracle.  It  runs  on  the  inner  side  of  a  white  band  (hyomandibular 
nerve)  which  is  located  just  posterior  to  the  spiracle.  Follow  the  hyoidean 
artery  to  the  walls  of  the  spiracle  and  note  its  branches  to  the  rudimentary  gill 
iu  the  spiracle.  (In  the  skate  there  are  numerous  branches  to  adjacent  muscles 


218       LABORATORY  MANUAL  FOR  VERTEBRATE  ANATOMY 

and  only  very  small  branches  to  the  walls  of  the  spiracle.)  Now  turn  the  animal 
ventral  side  up,  remove  the  mucous  membrane  between  the  spiracle  and  the 
upper  jaw,  and  shave  away  the  cartilage  about  halfway  between  the  upper  jaw 
and  the  common  carotid  artery  An  artery  of  moderate  size  will  be  revealed. 
Trace  it  toward  the  spiracle  and  note  that  it  is  the  continuation  of  the  hyoidean 
artery  already  seen,  formed  by  the  reunion  of  the  branches  in  the  spiracular 
gill.  It  passes  into  the  skull  ventral  to  the  external  carotid  artery,  and  is  then 
known  as  the  ventral  carotid  artery.  On  tracing  it  into  the  skull  by  scraping 
away  the  cartilage,  the  ventral  carotid  artery  will  be  found  to  join  the  internal 
carotid. 

The  efferent  branchial  arteries  pass  to  the  median  dorsal  line  of  the  roof  of 
the  pharyngeal  cavity.  In  the  skate  the  first  and  second  join  to  one,  and  shortly 
posterior  to  this  junction  a  vertebral  artery  arises  on  each  side  and  passes  into  the 
cartilage  of  the  skull,  where  it  is  distributed  to  the  brain  and  spinal  cord.  The 
efferent  branchial  arteries  join  in  pairs  in  the  median  dorsal  line  and  form  a 
large  trunk,  the  dorsal  aorta,  which  passes  into  the  pleuroperitoneal  cavity. 

Draw,  showing  efferent  branchial  arteries,  their  branches,  and  their  distri- 
bution to  the  gills. 

We  are  now  in  a  position  to  compare  the  head  arteries  of  the  elasmobranch  with  the 
primitive  plan  explained  at  the  beginning  of  this  section.  The  afferent  branchial  arteries  are 
the  ventral  halves,  the  efferent  branchial  arteries  the  dorsal  halves  of  the  aortic  arches.  These 
arches  connect  the  ventral  aorta  springing  from  the  heart  with  the  dorsal  aorta,  and  run  in 
the  walls  of  the  pharynx,  one  to  each  visceral  arch.  In  elasmobranch  and  other  fishes  the 
central  part  of  each  aortic  arch  breaks  up  into  a  number  of  smaller  vessels  and  capillaries 
running  in  the  gills.  Theoretically  and  in  vertebrate  embryos  there  are  six  aortic  arches.  In 
elasmobranchs  there  are  five  ventrally  and  four  dorsally.  The  first  is  lacking  on  the  ventral 
side  and  also  absent  on  the  dorsal  side;  but  the  second  aortic  arch  is  imperfectly  represented 
by  the  hyoidean  artery  which  supplies  the  spiracle.  The  four  efferent  branchial  arteries  pres- 
ent in  their  full  development  in  elasmobranchs  are,  then,  the  third,  the  fourth,  fifth,  and  sixth 
aortic  arches.  It  should  be  noted  that  the  carotid  artery  springs  from  the  third  aortic  arch,  a 
condition  universal  among  vertebrates.  The  first  aortic  arch  is  missing  in  all  adult  vertebrates 
(except  cyclostomes) ,  and  above  fishes  the  second  has  also  vanished. 

7.  The  dorsal  aorta  and  its  branches. — Separate  the  esophagus  from  the  body 
wall  on  the  left  side  and  follow  the  dorsal  aorta  posteriorly.  From  the  dorsal 
aorta  between  the  points  where  the  third  and  fourth  pairs  of  efferent  branchial 
arteries  unite  with  it  a  subdaman  artery  is  given  off  on  each  side.  Trace  the  left 
one  into  the  pectoral  fin.  It  proceeds  obliquely  caudad  and  laterad  passing  on 
the  dorsal  wall  of  the  large  bag  formed  by  the  posterior  cardinal  sinus.  (In  the 
skate  it  gives  off  the  posterior  coronary  artery  and  passes  internal  to  a  large  white 
band,  the  nerve  of  the  pectoral  fin.  This  may  be  cut  through.)  At  the  lateral 
boundary  of  the  posterior  cardinal  sinus,  the  subclavian  artery  gives  rise  to  the 
small  lateral  artery  which  branches  into  the  body  wall  and  usually  proceeds  pos- 
teriorly along  the  body  wall  in  a  position  on  a  level  with  the  lateral  line.  Farther 


COMPARATIVE  ANATOMY  OF  THE  CIRCULATORY  SYSTEM  219 

laterally  at  the  point  where  the  lateral  vein  enters  the  common  cardinal  vein, 
the  subclavian  artery  gives  rise  to  the  ventral  abdominal  artery  which  proceeds 
posteriorly  halfway  between  the  lateral  vein  and  the  midventral  line.  This 
artery  is  somewhat  irregular  in  the  smooth  dogfish  but  conspicuous  in  the  spiny 
dogfish  and  skate.  It  gives  off  branches  segmentally  into  the  body  wall  and  at 
the  posterior  end  of  the  pleuroperitoneal  cavity  anastomoses  with  the  vessels 
supplying  the  pelvic  fins.  After  giving  off  these  branches  into  the  body  wall 
the  subclavian  artery,  now  named  the  brachial  artery,  proceeds  into  the  pec- 
toral fin. 

The  dorsal  aorta  is  a  very  large  vessel  situated  in  the  mid-dorsal  line  of  the 
pleuroperitoneal  cavity.  It  has  median  unpaired  visceral  or  splanchnic  branches 
to  the  viscera,  lateral  visceral  branches  to  the  urogenital  system,  and  somatic 
branches  to  the  body  wall.  The  median  visceral  branches  are  as  follows.  Work 
on  the  left  side  turning  the  viscera  to  the  right. 

Spiny  dogfish:  Just  after  it  has  penetrated  the  pleuroperitoneal  cavity,  the 
dorsal  aorta  gives  rise  to  the  large  coeliac  artery  which  distributes  blood  to  the 
gonads,  stomach,  and  liver.  Near  its  origin  the  coeliac  artery  gives  off  small 
branches  into  the  adjacent  gonads,  esophagus,  and  cardiac  end  of  the  stomach. 
It  then  runs  posteriorly  for  a  considerable  distance  without  branching;  it  enters 
the  gastrohepatic  ligament  and  gives  rise  to  three  branches:  the  gastric,  the 
hepatic,  and  the  pancreatico-mesenteric  artery.  The  gastric  artery  passes  to  the 
stomach  and  divides  into  dorsal  and  ventral  gastric  arteries  which  branch  on 
the  surface  of  the  stomach  and  penetrate  its  walls.  The  hepatic  artery  turns 
anteriorly,  runs  alongside  the  bile  duct,  and  enters  the  substance  of  the  liver. 
The  pancreatico-mesenteric  artery  passes  dorsal  to  the  pylorus,  gives  off  small 
branches  into  the  pyloric  portion  of  the  stomach  and  the  ventral  lobe  of  the 
pancreas,  a  moderately  large  duodenal  artery  into  the  duodenum,  and  a  large 
anterior  mesenteric  artery  along  the  right  side  of  the  small  intestine  to  which  it 
gives  off  branches  at  the  rings  of  attachment  of  the  spiral  valve.  The  dorsal 
aorta  after  giving  rise  to  the  coeliac  artery  runs  without  further  visceral  branches 
to  the  free  edge  of  the  dorsal  mesentery.  Here  it  gives  off  two  arteries  which 
course  in  the  border  of  the  mesentery.  One  of  these,  the  gastrosplenic  artery, 
passes  to  the  spleen  and  bend  of  the  stomach.  The  other  vessel  is  the  superior 
mesenteric  artery.  It  passes  to  the  small  intestine  and,  as  the  posterior  mesenteric 
artery,  runs  posteriorly  on  the  left  side  of  the  intestine  with  branches  correspond- 
ing to  those  of  the  anterior  mesenteric.  Beyond  the  gap  in  the  dorsal  mesentery, 
the  dorsal  aorta  gives  off  the  inferior  mesenteric  artery  which  passes  along  the 
free  anterior  border  of  the  mesorectum  into  the  rectal  gland. 

Smooth  dogfish:  Shortly  after  entering  the  pleuroperitoneal  cavity,  the 
dorsal  aorta  gives  rise  to  the  large  coeliac  artery.  This  has  small  branches  into 
the  adjacent  gonads  and  soon  divides  into  gastric  and  pancreatico-mesenteric 
arteries.  The  gastric  artery  immediately  forks  into  a  smaller  anterior  gastric 


220       LABORATORY  MANUAL  FOR  VERTEBRATE  ANATOMY 

artery  which  passes  to  the  walls  of  the  anterior  part  of  the  stomach  and  a  pos- 
terior gastric  artery  which  runs  posteriorly  in  the  gastrohepatic  ligament  where 
it  branches  to  both  limbs  of  the  stomach.  The  posterior  gastric  artery  near  its 
origin  from  the  coeliac  sends  about  two  hepatic  arteries  to  the  liver.  The 
pancreatico-mesenteric  artery  sends  pancreatic  branches  into  the  pancreas,  a 
large  duodenal  branch  into  the  duodenum,  a  posterior  gastrosplenic  artery  which 
runs  posteriorly  along  the  narrowed  part  of  the  spleen  and  adjacent  stomach 
wall,  and  a  large  anterior  mesenteric  artery  which  passes  along  the  right  side  of 
the  small  intestine  and  branches  at  the  lines  of  attachment  of  the  spiral  valve. 
Shortly  posterior  to  the  point  of  origin  of  the  coeliac  artery  two  arteries  arise 
from  the  dorsal  aorta.  They  are  the  gastrosplenic  and  the  superior  mesenteric. 
The  gastrosplenic  proceeds  to  the  thick  portion  of  the  spleen  and  the  adjacent 
stomach  wall.  The  superior  mesenteric  supplies  branches  to  the  gonads  and 
then,  as  the  posterior  mesenteric  artery,  passes  along  the  left  side  of  the  small 
intestine,  branching  at  the  lines  of  attachment  of  the  coils  of  the  spiral  valve. 
The  dorsal  aorta  proceeds  unbranched  for  some  distance  and  then  gives  off  the 
inferior  mesenteric  artery.  This  passes  into  the  adjacent  gonads  to  which  it 
supplies  some  branches  and  then  emerging  from  the  gonad  proceeds  to  the  rectal 
gland  where  it  forms  a  network  of  branches. 

Skate:  Shortly  after  entering  the  pleuroperitoneal  cavity,  the  dorsal  aorta 
gives  off  the  coeliac  artery,  which  supplies  a  number  of  organs.  It  has:  a  hepatic 
branch  to  the  liver;  an  anterior  gastric  branch  which  divides  into  dorsal  and 
ventral  gastric  arteries  to  the  stomach  wall;  splenic  branches  to  the  spleen; 
and  a  gastroduodenal  branch,  from  which  arise  a  posterior  gastric  artery  to  the 
posterior  part  of  the  stomach,  pancreatic  branches  to  the  pancreas,  and  a  duodenal 
branch  to  the  pylorus  and  duodenum.  Shortly  posterior  to  the  origin  of  the 
coeliac  artery,  the  dorsal  aorta  gives  rise  to  the  superior  mesenteric  artery,  which 
after  some  small  branches  to  the  pancreas  and  spleen  proceeds  posteriorly  along 
the  small  intestine  to  which  it  supplies  branches  at  the  lines  of  attachment  of 
the  turns  of  the  spiral  valve.  Shortly  caudad  of  the  origin  of  the  superior 
mesenteric  artery,  the  inferior  mesenteric  artery  branches  from  the  dorsal  aorta. 
It  sends  genital  arteries  to  the  gonads  and  their  ducts  and  then  passes  in  the 
mesentery  to  the  rectal  gland. 

The  lateral  visceral  and  somatic  branches  of  the  dorsal  aorta  are  similar  in 
the  three  forms  under  consideration.  The  former  consist  of  the  genital  arteries 
already  noted  (but  completely  developed  only  in  mature  specimens)  and  the 
renal  arteries  into  the  kidneys.  The  latter  are  seen  by  loosening  the  kidney  from 
the  dorsal  body  wall  and  looking  on  the  dorsal  surface  of  the  organ.  The  somatic 
branches  consist  of  paired  parietal  arteries  to  the  body  wall,  passing  out  along 
the  myosepta.  The  arteries  to  the  paired  fins  are  merely  enlarged  parietal 
vessels.  The  subclavian  to  the  pectoral  fin  was  already  seen.  The  paired 
iliac  arteries  to  the  pelvic  fins  arise  from  the  dorsal  aorta  shortly  in  front  of 


COMPARATIVE  ANATOMY  OF  THE  CIRCULATORY  SYSTEM  221 

the  cloaca.  They  course  along  the  body  wall,  resembling  the  parietal  arteries, 
and  after  giving  off  a  network  of  branches  into  the  walls  of  the  cloaca  and  anas- 
tomising  anteriorly  with  the  posterior  end  of  the  ventral  abdominal  artery,  they 
enter  the  pelvic  fins.  The  dorsal  aorta  continues  into  the  tail  as  the  caudal 
artery  which  is  situated  in  the  haemal  canal  immediately  ventral  to  the  centra 
of  the  vertebrae. 

Draw  the  dorsal  aorta  and  its  branches. 

8.  The  structure  of  the  heart. — The  heart  of  elasmobranchs  is  a  tube  bent 
into  an  S-shape  and  differentiated  into  four  chambers.  These  chambers  have 
already  been  named.  They  were  originally  arranged  in  a  straight  line,  but  the 
bending  of  the  heart  tube  brings  the  ventricle  in  contact  with  the  sinus  venosus 
and  the  auricle  in  contact  with  the  conus  arteriosus.  The  sinus  venosus  has 
already  been  examined.  It  is  a  thin-walled  chamber,  triangular  in  form  in  the 
dogfishes,  tubular  in  the  skate.  Cut  across  the  connections  of  the  sinus  venosus 
with  the  transverse  septum  and  also  across  the  base  of  the  ventral  aorta,  and 
remove  the  heart  from  the  body.  Look  into  the  previously  opened  sinus 
venosus  and  find  the  large  sin-auricular  aperture  which  leads  into  the  auricle. 
It  is  guarded  by  a  pair  of  valves  formed  of  the  smooth  free  edges  of  the  sinus 
wall.  Note  the  shape  of  the  auricle.  It  is  a  broad  thin-walled  chamber  with 
large  lateral  expansions  on  each  side  of  the  ventricle.  Slit  open  the  auricle  and 
wash  out  the  contained  blood  clots.  Note  the  folds  in  its  wall.  Find  the 
auriculo-ventricular  opening  into  the  ventricle.  It  is  guarded  by  two  valves. 
Each  of  these  is  a  pocket  of  thin  tissue,  the  opening  of  the  pocket  being  directed 
into  the  ventricle.  Cut  off  the  ventral  half  of  the  ventricle  and  also  slit  open 
the  conus  arteriosus  by  a  longitudinal  ventral  incision.  Note  the  small  U-shaped 
cavity  of  the  ventricle  and  its  very  thick,  spongy  walls  forming  numerous  cavi- 
ties and  crevices  in  which  the  blood  is  held.  Examine  the  auriculo-ventricular 
valve  from  the  ventricular  side,  and  note  the  two  pockets  of  which  it  is  composed 
and  the  attachment  of  the  pockets  to  the  ventricular  wall.  On  the  wall  of  the 
conus  arteriosus  note  the  pocket-shaped  semilunar  valves,  the  pockets  opening 
anteriorly.  In  the  spiny  dogfish  there  are  nine  valves  in  three  circles  of  three  each ; 
two  circles  are  near  the  posterior  end  of  the  conus,  while  the  third  circle  composed 
of  larger  and  stronger  valves  is  near  the  junction  of  the  conus  with  the  ventral 
aorta.  In  the  smooth  dogfish  there  are  two  circles  of  valves  of  three  each,  situ- 
ated at  the  posterior  end  of  the  conus.  The  conus  arteriosus  of  the  skate  bears 
three  longitudinal  rows  of  valves  with  five  valves  in  each  row.  To  distinguish 
the  valves,  run  the  point  of  a  probe  along  the  conus  wall  from  the  anterior  end 
backward,  thus  opening  the  pockets. 

Make  a  drawing  to  show  the  structure  of  the  heart. 

The  heart  of  elasmobranchs  contains  only  venous  blood.  This  enters  the  sinus  venosus 
from  the  systemic  veins  and  passes  in  turn  through  the  auricle,  ventricle,  and  conus  arteriosus, 
and  out  into  the  ventral  aorta  which  distributes  the  blood  to  the  gills  by  way  of  the  afferent 


222       LABORATORY  MANUAL  FOR  VERTEBRATE  ANATOMY 

branchial  arteries.  On  passing  through  the  gill  filaments  the  blood  is  aerated  and  becomes 
arterial  blood.  It  is  then  collected  by  the  efferent  branchial  arteries  and  passed  into  the 
dorsal  aorta  which  distributes  it  to  all  parts  of  the  body.  The  venous  blood  is  returned  to 
the  heart  by  means  of  the  systemic  veins  and  the  two  portal  systems.  There  is  thus  in  the 
elasmobranchs  (and  all  fishes)  a  single  circulation  through  the  heart.  Since  the  heart  contains 
only  venous  blood,  the  heart  must  obtain  arterial  blood  from  some  outside  source.  This  is 
accomplished  by  means  of  the  coronary  artery  which  arises  from  the  efferent  branchial  vessels 
and  returns  to  the  heart.  As  we  shall  see  the  coronary  artery  originates  in  a  different  manner 
in  fish  than  in  other  vertebrates. 

9.  The  pericardio-peritoneal  canals.— Inspect  the  posterior  wall  of  the  peri- 
cardial  cavity  after  the  removal  of  the  heart.  In  the  dogfishes,  a  large  opening 
will  be  found  in  the  posterior  wall  dorsal  to  the  previous  attachment  of  the 
sinus  venosus.  On  probing  into  it,  it  will  be  found  to  lead  into  a  canal,  the 
pericardio-peritoneal  canal,  situated  along  the  ventral  wall  of  the  esophagus  inter- 
nal to  the  visceral  peritoneum  of  the  esophagus.  Eventually  the  canal  opens 
into  the  pleuroperitoneal  cavity  by  a  small  slit.  In  the  skate  there  is  an  open- 
ing of  moderate  size  in  the  center  of  the  posterior  wall  of  the  sinus  venosus. 
On  probing  into  this  it  will  be  found  to  lead  into  a  canal,  the  pericardio-peritoneal 
canal,  which  passes  through  the  center  of  the  hepatic  sinus.  On  probing  into  this 
it  will  be  found  to  fork  into  two  canals  lying  on  the  ventral  wall  of  the  esopha- 
gus internal  to  its  serosa.  They  open  into  the  pleuroperitoneal  cavity  by  minute 
slits.  The  pericardio-peritoneal  canals  serve  to  connect  the  pericardial  and 
pleuroperitoneal  cavities,  and  arise  through  the  failure  of  the  transverse  septum 
to  close  completely  across  the  coelom. 

C.      THE   CIRCULATORY   SYSTEM  OF  NECTURUS 

i.  The  chambers  of  the  heart. — The  pericardial  cavity  has  already  been 
exposed;  if  a  new  specimen  is  provided,  the  pericardial  and  pleuroperitoneal 
cavities  are  to  be  opened  as  before.  The  parts  of  the  heart  visible  in  ventral 
view  are  the  ventricle,  the  auricles,  and  the  conus  arteriosus.  The  ventricle  is 
thick  walled  and  conical  in  form.  Anterior  to  the  ventricle  on  either  side  is  a 
thin-walled  auricle.  Springing  from  the  base  of  the  ventricle  and  passing  for- 
ward between  the  two  auricles  is  the  tubular  conus  arteriosus.  Anteriorly  the 
conus  passes  into  the  enlarged  beginning  of  the  ventral  aorta;  this  muscular 
expansion  of  the  base  of  the  ventral  aorta  is  named  the  bulbus  arteriosus.  Lift 
the  apex  of  the  ventricle  and  note  the  sinus  venosus  situated  dorsad  to  the 
ventricle.  The  sinus  venosus  receives  two  large  venous  channels  lying  in  the 
dorsal  wall  of  the  pericardial  cavity.  These  are  formed  by  the  union  of  the  two 
hepatic  sinuses  with  the  two  common  cardinal  veins.  The  hepatic  sinuses  are  the 
two  large  veins  which  emerge  from  the  transverse  septum  and  pass  into  the 
sinus.  The  common  cardinal  veins  join  the  hepatic  sinuses  on  their  lateral 
surfaces. 


COMPARATIVE  ANATOMY  OF  THE  CIRCULATORY  SYSTEM  223 

2.  The  hepatic  portal  system  and  the  ventral   abdominal  vein. — In  the 
median  ventral  line  of  the  body  wall,  posterior  to  the  liver,  inclosed  in  the  falci- 
form ligament  is  situated  the  ventral  abdominal  vein.     It  is  homologous  with 
the  lateral  abdominal  veins  of  the  elasmobranchs.     It  receives  parietal  branches 
from  the  body  wall.     At  the  level  of  the  posterior  end  of  the  liver  it  leaves  the 
body  wall  and  passes  into  the  liver  where  it  is  situated  on  the  dorsal  surface. 
After  a  short  course  it  joins  the  hepatic  portal  vein  at  the  place  of  attachment 
of  the  hepatoduodenal  ligament. 

The  hepatic  portal  vein  is  the  vein  which  collects  the  blood  from  the  digestive 
tract.  It  is  formed  by  the  union  of  branches  from  the  intestine,  pancreas,  spleen, 
and  stomach.  Stretch  out  the  dorsal  mesentery  of  the  small  intestine.  In 
this  mesentery  about  halfway  between  the  body  wail  and  the  intestine  runs  a 
conspicuous  vein,  the  mesenteric  vein.  Trace  it  posteriorly  and  note  its  begin- 
ning in  the  wall  of  the  large  intestine.  As  it  passes  forward  in  the  mesentery  it 
receives  numerous  intestinal  veins  from  the  small  intestine.  It  then  passes  into 
the  substance  of  the  pancreas  receiving  small  pancreatic  veins  from  that  organ. 
At  the  level  of  the  pancreas  the  large  gastrosplenic  vein  also  joins  the  mesenteric 
from  the  left.  On  following  the  gastrosplenic  to  the  spleen  it  will  be  found  to 
be  formed  of  splenic  branches  from  the  spleen  and  gastric  veins  from  the  stomach. 
The  union  of  the  mesenteric,  gastrosplenic,  and  pancreatic  veins  produces  a 
large  vessel,  the  hepatic  portal  vein,  which  lies  along  the  center  of  the  dorsal 
face  of  the  liver.  It  also  receives  the  ventral  abdominal  vein  as  already  noted. 
Follow  it  along  the  surface  of  the  liver.  It  branches  into  the  liver  substance 
and  in  its  course  also  receives  additional  gastric  veins  from  the  stomach  and  also 
veins  from  the  ventral  body  wall  which  pass  into  the  liver  by  way  of  the  falci- 
form ligament.  (These  have  probably  been  destroyed  in  the  study  of  the  diges- 
tive tract.)  The  hepatic  portal  vein  subdivides  in  the  substance  of  the  liver  and 
eventually  passes  into  capillaries. 

Draw  the  hepatic  portal  system. 

3.  The  renal  portal  system. — Trace  the  ventral  abdominal  vein  posteriorly. 
It  soon  receives  some  vesical  veins  from  the  bladder.     Shortly  anterior  to  the 
hind  limbs  the  abdominal  vein  is  seen  to  be  formed  by  the  union  of  the  two 
pelvic  veins  which  run  along  the  inner  surface  of  the  lateral  body  wall  just  in 
front  of  the  pelvic  girdle.     Follow  one  of  the  pelvic  veins.     It  is  joined  by  the 
femoral  vein  from  the  hind  limb.     The  vein  formed  by  this  union  is  the  renal 
portal  vein.     It  passes  at  once  to  the  dorsal  surface  of  the  adjacent  kidney.     In 
male  specimens  the  kidney  is  a  brownish  organ  of  considerable  size  situated  at 
the  side  of  the  intestine.     In  female  specimens  the  kidney  is  much  smaller  and 
more  slender,  and  is  situated  at  the  common  point  of  attachment  of  the  mesova- 
rium  and  mesotubarium.     It  will  be  located  by  laying  the  ovary  to  one  side  and 
the  oviduct  to  the  other  side.     At  the  posterior  end  of  the  pleuroperitoneal  cavity 
the  kidney  in  the  female  lies  between  the  intestine  and  the  oviduct.     Having 


224 


LABORATORY  MANUAL  FOR  VERTEBRATE  ANATOMY 


located  the  kidney,  identify  the  renal  portal  vein  near  the  lateral  margin  of  its 
dorsal  surface.  At  the  place  where  the  renal  portal  vein  passes  from  the  body 
wall  to  the  surface  of  the  kidney,  the  vein  receives  the  caudal  vein,  which  ascends 
from  the  tail,  forks,  and  passes  to  the  surface  of  the  posterior  end  of  each  kidney. 


D 


FIG.  57. — Diagrams  to  show  the  origin  of  the  postcaval  vein  and  the  changes  in  the  abdominal 
vein  in  amphibians  and  reptiles.  A,  elasmobranch  stage,  same  as  in  Figure  556;  the  lateral  abdominal 
veins  i  enter  the  common  cardinal  veins  c  and  are  not  connected  with  the  renal  portal  veins  p.  B,  the 
lateral  abdominals  i  have  joined  the  renal  portals  at  /  posteriorly,  and  anteriorly  pass  into  the  liver  /, 
where  they  unite  with  the  hepatic  portal  vein  h;  a  new  vein,  the  postcaval  vein  g,  is  seen  growing  caudad 
from  the  liver/  where  it  arises  from  the  hepatic  veins  o.  C,  condition  in  the  adults  of  urodele  amphibi- 
ans; the  postcaval  vein  g  has  reached  and  fused  with  the  posterior  cardinals  e  and  the  subcardinals  j 
at  the  point  r;  the  two  lateral  abdominal  veins  have  united  to  form  the  ventral  abdominal  vein  i  which 
empties  into  the  hepatic  portal  h.  D,  condition  in  adult  reptiles;  the  anterior  portions  of  the  posterior 
cardinal  veins  n  are  obliterated,  leaving  the  postcaval  vein  g  as  the  sole  drainage  for  the  subcardinals  j 
and  the  kidneys  k;  the  two  lateral  abdominal  veins  remain  separate  as  in  elasmobranchs.  a,  anterior 
cardinal  vein;  b,  sinus  venosus;  c,  common  cardinal  vein;  d,  subclavian  vein;  e,  posterior  cardinal 
vein;  /,  liver;  g,  postcaval  vein;  h,  hepatic  portal  vein;  i,  lateral  (or  in  C  ventral)  abdominal  vein; 
j,  subcardinal  vein;  k,  kidney;  /,  iliac  or  femoral  vein;  m,  caudal  vein;  n,  obliterated  part  of  the  pos- 
terior cardinals;  o,  hepatic  veins;  p,  renal  portal  veins;  q,  pelvic  veins;  r,  union  of  postcaval,  posterior 
cardinals,  and  subcardinals;  s,  union  of  postcaval  and  subcardinals;  /,  union  of  abdominal  vein  with 
renal  portal  system. 

The  renal  portal  vein  runs  forward  along  the  surface  of  the  kidney,  into  the  sub- 
stance of  which  it  sends  numerous  branches.  It  also  receives  branches  from  the 
body  wall. 

Draw  the  renal  portal  system. 

It  will  be  seen  that  the  ventral  abdominal  vein  forms  a  connection  between 
the  renal  portal  and  hepatic  portal  systems.  Blood  from  the  hind  legs  and  tail 


COMPARATIVE  ANATOMY  OF  THE  CIRCULATORY  SYSTEM  225 

may  pass  into  the  abdominal  vein  and  so  into  the  hepatic  portal  vein,  or  may 
pass  into  the  renal  portal  vein.  This  arrangement  appears  to  be  a  device  to 
assist  the  return  of  the  blood  from  the  posterior  regions  of  the  body.  In  the 
elasmobranchs  there  is  no  such  connection  between  the  two  portal  systems 
(although  connections  have  been  reported  in  some  specimens  as  individual 
variations).  Further,  in  elasmobranchs  the  lateral  abdominal  veins  enter  the 
cardinal  system  of  veins,  while  in  Necturus  their  homologue,  the  ventral  abdomi- 
nal vein,  empties  into  the  hepatic  portal  system  (Fig.  57). 
4.  The  systemic  veins. — 

a)  The  anterior  systemic  veins:  It  has  already  been  noted  that  the  common 
cardinal  vein  joins  the  hepatic  sinus  on  each  side  in  the  pericardial  cavity. 
Turn  to  the  pericardial  cavity  and  locate  the  common  cardinal  veins.     Trace 
one  of  them  laterally,  removing  the  muscles  between  the  pericardial  cavity  and 
the  base  of  the  fore  limb.    Just  outside  of  the  pericardial  cavity  the  common 
cardinal  receives  the  jugular  and  subclavian  veins.     The  latter  lies  ventral  and 
anterior  to  the  former.     Trace  the  subclavian  into  the  fore  limb  by  removing 
the  skin  from  the  outer  surface  of  the  limb.     The  subclavian  is  seen  to  be  formed 
at  the  shoulder  by  the  union  of  the  cutaneous  vein  from  the  skin  and  the  brachial 
vein  which  runs  along  the  surface  of  the  limb  muscles.     The  jugular  vein  is 
homologous  with  the  anterior  cardinal  vein  of  the  elasmobranchs.     Follow  it 
forward.     It  is  formed  by  the  union  of  the  external  and  internal  jugular  veins. 
The  external  jugular  vein  first  receives  branches  from  the  floor  of  the  mouth. 
The  main  vein  then  passes  dorsally  immediately  behind  the  gills.     It  may  be 
picked  up  here  by  removing  the  skin  behind  the  last  gill.     The  external  jugular 
may  then  be  traced  forward  above  the  gills,  where  it  enlarges,  forming  the 
jugular  sinus.    This  sinus  receives  tributaries  from  the  head  and  jaws.     The 
internal  jugular  vein  is  a  small  vein  which  joins  the  external  jugular  posterior 
to  the  jugular  sinus.     It  is  difficult  to  find,  and  part  of  it  will  be  seen  later  in 
the  roof  of  the  mouth. 

The  common  cardinal  vein  also  receives  a  lateral  vein  from  the  body  wall. 
Remove  the  skin  from  the  lateral  line  shortly  posterior  to  the  fore  limb.  Cut 
through  the  shoulder  muscles  so  as  to  reveal  the  partition  (horizontal  skeletoge- 
nous  septum)  between  the  epaxial  and  hypaxial  muscles.  The  lateral  vein  will 
be  found  situated  along  this  partition  and  can  be  followed  forward  into  the 
common  cardinal  vein. 

b)  The  postcaval  vein:   Turn  to  the  pleuroperitoneal  cavity.     Examine  the 
dorsal  mesentery  of  the  small  intestine  at  its  junction  with  the  dorsal  body  wall 
(in  female  specimens  spread  the  ovaries  apart,  laying  one  to  each  side).     In  the 
mesentery  runs  a  large  vein,  the  postcaval  vein.     It  passes  forward,  receiving 
numerous  genital  veins  from  the  adjacent  gonads  and  renal  veins  from  the  kidneys. 
Trace  it  forward.     At  about  the  level  of  the  spleen  it  turns  ventrally  and  enters 
the  dorsal  surface  of  the  right  side  of  the  liver.     It  is  best  seen  by  laying  the 


226       LABORATORY  MANUAL  FOR  VERTEBRATE  ANATOMY 

stomach  to  the  left  and  the  liver  to  the  right.  It  passes  forward  imbedded  in 
the  liver  substance  and  should  be  followed  by  picking  away  the  liver  tissue.  It 
receives  several  hepatic  veins  from  the  liver;  one  of  the  larger  of  these  lies  along 
the  midventral  line  of  the  liver  and  joins  the  postcaval  near  the  anterior  end  of 
the  liver.  At  the  anterior  end  of  the  liver  the  postcaval  vein  emerges  as  a  very 
large  vessel  situated  in  the  coronary  ligament.  It  pierces  the  transverse  septum 
and  forks  into  the  two  hepatic  sinuses  which,  after  being  joined  by  the  common 
cardinal  veins,  enter  the  sinus  venosus.  The  origin  of  the  postcaval  vein  is 
discussed  below. 

c)  The  posterior  cardinal  veins:  At  the  place  where  the  postcaval  vein  turns 
ventrally  toward  the  liver  it  is  connected  with  a  pair  of  veins,  the  posterior 
cardinal  veins.  Trace  these  anteriorly.  They  lie  very  near  the  mid-dorsal  line 
of  the  anterior  half  of  the  pleuroperitoneal  cavity,  one  to  either  side  of  the  dorsal 
aorta.  In  females  they  are  situated  in  the  mesotubarium,  along  the  line  where 
this  unites  with  the  dorsal  wall.  Trace  the  posterior  cardinals  posteriorly  and 
note  connections  between  them  and  the  renal  portal  veins  (which  are  of  course 
the  original  posterior  ends  of  the  posterior  cardinals,  as  shown  in  Fig.  57).  Note 
also  the  parietal  veins  which  enter  the  posterior  cardinals  in  their  course  along 
the  body  wall.  The  posterior  cardinals  may  be  traced  to  the  transverse  septum. 
Shortly  before  reaching  this  they  diverge  from  each  other  and  penetrating  the 
lateral  portions  of  the  septum  enter  the  common  cardinal  vein  practically  at  the 
same  point  as  the  entrance  of  the  jugular  and  the  subclavian. 

5.  The  pulmonary  veins. — The  pulmonary  vein  is  a  large  vessel  situated 
along  the  ventral  side  of  each  lung,  i.e.,  the  side  opposite  that  which  is  attached 
to  the  dorsal  wall.     The  two  pulmonary  veins  run  forward  in  the  walls  of  the 
lungs  and  shortly  caudad  of  the  transverse  septum  converge  and  at  the  septum 
unite  to  one  vessel.     This  passes  through  the  transverse  sept'im  and  running 
forward  in  the  dorsal  wall  of  the  left  hepatic  sinus  enters  the  left  auricle. 

6.  The  ventral  aorta  and  the  aortic  arches. — The  conus  arteriosus  passes 
anteriorly  into  the  ventral  aorta.     The  greater  part  of  the  ventral  aorta  lies 
within  the  pericardial  cavity  and  owing  to  the  fact  that  this  portion  of  the  aorta 
is  expanded  and  possesses   thickened  muscular  walls,  it  is  named  the  bulbus 
arteriosus.1     Trace  the  ventral  aorta  forward  out  of  the  pericardial  chamber 
by  dissecting  away  the  anterior  wall  of  the  chamber.     The  ventral  aorta  very 
soon  forks  into  two  vessels  which  pass  to  the  right  and  left.     Trace  the  right  one, 
since  the  visceral  arches  have  been  left  intact  on  that  side.     Follow  it  toward 
the  gill  arches.     It  soon  divides  into  two  vessels  and  subsequently  the  posterior 

1  The  term  bulbus  arteriosus  is  very  ambiguously  used  in  many  texts  of  vertebrate  anatomy  and 
embryology.  The  term  should  be  applied  only  to  the  expanded  muscular  base  of  the  ventral  aorta. 
The  bulbus  arteriosus  does  not  take  part  in  the  heart  beat  and  is  not  a  chamber  of  the  heart  but  a  portion 
of  the  ventral  aorta.  This  definition  does  not  correspond  with  the  one  given  in  K.  Very  few  groups 
of  vertebrates  have  a  bulbus  arteriosus;  the  chief  group  possessing  it  is  the  Teleostei.  The  term  truncus 
arteriosus  is  another  ambiguous  name.  It  should  probably  be  used  as  synonymous  with  ventral  aorta. 


COMPARATIVE  ANATOMY  OF  THE  CIRCULATORY  SYSTEM  227 

one  again  divides  in  two,  making  a  total  of  three  afferent  branchial  arteries,  one 
to  each  of  the  gills.  Trace  each  one  into  the  gill,  removing  the  skin  from  the 
gill.  At  the  entrance  into  the  gill  the  first  afferent  branchial  artery  gives  off 
an  external  carotid  artery  which  turns  medially,  running  beside  the  branchial 
artery,  and  then  branches  into  the  floor  of  the  mouth.  Within  the  gill  each 
branchial  artery  sends  up  a  loop  which  branches  among  the  gill  filaments,  from 
which  other  branches  collect  into  a  loop  on  the  other  side  of  the  gill,  this  loop 
joining  the  branchial  artery  again.  In  addition  to  the  two  loops,  a  short  con- 
necting branch  runs  through  the  base  of  each  gill. 

Next  turn  back  the  flap,  previously  formed,  of  the  floor  of  the  mouth  and 
pharyngeal  cavities.  Extend  the  incision  posteriorly  along  the  left  side  of  the 
esophagus.  Strip  off  the  mucous  membrane  from  the  roof  of  the  mouth  and 
pharyngeal  cavities.  In  the  roof  will  be  seen  a  pair  of  large  vessels,  the  roots 
or  radices  of  the  aorta,  which  pass  obliquely  posteriorly  and  unite.  Trace  the 
right  root  laterally  into  the  visceral  arches  dissecting  off  the  mucous  membrane 
from  the  latter.  Locate  from  the  inside  the  branchial  arteries  exposed  in  the 
preceding  paragraph  and  note  their  emergence  from  the  dorsal  ends  of  the  visceral 
arches  as  the  efferent  branchial  arteries.  The  second  and  third  efferent  branchial 
arteries  unite  as  they  emerge  from  the  visceral  arches,  thus  forming  two  efferent 
branchial  arteries  on  each  side.  From  the  common  vessel  formed  by  the  union 
of  the  second  and  third  efferent  branchial  arteries  an  artery  arises  which  passes 
posteriorly.  Trace  it  into  the  pleuroperitoneal  cavity  and  note  that  it  courses 
along  the  dorsal  or  attached  side  of  the  lung.  It  is  the  pulmonary  artery.  From 
the  first  efferent  branchial  artery  very  near  the  place  where  it  joins  the  second 
and  third  arises  the  internal  carotid  artery  which  passes  forward  on  the  roof  of 
the  mouth.  Accompanying  the  internal  carotid  artery  is  the  internal  jugular 
(anterior  cardinal)  vein.  The  efferent  branchial  arteries  medial  to  the  origin 
of  the  pulmonary  and  internal  carotid  arteries  unite  to  form  the  root  of  the  aorta 
on  each  side.  From  this  springs  the  vertebral  artery  which  passes  at  once  dorsally 
into  the  skull.  The  two  roots  of  the  aorta  then  pass  obliquely  caudad  and 
unite  in  the  mid-dorsal  line  to  form  the  large  dorsal  aorta. 

Draw  the  afferent  and  efferent  branchial  arteries  and  their  branches. 

The  afferent  and  efferent  branchial  arteries  are  as  in  the  elasmobranchs,  the  ventral  and 
dorsal  halves,  respectively,  of  the  aortic  arches.  Besides  the  network  in  the  gills,  each  arch 
forms  a  complete  semicircle  by  means  of  the  direct  connecting  branch  in  the  base  of  the  gill. 
The  three  aortic  arches  present  in  Necturus  correspond  to  the  third,  fourth,  and  sixth  arches 
of  the  original  six.  Note  the  origin  of  the  external  carotid  from  the  ventral  part,  of  the  internal 
carotid  from  the  dorsal  part  of  the  third  aortic  arch,  and  of  a  new  vessel,  the  pulmonary  artery, 
from  the  last  or  sixth  arch  (see  Fig.  58C,  p.  267). 

7.  The  branches  of  the  dorsal  aorta. — Trace  the  dorsal  aorta  posteriorly 
into  the  pleuroperitoneal  cavity.  It  runs  in  the  median  dorsal  line.  Immedi- 
ately beyond  its  origin  it  gives  off  a  subclavian  artery  on  each  side.  Trace  one 


228       LABORATORY  MANUAL  FOR  VERTEBRATE  ANATOMY 

of  them.  It  passes  laterally.  It  gives  off  a  conspicuous  cutaneous  artery  which 
lies  on  the  inner  surface  of  the  pectoral  girdle.  It  branches  to  nearby  muscles 
and  the  skin.  The  subclavian  then  gives  off  an  artery  to  the  shoulder  and  as 
the  brachial  artery  passes  into  the  fore  limb  where  it  branches  extensively. 

In  its  course  along  the  pleuroperitoneal  cavity  the  dorsal  aorta  gives  off  both 
visceral  and  somatic  branches.  The  median  visceral  branches  will  be  described 
first. 

The  first  visceral  branch  of  the  dorsal  aorta  is  the  gastric  artery.  It  passes 
to  the  stomach  and  forks  into  the  dorsal  and  ventral  gastric  arteries  supplying  the 
corresponding  walls  of  the  stomach.  The  ventral  gastric  artery  also  furnishes 
a  few  small  branches  to  the  spleen.  Some  distance  posterior  to  the  origin  of 
the  gastric  artery,  the  coeliaco-mesenteric  artery  springs  from  the  aorta.  It 
passes  ventrally  in  the  mesentery  giving  rise  to  some  mesenteric  branches  to  the 
beginning  of  the  small  intestine;  it  then  proceeds  to  the  region  of  the  hepa to- 
duodenal  ligament  where  it  branches  into  a  splenic  artery  to  the  spleen,  a 
pancreatico-duodenal  artery  to  the  pancreas,  duodenum  and  pyloric  region  of 
the  stomach,  and  a  hepatic  artery,  which  runs  along  the  dorsal  surface  of  the 
liver  in  contact  with  the  hepatic  portal  vein  and  supplies  numerous  branches 
to  the  liver  substance.  Posterior  to  the  point  of  origin  of  the  coeliaco- 
mesenteric  vessel,  the  dorsal  aorta  gives  off  a  number  of  mesenteric  arteries  into 
the  intestine. 

The  lateral  visceral  branches  of  the  dorsal  aorta  consist  of  numerous  genital 
arteries  to  the  testes  in  the  male,  and  ovaries  and  oviducts  in  the  female,  and  of 
renal  arteries  to  the  kidneys.  The  somatic  branches  of  the  aorta  consist  of  the 
parietal  or  intercostal  arteries.  These  arise  from  the  dorsal  side  of  the  aorta  at 
segmental  intervals;  they  pass  dorsally  and  divide  in  two,  one  branch  going  to 
each  side  of  the  body.  These  branches  pass  laterally  along  the  internal  surface 
of  the  body  wall  and  supply  the  body  musculature. 

Near  the  posterior  end  of  the  pleuroperitoneal  cavity,  the  aorta  gives  off 
on  each  side  an  iliac  artery,  which  passes  laterally  alongside  the  femoral  vein 
toward  the  hind  limb.  It  gives  off  an  epigastric  artery  which  runs  anteriorly 
along  the  body  wall,  a  hypo  gastric  branch  to  the  urinary  bladder  and  cloaca,  and 
as  the  femoral  enters  the  hind  limb,  into  which  it  should  be  traced.  It  runs 
along  the  medial  side  of  the  leg  and  at  the  knee  gives  rise  to  a  number  of  branches. 
The  dorsal  aorta  proceeds  into  the  tail  as  the  caudal  artery,  giving  off  a  pair  of 
cloacal  arteries  into  the  cloaca  as  it  passes  that  region. 

Draw  the  branches  of  the  dorsal  aorta. 

8.  The  chambers  of  the  heart. — Remove  the  heart  from  the  pericardial 
cavity  by  cutting  across  both  ends.  The  chambers  of  heart  were  previously 
named.  The  sinus  venosus  is  a  chamber  with  very  thin,  delicate  walls.  It 
receives  from  behind  the  two  large  trunks  formed  by  the  union  of  the  common 
cardinal  vein  and  hepatic  sinus  on  each  side.  Anteriorly  the  sinus  passes  into 


COMPARATIVE  ANATOMY  OF  THE  CIRCULATORY  SYSTEM  229 

the  auricle.  The  sinus  connects  chiefly  with  the  right  auricle  and  is  slightly 
displaced  to  the  right.  By  cutting  open  the  sinus  locate  the  sin-auricular  open- 
ing guarded  by  a  pair  of  valves.  Cut  into  one  of  the  auricles  and  wash  out 
its  contents.  Looking  into  the  auricle  note  the  inter  auricular  septum  which 
separates  the  two  auricles;  the  septum  is  very  incomplete,  being  perforated  by 
a  number  of  openings.  Remove  the  ventral  half  of  the  ventricle  and  also 
slit  open  the  conus  and  bulbus  arteriosus  by  a  longitudinal  incision.  Note  the 
thick  spongy  walls  of  the  ventricle  and  the  numerous  muscle  strands  in  the 
interior.  Locate  the  single  auricula-ventricular  opening  between  the  auricles 
and  ventricle.  It  is  on  the  left  side.  It  is  guarded  by  a  pair  of  valves.  In  the 
base  of  the  opened  conus  arteriosus  note  the  transverse  row  of  three  semilunar 
valves.  In  the  bulbus  arteriosus  is  a  longitudinal  partition,  dividing  the  interior 
into  right  and  left  channels. 

The  heart  of  Necturus  and  of  all  Amphibia  receives  both  arterial  and  venous  blood,  and 
consequently  there  is  a  double  circulation  through  the  heart.  The  blood  from  the  systemic 
veins  enters  the  sinus  venosus  and  is  passed  on  chiefly  to  the  right  auricle.  The  blood  from 
the  pulmonary  veins,  which  is  aerated  blood,  returns  to  the  left  auricle.  Owing  to  the 
incomplete  nature  of  the  interauricular  septum  there  is  some  mixing  of  arterial  and  venous 
blood.  The  blood  from  both  auricles  passes  into  the  single  ventricle  where  it  is  further  mixed, 
and  the  mixed  blood  exits  by  way  of  the  conus  arteriosus  and  ventral  aorta.  Passing  to  the 
gills  the  blood  is  aerated  and  as  arterial  blood  enters  the  dorsal  aorta  and  pulmonary  artery. 
As  the  pulmonary  artery  already  contains  aerated  blood  before  it  reaches  the  lungs,  it  is  evi- 
dent that  the  lungs  are  but  slightly  functional  in  the  gilled  urodeles. 

9.  Comparison  of  the  circulatory  system  of  Necturus  and  the  elasmobranchs. — It  is 
evident  that  the  circulatory  system  of  Necturus  has  undergone  some  modifications  from  the 
condition  seen  hi  elasmobranchs.  In  the  arterial  system  the  chief  changes  concern  the  aortic 
arches.  Of  the  five  arches  present  in  elasmobranchs  but  three  have  persisted  hi  Necturus.  The 
persistent  arches  are  the  third,  fourth,  and  sixth,  the  second  and  fifth  having  vanished.  In 
addition  we  note  the  origin  of  a  new  vessel,  the  pulmonary  artery,  as  a  branch  from  the  last 
(sixth)  aortic  arch.  The  aortic  arches  of  Necturus  have  still  the  form  of  an  arch  which  passes 
through  the  substance  of  a  visceral  arch.  In  the  venous  system  the  changes  are  more  pro- 
nounced. The  anterior  cardinal  vein  still  remains  the  chief  vein  of  the  head,  but  the  posterior 
cardinal  vein  is  much  decreased  in  importance.  The  functions  of  the  posterior  cardinal  vein 
are  taken  over  by  a  new  vein,  the  postcaval  vein,  which  is  absent  in  fishes  (except  in  the  Dipnoi). 
The  origin  of  the  postcaval  vein  is  somewhat  complicated  but  from  a  study  of  its  relations  in 
Necturus  it  is  evident  that  the  anterior  part  is  derived  from  the  hepatic  veins  or  sinuses  of  the 
elasmobranchs  (and  these  in  turn  are  the  proximal  portions  of  the  vitelline  veins),  and  that 
the  posterior  part  of  the  postcaval  is  formed  from  the  subcardinal  veins,  chiefly  the  right  one 
(Fig.  57,  p.  224).  The  posterior  portions  of  the  posterior  cardinal  veins  are  functioning  as  the 
renal  portal  veins  as  in  fishes.  The  anterior  portions  of  the  posterior  cardinals  are  diminished 
in  importance  but  have  the  same  relations  as  in  fishes;  posteriorly  they  are  connected  with  the 
subcardinals  (postcaval)  as  also  in  elasmobranchs  (Fig.  55,  p.  205).  It  will  also  be  noted  that 
the  renal  portal  veins  (posterior  cardinals)  have  increased  their  posterior  connections.  Whereas 
in  fishes  they  collect  only  from  the  tail,  in  Necturus  they  collect  from  both  tail  and  posterior 
appendages.  This  is  due  to  a  union  between  the  renal  portal  and  abdominal  veins  (Fig.  57). 
Meantime  the  ventral  abdominal  vein  (same  as  the  lateral  abdominal  veins  of  elasmobranchs) 


230       LABORATORY  MANUAL  FOR  VERTEBRATE  ANATOMY 

has  shifted  its  anterior  connections.  Whereas  in  fishes  it  opens  into  the  common  cardinal 
vein,  in  Amphibia  it  passes  into  the  hepatic  portal  system.  The  abdominal  vein  thus  becomes 
a  connection  between  the  renal  and  hepatic  portal  systems  and  provides  two  outlets  for  the 
blood  from  legs  and  tail.  The  pulmonary  veins  are  new  formations. 


D.      THE   CIRCULATORY   SYSTEM   OF  THE   TURTLE 

Specimens  for  the  study  of  this  system  should  have  been  doubly  injected, 
that  is,  into  the  arterial  system,  and  in  both  directions  into  one  ventral  abdominal 
vein.  Remove  the  plastron.  With  the  bone  scissors  cut  away  the  sides  of 
the  carapace  between  fore  and  hind  limbs. 

1.  The  chambers  of  the  heart. — The  heart  of  the  turtle  possesses  but  three 
different  chambers,  in  contrast  to  the  four  chambers  present  in  the  forms  already 
considered;  one  of  the  chambers  is  divided  into  two  completely  separate  halves ; 
Examine  the  heart  in  the  pericardial  cavity,  removing  the  ventral  wall  of  the 
pericardial  sac  if  this  has  not  already  been  done.     From  the  ventral  view  the 
visible  parts  of  the  heart  are  the  ventricle — the  thick- walled,  conical  posterior 
part — and  the  auricles — thin-walled  chambers,  one  on  each  side  anterior  to  the 
ventricle.     The  two  auricles  are  entirely  separate  from  each  other,  as  will  be 
seen  later.     The  ventricle  is  attached  to  the  posterior  pericardial  wall  by  a 
ligament,  which  is  apparently  a  remnant  of  the  ventral  mesentery  or  ventral 
mesocardium  of  the  heart,  a  structure  which  was  considered  in  the  general  dis- 
cussion of  the  coelom.     Cut  through  this  ligament,  raise  the  ventricle,  and  press 
it  forward.    A  large  chamber,  the  sinus  venosus,  is  revealed  dorsal  to  the  auricles 
and  attached  to  the  right  auricle.     The  large  bases  of  the  systemic  veins  will  be 
seen  entering  the  sinus.     Put  the  ventricle  back  in  place.     Observe  the  large 
vessels,  arteries,  which  spring  directly  from  the  base  of  the  ventricle  without 
the  intervention  of  a  conus  arteriosus.     The  latter  is  lacking,  or  to  be  very 
accurate,  is  so  shortened  down  as  to  be  invisible.     The  bases  of  the  arteries  which 
spring  from  the  ventricle  correspond  to  the  ventral  aorta  of  elasmobranchs.     The 
ventral  aorta  is  thus  seen  to  have  split  into  several  separate  trunks.     The  arteries 
will  be  investigated  later  and  should  be  left  undisturbed  at  present. 

2.  The  ventral  abdominal  veins  and  the  renal  portal  system. — Running  in 
the  ventral  pleuroperitoneum  from  the  pelvic  girdle  up  to  the  heart  are  two 
large  veins,  the  ventral  abdominal  veins.     They  are  homologous  with  the  lateral 
abdominal  veins  of  elasmobranchs.     The  two  veins  are  generally  connected  just 
anterior  to  the  pelvic  girdle  by  a  cross-branch.     Trace  the  veins  forward.     They 
receive  pericardial  veins  from  the  pericardial  sac  and  then  each  turns  dorsally 
to  enter  the  liver.     Just  at  this  turn  each  vein  receives  a  pectoral  branch  from 
the  pectoral  muscles  of  that  side.     Trace  the  pectoral  vein  hi  to  the  muscle. 
Slit  the  pleuroperitoneum  alongside  each  abdominal  vein  and  by  lifting  the  cut 
edges  find  the  places  where  the  vein  of  each  side  penetrates  the  lobe  of  the  liver. 

Trace  the  ventral  abdominal  veins  posteriorly.  Make  a  longitudinal  slit 
in  the  pleuroperitoneum  midway  between  the  two  veins,  and  separating  the  cut 


COMPARATIVE  ANATOMY  OF  THE  CIRCULATORY  SYSTEM  231 

edges,  look  within  and  locate  the  urinary  bladder.  Note  the  small  vesical  vein 
passing  from  the  bladder  into  each  abdominal  vein.  Continue  to  trace  the 
abdominal  veins  posteriorly.  Each  passes  to  one  side  of  the  pointed  anterior 
extremity  of  the  pelvic  girdle  and  at  the  same  time  gradually  turns  laterally. 
As  it  turns  it  receives  a  pelvic  vein  which  runs  over  the  ventral  surface  of  the 
muscles  of  the  pelvic  girdle.  The  left  pelvic  vein  seems  to  be  usually  larger  than 
the  right  one. 

In  their  course  between  the  heart  and  pelvic  girdle  each  vein  gives  off  laterally 
one  or  more  small  branches  which  pass  to  the  borders  of  the  carapace  where  they 
join  the  margino-costal  vein  to  be  described  later. 

Draw  the  two  abdominal  veins  with  their  branches  thus  far  noted. 

Continue  to  trace  the  abdominal  veins  in  the  posterior  direction.  As  both 
have  identical  branches  it  is  necessary  to  follow  only  one,  selecting  the  one  which 
has  been  most  successfully  injected.  It  passes  along  the  dorsal  surface  of  the 
pelvic  girdle  near  the  anterior  margin  of  the  latter;  the  girdle  should  be  pulled 
toward  the  student  in  order  to  follow  the  vein.  Grasp  the  hind  leg  on  the  side 
on  which  you  are  dissecting  and  work  it  back  and  forth  until  it  is  freely  movable. 
Press  the  leg  away  from  the  carapace  of  that  side  and  cut  through  the  skin  between 
the  leg  and  the  carapace  back  to  the  end  of  the  tail.  Remove  the  skin  from  leg 
and  tail.  Now  trace  the  abdominal  vein  laterally  along  the  base  of  the  leg. 
Just  beyond  the  pelvic  vein  a  small  crural  from  thigh  muscles  and  a  larger  vein 
from  fat  enter  the  abdominal.  About  an  inch  and  one-half  lateral  to  this 
the  large  femoral  vein  emerges  from  the  leg  and  joins  the  abdominal  vein, 
now  designated  the  iliac  vein.  The  femoral  vein  should  be  followed  into  the 
leg  by  separating  the  muscles.  The  iliac  vein  is  now  situated  alongside  a  conspic- 
uous artery,  the  epigastric  artery,  both  being  imbedded  in  the  abdominal  wall  from 
which  small  veins  pass  into  the  iliac  vein.  After  a  short  distance  the  iliac  vein 
receives  the  epigastric  vein  which  accompanies  the  artery  of  the  same  name 
anteriorly  along  the  curve  of  the  carapace.  The  iliac  vein  now  turns  abruptly 
posteriorly  and  runs  between  the  base  of  the  leg  and  the  carapace,  deeply 
imbedded  in  some  loose  tissue.  This  tissue  should  be  cleared  away  and  the 
vein  followed.  It  receives  branches  from  the  carapace  and  near  the  posterior 
part  of  the  thigh  a  well-marked  sciatic  vein  from  the  thigh.  Posterior  to  this 
point  it  receives  several  small  branches  from  the  leg  and  as  the  caudal  vein 
passes  along  the  side  of  the  tail,  receiving  at  the  base  of  the  tail  a  cloacal  branch 
from  the  anal  region. 

Return  to  the  point  where  the  epigastric  vein  enters  the  iliac  vein.  At  this 
place  a  large  vein  continues  forward  from  the  anterior  and  dorsal  surface  of  the 
iliac.  This  vein,  the  renal  portal  vein,  runs  forward  and  dorsally,  penetrating  the 
pleuroperitoneum.  Cut  the  pleuroperitoneum  transversely  halfway  between 
the  heart  and  pelvic  girdle,  cutting  across  both  abdominal  veins.  Cut  also  into 
the  pleuroperitoneum  at  the  place  where  the  renal  portal  vein  passes  through  it. 
A  layer  of  muscle  will  be  found  outside  the  peritoneum  at  this  place.  Both 


232       LABORATORY  MANUAL  FOR  VERTEBRATE  ANATOMY 

muscles  and  membrane  should  be  slit  ventrally  to  meet  the  transverse  incision 
across  the  pleuroperitoneum.  In  this  way  free  access  is  gained  to  the  pleuro- 
peritoneal  cavity.  With  the  left  hand  carefully  press  all  of  the  viscera  forward. 
It  is  usually  necessary  to  detach  the  lung  from  the  dorsal  wall  and  push  it  for- 
ward also.  With  the  right  hand  press  the  pelvic  girdle  caudad.  A  space  cleared 
of  viscera  is  thus  left  dorsal  to  and  in  front  of  the  pelvic  girdle.  Look  into  this 
place  near  the  median  dorsal  line  for  a  somewhat  flattened  organ,  the  kidney, 
situated  against  the  median  dorsal  wall.  The  kidney  is  retroperitoneal,  that  is, 
dorsal  to  the  pleuroperitoneum.  This  latter  membrane  should  be  stripped  off 
from  the  ventral  face  of  the  kidney.  (In  male  specimens  the  rounded  yellow 
testis  and  black  coiled  epididymis  will  be  noted  attached  to  the  ventral  surface 
of  the  kidney.)  The  renal  portal  vein  may  now  be  followed  from  the  point  where 
it  leaves  the  iliac  through  the  pleuroperitoneum  toward  the  kidney.  Before 
reaching  the  kidney  it  receives  a  vein  from  the  carapace.  At  about  the  middle 
of  the  lateral  border  of  the  kidney  is  a  fissure;  the  renal  portal  vein  enters  this 
fissure  and  passes  onto  the  ventral  face  of  the  kidney  where  it  immediately 
forks.  One  of  its  branches,  the  vertebral  vein,  runs  forward  and  may  be  traced 
in  well-injected  specimens  by  separating  the  lung  from  the  carapace  and  raising 
the  lung  and  also  stripping  off  the  pleuroperitoneum  from  the  dorsal  wall.  The 
vertebral  vein  passes  anteriorly  dorsal  to  the  arches  of  the  ribs  and  receives 
laterally  an  intercostal  branch  at  each  suture  between  the  costal  plates  of  the 
carapace.  The  intercostal  veins  anastomose  with  each  other  in  the  curve  of 
the  carapace  by  means  of  a  longitudinal  vessel,  the  margino-costal  vein,  which 
is  the  anterior  continuation  of  the  epigastric  vein  previously  noted.  The 
margino-costal  vein  also  has  connections  with  the  abdominal  veins.  The 
posterior  branch  of  the  renal  portal  vein  passes  posteriorly  over  the  ventrai 
face  of  the  kidney  and  as  the  internal  iliac  or  hypogastric  vein  receives  branches 
from  the  reproductive  organs  (male),  bladder,  cloaca,  etc.  The  renal  portal 
vein  in  its  passage  along  the  ventral  face  of  the  kidney  gives  off  branches  into 
that  organ.  The  renal  portal  vein  is  the  posterior  part  of  the  posterior  cardinal 
vein  (Figs.  55  and  57).  The  vertebral  vein  is  formed  by  the  longitudinal  fusion 
of  segmental  branches  of  the  posterior  cardinal  vein  of  the  embryo. 

Draw  these  veins  as  far  as  you  have  seen  them,  adding  them  to  the  drawing 
of  the  abdominal  veins  already  made. 

The  student  should  consider  at  this  point  the  differences  between  the  con- 
nections of  the  ventral  abdominal  veins  of  the  turtle  and  their  homologues,  the 
lateral  abdominal  veins,  of  elasmobranchs.  In  the  turtle  these  veins  have  formed 
a  connection  with  the  renal  portal  system  posteriorly  while  anteriorly  they  enter 
the  liver  instead  of  the  cardinal  system. 

3.  The  hepatic  portal  system. — Lift  up  the  lobes  of  the  liver  separating  them 
gently  from  the  stomach  and  duodenum,  and  find  on  their  dorsal  surfaces  at  the 
place  where  the  gastro-hepato-duodenal  ligament  is  attached  to  the  liver  a  large 


COMPARATIVE  ANATOMY  OF  THE  CIRCULATORY  SYSTEM  233 

in,  the  hepatic  portal  vein.  It  runs  completely  across  the  liver  imbedded  in 
its  wall,  and  at  the  right,  at  the  point  where  the  bile  duct  enters  the  duodenum, 
turns  abruptly  posteriorly,  penetrating  the  mesentery.  On  the  left  note  the 
numerous  gastric  veins  entering  the  hepatic  portal  vein  from  the  stomach. 
Just  to  the  right  of  the  bridge  connecting  the  two  lobes  of  the  liver,  two  or  three 
anterior  pancreatic  veins  pass  from  the  pancreas  into  the  hepatic  portal  vein. 
Near  the  bile  duct  it  receives  cystic  veins  from  the  bile  duct,  posterior  pancreatic 
veins  from  the  right  end  of  the  pancreas,  and  a  long  duodenal  branch  from  the 
first  part  of  the  small  intestine.  The  hepatic  portal  vein  should  be  followed 
posteriorly;  it  is  imbedded  in  the  pancreas  and  at  the  bend  of  the  duodenum 
penetrates  the  mesentery  and  emerges  to  the  left  of  the  duodenum.  Liver  and 
duodenum  must  be  pressed  forward  to  follow  it.  The  vein  next  passes  to  the 
posterior  side  of  the  adjacent  loop  of  the  small  intestine  which  should  also  be 
pressed  forward.  The  vein  will  then  be  found  to  pass  on  the  left  side  of  the 
spleen  in  contact  with  that  organ  and  to  receive  numerous  splenic  tributaries 
from  it.  Shortly  posterior  to  the  spleen  the  hepatic  portal  vein  reaches  the 
central  point  of  the  mesentery  where  the  mesentery  is  thrown  into  a  coil.  At 
this  place  the  numerous  mesenteric  veins,  accompanied  by  arteries,  will  be  seen 
passing  in  the  mesentery  from  all  parts  of  the  intestine  into  the  hepatic  portal 
vein. 

Draw  the  hepatic  portal  vein  and  its  branches. 

Now  by  dissecting  away  the  liver  substance  trace  the  anterior  portions  of 
the  ventral  abdominal  veins  into  the  liver  and  find  their  union  with  the  hepatic 
portal  vein.  Note  how  the  hepatic  portal  vein  breaks  up  into  many  branches 
in  the  liver  substance.  As  in  other  vertebrates  the  direction  of  flow  in  the 
hepatic  portal  vein  is  from  the  digestive  tract  into  the  liver. 

Add  to  the  drawing  the  connections  of  the  ventral  abdominal  vein  in  the  liver. 

4.  The  systemic  veins. — Four  large  systemic  veins  enter  the  sinus  venosus. 
Turn  the  ventricle  forward  so  as  to  obtain  a  clear  view  of  the  sinus.  As  already 
noted,  it  is  not  symmetrically  placed  but  is  displaced  slightly  to  the  right,  con- 
necting with  the  right  auricle.  A  large  vein  enters  the  left  wall  of  the  sinus, 
passing  around  the  border  of  the  left  auricle.  This  is  the  left  precaval  vein 
(also  called  anterior  vena  cava  and  descending  vena  cava).  Another  vein,  the 
left  hepatic  vein,  emerges  from  the  bridge  of  the  liver  and  enters  the  left  angle  of 
the  posterior  wall  of  the  sinus.  The  very  large  vein  which  passes  into  the  right 
angle  of  the  posterior  wall  of  the  sinus  is  the  postcaval  vein  (also  named  posterior 
vena  cava  and  ascending  vena  cava) ;  it  emerges  from  the  right  lobe  of  the  liver. 
Just  in  front  of  the  entrance  of  the  postcaval  vein  and  best  seen  by  pressing  the 
heart  to  the  left,  the  right  precaval  vein  passes  into  the  right  anterior  angle  of 
the  sinus  venosus. 

a)  The  branches  of  the  precavals:  Each  precaval  enters  the  pericardial  cavity 
by  passing  through  the  anterior  wall  of  the  pericardial  sac.  From  this  point 


234       LABORATORY  MANUAL  FOR  VERTEBRATE  ANATOMY 

it  may  be  followed  forward.  As  both  have  identical  branches,  it  is  necessary 
to  follow  only  one.  The  one  whose  branches  appear  to  be  filled  with  blood 
should  be  selected;  the  left  one  is  usually  easier  to  follow.  In  specimens  which 
have  been  preserved  for  a  long  time  the  dissection  of  the  branches  of  the  precaval 
veins  is  generally  unsatisfactory  because  the  branches  are  often  empty,  but,  of 
those  named  below,  as  many  as  the  condition  of  the  specimen  permits  should 
be  identified.  Be  very  careful  not  to  injure  the  adjacent  arteries  springing  from 
the  ventricle.  Trace  the  precaval  forward  out  of  the  pericardial  sac.  Shortly 
anterior  to  the  place  where  the  precaval  penetrates  the  pericardial  sac  the  vein 
receives  practically  simultaneously  four  tributaries,  three  small  and  one  large, 
one.  The  most  medial  branch  is  the  small  thyreo-scapular  vein  which  collects 
a  branch  from  the  thyroid  gland  (the  gland  situated  in  the  fork  of  the  large 
arteries)  and  then  passes  to  the  inner  surface  of  the  shoulder  where  it  collects 
from  several  muscles.  Lateral  to  this  vein  is  the  slightly  larger  internal  jugular 
vein.  This  runs  anteriorly  along  the  side  of  the  neck  in  contact  with  a  white 
nerve  (vagus  or  tenth  cranial  nerve).  It  receives  medially  an  extensive  net- 
work of  branches  from  the  esophagus.  It  may  be  traced  anteriorly  to  the  base 
of  the  skull  from  which  it  issues,  making  an  anastomosis  with  the  external  jugular 
vein,  soon  to  be  described.  The  third  tributary  of  the  precaval  vein  is  the  large 
subclavian  vein,  by  far  the  largest  of  the  four  branches  which  enter  the  precaval. 
It  passes  along  the  side  of  the  neck  and  as  the  axillary  vein  turns  toward  the 
shoulder.  Here  it  is  seen  to  be  formed  by  the  union  of  two  large  branches, 
the  external  jugular  vein  from  the  neck  and  the  brachial  from  the  fore  limb.  The 
external  jugular  lies  along  the  side  of  the  neck,  lateral  and  dorsal  to  the  internal 
jugular.  It  collects  from  the  head,  and  in  its  passage  posteriorly  along  the  neck 
has  at  regular  intervals  vertebral  veins  passing  into  it  from  between  the  vertebrae. 
Near  its  junction  with  the  brachial  it  receives  the  last  of  the  vertebral  veins 
which  descends  from  the  junction  between  last  cervical  and  first  trunk  vertebra, 
where  it  connects  with  the  anterior  end  of  the  vertebral  vein  described  with  the 
renal  portal  system.  The  external  jugular  vein  also  receives  branches  from  the 
skin  and  muscles  of  the  shoulder  region.  The  fourth  and  most  lateral  and  dorsal 
of  the  tributaries  of  the  precaval  is  a  small  scapular  vein  which  comes  from  the 
muscles  covering  the  scapula. 

Draw  the  branches  of  the  precaval  as  far  as  you  have  found  them. 

b)  The  left  hepatic  vein:  The  left  hepatic  vein  should  be  traced  into  the  left 
lobe  of  the  liver  from  which  it  collects  venous  blood.     To  do  this,  clear  away  the 
intervening  posterior  wall  of  the  pericardial  sac  and  pleuroperitoneum. 

c)  The  postcaval  vein:  Trace  the  postcaval  vein  posteriorly  into  the  right  lobe 
of  the  liver.     Its  course  may  be  followed  by  making  a  slight  hole  in  the  vein 
where  it  enters  the  sinus  and  probing  posteriorly  into  the  hole,  dissecting  away 
the  liver  substance  along  the  probe.     Note  the  numerous  hepatic  veins  which 
enter  the  postcaval  during  its  passage  through  the  liver.     Find  where  the  post- 


COMPARATIVE  ANATOMY  OF  THE  CIRCULATORY  SYSTEM  235 

caval  enters  the  liver  from  behind  to  the  right  of  the  hepatic  portal  vein.  At  this 
point  the  serosa  of  the  liver  is  fused  to  the  pleuroperitoneal  membrane  over  the 
ventral  face  of  the  lung.  This  fusion  should  be  broken  and  the  postcaval  vein 
freed.  On  following  it  posteriorly  it  will  be  found  to  swerve  toward  the  median 
line  where  it  runs  alongside  a  large  artery  (dorsal  aorta).  The  postcaval  may 
be  traced  to  the  posterior  end  of  the  pleuroperitoneal  cavity.  Its  relations  there 
will  be  described  later. 

Add  the  left  hepatic  and  postcaval  veins  to  your  drawing  of  the  precaval  vein. 

5.  The  pulmonary  veins. — A  pulmonary  vein  passes  from  each  lung  to  the 
left  auricle  of  the  heart.     It  is  situated  posterior  to  the  bronchus  where  it  should 
be  identified.     Follow  it  toward  the  heart.     It  passes  dorsal  to  the  precaval 
vein.     The  right  pulmonary  runs  in  the  dorsal  wall  of  the  pericardial  sac  anterior 
to  the  sinus  venosus  and  joins  the  left  vein  at  the  entrance  of  both  into  the  left 
auricle.     The  point  of  entrance  is  near  the  left  precaval  vein. 

6.  The  aortic  arches  and  their  branches. — From  the  ventricle  three  large 
arterial  trunks  extend  forward.   Together  they  constitute  the  ventral  aorta  which 
must  be  conceived  of  as  having  split  into  three  trunks.     Clean  away  the  connec- 
tive tissue  from  these  arteries  and  separate  them  from  each  other.     The  trunk 
farthest  to  the  left  is   the  pulmonary  artery;   the  vessel  next  to  it  is  the 
left  aorta;   the  third  and  right-hand  trunk  is  the  right  aorta,  but  it  is  concealed 
from  view  by  the  large  branch,  the  brachiocephalic  (innominate)  artery  which 
it  gives  off  immediately  on  leaving  the  heart.     Note  the  small  coronary  arteries 
springing  from  the  base  of  the  brachiocephalic  artery  and  branching  over  the 
surface  of  the  heart.     The  brachiocephalic  artery  lies  in  the  median  line  and 
forks  at  once  into  large  branches.     In  the  angle  of  the  fork  lies  a  reddish  body, 
the  thyroid  gland. 

a)  The  branches  of  the  brachiocephalic  artery:  We  shall  follow  this  vessel  first; 
it  divides  at  once  into  four  trunks;  the  large  medial  ones  are  the  right  and  left 
subclavian  arteries,  the  much  smaller  lateral  ones  are  the  right  and  left  carotid 
arteries.  Clean  away  connective  tissue  from  these  vessels  and  follow  their 
courses.  The  two  subclavians  embrace  the  thyroid  gland  between  their  bases 
and  supply  small  thyroid  arteries  into  this  gland.  Each  subclavian  next  gives 
off  branches  to  the  ventral  side  of  the  neck  and  to  the  trachea,  of  which  the  chief 
one  is  the  ventral  cervical  artery,  a  vessel  arising  from  the  subclavian  about  one- 
half  inch  beyond  the  thyroid  gland  and  branching  profusely  into  the  esophagus, 
trachea,  muscles  of  the  neck,  and  thymus  gland.  The  thymus  gland  is  a  yellowish 
mass  lateral  to  the  ventral  cervical  artery  and  receiving  branches  from  it.  The 
subclavian  artery,  now  named  the  axillary,  turns  laterally  and  passes  to  the  inner 
surface  of  the  pectoral  girdle,  where  a  large  branch  arises  and  branches  extensively 
into  the  pectoral  and  shoulder  muscles.  The  axillary  then  turns  abruptly  posteri- 
orly and  about  an  inch  beyond  the  turn  gives  off  the  small  dorsal  cervical  into  the 
neck,  the  first  intercostal  laterally,  and  the  vertebral  caudally.  The  first  inter- 


236       LABORATORY  MANUAL  FOR  VERTEBRATE  ANATOMY 

costal  runs  laterally  and  then  turns  posteriorly,  joining  the  margino-costal  artery, 
which  courses  along  the  curve  of  the  carapace.  The  vertebral  passes  backward 
along  the  vertebral  column  dorsal  to  the  ribs  alongside  the  vertebral  vein  and 
gives  off  at  the  sutures  of  the  costal  plates  the  intercostal  arteries,  which  run 
laterally  into  the  margino-costal  artery.  At  the  point  where  the  first  intercostal 
and  vertebral  arise,  the  axillary  bends  sharply  laterally  and  as  the  brachial  artery 
passes  into  the  fore  limb  alongside  the  brachial  vein. 

Each  carotid  artery  passes  forward  along  the  ventral  side  of  the  neck,  soon 
crossing  dorsal  to  the  subclavian  and  then  coming  to  lie  medial  to  the  subclavian. 
In  specimens  in  which  the  neck  is  drawn  into  the  shell  the  carotids  usually  make 
loops  in  the  neck  region.  As  the  carotid  artery  passes  the  thymus  gland  it  gives 
branches  into  the  gland.  It  then  proceeds,  without  branching,  the  entire  length 
of  the  neck  in  contact  with  the  internal  jugular  vein  and  the  vagus  nerve,  and 
enters  the  skull  by  a  foramen  in  front  of  the  auditory  region. 

Draw  the  brachiocephalic  artery  and  its  branches. 

b)  The  pulmonary  arteries:  The  pulmonary  artery  is  the  one  farthest  to  the 
left  of  the  three  arterial  trunks  which  spring  from  the  ventricle.     It  divides 
immediately  into  right  and  left  pulmonary  arteries.     To  see  this  division  lift  the 
pulmonary  trunk  and  look  on  its  dorsal  side.  Trace  the  left  pulmonary  first.  It 
proceeds  laterally  posterior  to  the  left  aorta  to  which  it  is  more  or  less  bound 
by  connective  tissue,  forming  the  arterial  ligament  or  ligament  of  Botallus.     (The 
significance  of  this  ligament  will  be  explained  later.)     The  pulmonary  proceeds 
directly  to  the  left  lung  in  company  with  the  left  bronchus  and  left  pulmonary 
vein.     Trace  the  right  pulmonary  in  the  same  way.     It  is  bound  to  the  right 
aorta  by  the  right  arterial  ligament. 

Add  these  to  the  preceding  drawing. 

c)  The  right  and  left  aortae:  Trace  both  of  these  arteries  away  from  the  heart. 
Each  makes  a  curve  as  it  leaves  the  heart  and  turns  posteriorly,  passing  dorsal 
to  the  precaval  vein,  the  bronchi,  and  the  pulmonary  vessels,  and  disappearing 
dorsal  to  the  lobes  of  the  liver.     Vessels  already  studied  may  be  cut  to  follow 
the  aortae  posteriorly.     Trace  the  left  aorta  first.     Grasp  the  stomach  and  left 
lobe  of  the  liver  and  press  them  to  the  right,  separating  the  cardiac  end  of  the 
stomach  from  the  lung.     The  left  aorta  will  be  found  passing  to  the  left  of  the 
esophagus  and  dorsal  to  the  stomach.     It  gives  off  simultaneously  three  large 
branches.     One  of  these  is  the  gastric  artery  which  passes  to  the  stomach  in  the 
cardiac  region  and  follows  the  curve  of  the  stomach  along  the  length  of  this  organ. 
After  a  short  distance  it  forks  into  anterior  and  posterior  gastric  arteries  which 
supply  the  lesser  and  greater  curvatures  of  the  stomach  respectively.    Another 
branch  from  the  left  aorta  is  the  coeliac  artery.     It  soon  forks  into  anterior 
and  posterior  pancreatico-duodenal  arteries.     The  anterior  pancreatico-duodenal 
artery  passes  to  the  left  end  of  the  pancreas,  gives  off  there  branches  into  the 
pvloric  end  of  the  stomach  and  to  the  liver,  then  turns  to  the  right  and  runs 


COMPARATIVE  ANATOMY  OF  THE  CIRCULATORY  SYSTEM  237 

along  the  pancreas  supplying  the  liver,  pancreas,  and  duodenum  with  many 
small  branches.  The  posterior  pancreatico-duodenal  artery  enters  the  right 
end  of  the  pancreas  and  passing  along  the  pancreas  supplies  branches  to  the 
liver,  pancreas,  duodenum,  and  gall  bladder.  The  third  branch  of  the  left 
aorta  is  the  superior  mesenteric  artery.  It  runs  posteriorly  in  the  mesentery; 
trace  it,  tearing  the  mesentery,  to  the  center  of  the  coils  of  the  mesentery.  At 
this  point  the  artery  breaks  up  in  a  fanlike  manner  into  many  radiating  branches 
which  traverse  the  mesentery  to  all  parts  of  the  small  intestine.  One  branch, 
the  inferior  mesenteric,  passes  to  the  large  intestine  and  accompanies  it  to  the 
cloaca. 

Now  follow  the  left  aorta  posterior  to  the  point  where  it  gives  rise  to  the 
superior  mesenteric  artery.  It  becomes  smaller  and  very  soon  meets  another 
vessel  coming  from  the  right.  The  two  join  in  a  V-shaped  manner  and  form  one 
vessel,  the  dorsal  aorta,  which  continues  posteriorly  in  the  median  dorsal  line. 
Follow  the  vessel,  which  meets  the  left  aorta,  anteriorly,  to  discover  its  identity. 
Separate  the  right  lobe  of  the  liver  from  the  right  lung,  and  turn  the  liver  and 
duodenum  to  the  left.  The  vessel  in  question  can  then  be  traced  anteriorly 
dorsal  to  the  right  bronchus  and  pulmonary  vessels  to  the  heart.  It  is  there- 
fore the  right  aorta.  Immediately  beyond  its  origin  from  the  heart  the  right 
aorta  gives  rise  to  the  large  brachiocephalic  artery  whose  branches  were  followed 
above.  It  has  no  other  branches. 

Draw  the  right  and  left  aortae  and  the  branches  of  the  latter. 

7.  The  dorsal  aorta  and  the  postcaval  vein. — The  digestive  tract  may  now 
be  removed,  except  the  large  intestine  which  is  to  be  left  in  place.  Follow  the 
dorsal  aorta  posteriorly.  It  runs  in  the  median  line  ventral  to  some  long  muscles 
and  in  company  with  the  postcaval  vein  which  courses  at  first  to  its  right  and 
later  comes  to  lie  ventral  to  the  aorta.  We  shall  study  the  branches  of  both 
vessels.  The  postcaval  vein  is  seen  to  be  formed  by  two  vessels  running  along 
the  medial  side  of  the  kidneys.  Each  of  these  receives  numerous  renal  and 
genital  veins  from  the  kidneys  and  reproductive  organs  respectively.  The  post- 
caval vein  is  thus  seen  to  originate  between  the  kidneys.  After  adding  these 
branches  to  your  drawing  of  the  postcaval  vein,  the  vein  may  be  removed  and 
the  dorsal  aorta  studied.  The  aorta  gives  off  a  number  of  small  branches  into 
the  muscles  on  which  it  rests  and  then  passes  between  the  two  kidneys.  Hold  the 
large  intestine  backward  and  clear  away  the  connective  tissue  from  between 
the  two  kidneys.  The  dorsal  aorta  is  seen  to  give  numerous  renal  arteries  into 
the  kidneys  and  genital  arteries  to  the  reproductive  system.  At  the  posterior 
end  of  the  kidneys  it  forks  into  the  right  and  left  common  iliac  arteries. 

Separate  one  kidney  from  the  carapace  and  press  it  and  the  reproductive 
organs  to  the  other  side.  Two  large  arteries  will  be  seen  emerging  dorsal  to  the 
kidney.  The  anterior  one  is  the  epigastric  artery;  the  posterior  one,  the  common 
iliac  mentioned  in  the  preceding  paragraph.  Follow  the  epigastric.  (If  it  was 


238       LABORATORY  MANUAL  FOR  VERTEBRATE  ANATOMY 

injured  in  the  dissection  of  the  renal  portal  system,  try  the  other  side.)  It  runs 
laterally  to  the  point  where  the  renal  portal  vein  enters  the  pleuroperitoneal 
cavity.  At  this  point  it  divides.  The  anterior  branch  continues  to  the  carapace 
and  runs  forward  along  the  curve  of  the  carapace,  supplying  the  fat  bodies  and 
becoming  continuous  with  the  margino-costal  artery  described  above.  The 
posterior  branch  turns  and  passes  medially  parallel  to  the  ventral  abdominal 
vein.  It  supplies  the  base  of  the  leg  and  the  pelvic  muscles  and  terminates  on 
the  ventral  surface  of  the  pelvis. 

Next  follow  the  common  iliac  artery  of  the  same  side.  It  divides  at  once 
before  it  has  emerged  from  above  the  kidney  into  an  internal  iliac  and  an  external 
iliac  artery.  The  external  iliac  forks  after  a  short  distance.  The  medial  and  larger 
branch  supplies  the  muscles  of  the  pelvis  and  as  the  femoral  artery  enters  the 
thigh.  The  smaller  and  lateral  branch  passes  deep  dorsally  to  the  point  where 
the  ilium  is  articulated  to  the  sacral  ribs;  here  it  passes  dorsal  to  a  nerve  and 
turns  ventrally  as  the  sciatic  artery  into  the  hind  leg,  running  along  the  medial 
surface  of  the  ilium.  The  internal  iliac  is  best  followed  by  replacing  the  kidney 
against  the  dorsal  wall,  pulling  the  large  intestine  backward  and  locating  the 
point  of  origin  of  the  internal  iliacs  from  the  common  iliac.  The  chief  branch 
of  the  internal  iliac  is  the  hemorrhoidal  artery  which  passes  forward  along  the 
side  of  the  large  intestine;  in  addition  there  are  branches  to  the  bladder,  the 
reproductive  organs,  and  the  pelvic  region  in  general. 

Draw  the  dorsal  aorta  and  its  branches. 

8.  The  structure  of  the  heart.— Separate  the  heart  of  the  turtle  by  cutting 
across  the  great  vessels  and  remove  it  from  the  body.  The  posterior  chamber 
of  the  heart  is  the  sinus  venosus  which  receives  the  four  great  systemic  veins. 
Clean  out  the  blood  from  the  sinus.  It  is  a  thin-walled  chamber  attached  to  the 
right  auricle,  into  which  it  opens  by  the  sin-auricular  opening  guarded  by  a  pair 
of  thin  valves.  Open  each  auricle  by  making  a  slit  in  the  margin  and  washing 
out  the  blood  clots.  The  walls  of  the  auricles  are  somewhat  spongy.  Look  into 
the  left  auricle  and  note  the  thin  inter  auricular  septum  which  completely  separates 
the  cavity  of  the  left  auricle  from  that  of  the  right  one.  Find  the  opening  of  the 
pulmonary  veins  into  the  dorsal  wall  of  the  left  auricle  near  the  septum.  Find 
on  each  side  the  large  auricula-ventricular  opening  between  each  auricle  and  the 
ventricle.  Make  a  cut  all  of  the  way  around  the  margin  of  the  ventricle  so  as 
to  make  dorsal  and  ventral  flaps  of  the  ventricle.  Spread  apart  the  two  flaps 
cautiously  extending  your  cut  inward  until  the  two  flaps  are  attached  only  along 
the  base  of  the  ventricle.  Note  the  exceedingly  thick  walls  of  the  ventricle  and 
the  muscular  columns  projecting  into  the  interior.  The  cavity  of  the  ventricle 
is  a  broad  but  flattened  cavity  usually  containing  a  spongy  network  which  may 
be  cleaned  out.  Spreading  the  two  flaps  widely,  note  in  the  base  of  the  ventricle 
a  band  passing  across  from  one  side  to  the  other.  On  each  side  of  this  band  is 
an  auriculo- ventricular  opening.  The  band  is  a  continuation  of  the  interauricular 


COMPARATIVE  ANATOMY  OF  THE  CIRCULATORY  SYSTEM  239 

septum  and  forms  a  fold  or  valve  on  each  side,  which  partially  occludes  the 
auriculo-ventricular  opening.  The  right  valve  continues  ventrally  into  a  ridge 
which  is  on  the  ventral  flap  of  the  specimen.  This  ridge  is  the  incomplete 
intervenlricular  septum.  On  bringing  the  two  flaps  of  the  specimen  together, 
it  will  be  seen  that  the  interventricular  septum  was  connected  with  the  muscular 
wall  of  the  dorsal  flap  and  that  a  space  is  left  dorsal  to  the  septum  by  which  the 
right  and  left  ventricles  communicate  with  each  other.  The  right  ventricle  to 
the  right  of  the  septum  is  very  small,  while  the  left  ventricle  is  much  larger  and 
communicates  with  the  cavity  of  both  auricles  owing  to  the  incomplete  character 
of  the  interventricular  septum.  Spread  the  flaps  of  the  specimen  again  and 
pass  a  probe  ventral  to  the  interventricular  septum.  The  probe  emerges  in 
the  pulmonary  artery.  Probe  into  the  other  arterial  trunks  and  find  their  open- 
ings into  the  ventricle.  The  opening  of  the  left  aorta  is  to  the  right  of  the  inter- 
ventricular septum,  into  the  small  right  ventricle,  while  that  of  the  right  aorta 
is  to  the  left  of  the  septum,  into  the  left  ventricle;  however,  owing  to  the  gap 
dorsal  to  the  septum,  the  left  aorta  can  also  obtain  blood  from  the  left  ventricle. 
By  slitting  open  the  arterial  trunks  find  the  little  pocket-like  semilunar  valves 
which  guard  their  exits  from  the  ventricle.  They  represent  the  remains  of  the 
conus  arteriosus. 

We  may  now  attempt  to  explain  the  course  of  the  circulation  through  the  turtle's  heart. 
The  matter  is  somewhat  complicated  and  further  details  will  be  found  in  P  and  H,  page  359. 
We  have  noted  that  all  of  the  venous  blood  returns  to  the  sinus  venosus  and  that  this  in  turn 
connects  with  the  right  auricle  which  passes  it  on  into  the  right  side  of  the  ventricle.  Although 
this  is  imperfectly  separated  from  the  left  side  of  the  ventricle,  the  venous  blood  is  well  retained 
in  the  right  side  owing  to  the  spongy  nature  of  the  ventricular  walls.  Meantime,  the  two 
pulmonary  veins  have  returned  the  blood  from  the  lungs  to  the  left  auricle.  Since  the  function 
of  the  lungs  is  to  aerate  the  blood,  this  blood  is  arterial.  From  the  structure  and  relations  of 
the  heart,  the  right  auricle  always  contains  venous  blood,  and  the  left  auricle  arterial  blood. 
The  left  auricle  passes  the  arterial  blood  into  the  left  side  of  the  ventricle.  There  is  some 
slight  mixture  of  venous  and  arterial  blood  in  the  ventricle.  As  the  ventricle  contracts  both 
kinds  of  blood  are  moved  toward  the  arterial  trunks.  We  have  noted  that  the  pulmonary 
artery  springs  from  the  small  right  ventricle  and  that  the  opening  into  this  artery  is  to  the 
right  of  and  somewhat  concealed  by  the  interventricular  septum.  When  the  ventricle  con- 
tracts, the  pressure  practically  closes  the  septum  so  that  most  of  the  venous  blood  passes  out 
into  the  pulmonary  artery.  Simultaneously,  the  nearly  pure  arterial  blood  in  the  left  ven- 
tricle passes  into  the  base  of  the  right  aorta,  since  that  is  connected  with  the  left  ventricle 
and  since  the  communication  with  the  left  aorta  is  closed  temporarily  by  the  interventricular 
septum.  Toward  the  end  of  the  contraction  the  remaining  blood  in  both  ventricles  passes 
into  the  left  aorta,  as  the  diminished  pressure  again  opens  up  the  gap  in  the  septum.  It 
thus  happens  that  the  brachiocephalic  artery  passing  to  the  anterior  part  of  the  body  con- 
tains nearly  pure  arterial  blood,  while  the  left  aorta  contains  mixed  blood.  On  account  of  the 
function  of  right  and  left  aortae,  the  dorsal  aorta  also  carries  mixed  blood.  It  is  universally 
true  among  vertebrates  that  the  arrangement  of  the  circulatory  system  is  such  that  the 
purest  blood  is  received  by  the  head;  and  this  is  no  doubt  due  to  the  greater  oxygen  require- 
ments of  the  nervous  parts  of  the  head. 


240       LABORATORY  MANUAL  FOR  VERTEBRATE  ANATOMY 

We  now  see  that  whereas  in  fishes  there  is  a  single  circulation  through  the  heart,  which 
carries  only  venous  blood,  there  is  in  the  turtle  a  double  circulation,  one  half  of  the  heart 
(always  the  left)  conveying  arterial  blood  and  one  half  (always  the  right)  venous  blood.  This 
change  is  due  to  the  development  of  the  lung  method  of  breathing  with  the  consequent  shift- 
ing of  the  sinus  venosus  to  the  right  side  and  the  division  of  the  auricle  into  two  separate 
chambers.  The  blood  could  not  go  through  the  lungs  and  then  to  the  body  because  all  of  the 
force  of  the  heart  beat  would  be  lost  in  the  passage  of  the  blood  through  the  capillaries  of  the 
lungs,  and  the  circulation  would  stagnate  in  the  lungs.  The  aerated  blood  returns  to  the  heart 
to  take  advantage  of  the  driving  action  of  the  heart.  Thus,  the  double  circulation  arose. 
We  note,  however,  that  the  method  employed  by  the  turtle  (and  other  reptiles)  is  imperfect 
in  that  the  arterial  and  the  venous  blood  are  mixed  in  the  heart.  A  little  consideration  will 
show  that  this  situation  cannot  be  remedied  merely  be  completing  the  interventricular  septum 
(which  has  indeed  happened  in  the  crocodiles  and  alligators)  because  the  left  aorta  opens  into 
the  right  ventricle  and  would  still  continue  to  receive  venous  blood.  The  difficulty  is  in  reality 
due  to  the  presence  of  the  three  arterial  trunks  formed  by  the  splitting  of  the  ventral  aorta. 
We  can  see  that  if  the  ventral  aorta  would  split  into  but  two  trunks,  one  of  which  (pulmonary) 
is  connected  with  the  right  side  of  the  heart  and  the  other  (aorta)  with  the  left  side  and  if, 
further,  the  interventricular  septum  would  be  completed,  the  difficulty  would  be  overcome 
and  no  venous  blood  could  get  into  the  arterial  system.  This  is  precisely  what  has  happened 
in  birds  and  mammals  in  which  the  double  circulation  is  complete  and  perfect.  It  follows 
from  this  that  birds  and  mammals  could  not  have  evolved  from  any  living  groups  of  reptiles 
but  must  have  arisen  far  back  in  the  reptilian  line  before  the  splitting  of  the  ventral  aorta 
occurred. 

9.  Comparison  of  the  circulatory  system  of  the  turtle  with  preceding  forms. — The 
student  can  hardly  fail  to  have  noted  marked  differences  between  the  circulatory  system  of  the 
turtle  and  of  the  elasmobranchs  and  Amphibia.  These  changes  are  associated  with  the  adop- 
tion of  the  air-breathing  habit.  In  the  arterial  system  profound  changes  have  occurred  in  the 
aortic  arches  (Fig.  58,  p.  267).  We  have  learned  that  in  vertebrate  embryos  there  are  six  aortic 
arches;  in  elasmobranchs  the  first  is  missing,  the  second  incomplete,  and  four  complete  ones 
persist;  in  Necturus  the  first,  second,  and  fifth  have  vanished,  only  the  third,  fourth,  and  sixth 
remaining.  In  the  turtle  we  note  that  but  one  pair  of  aortic  arches  has  persisted;  these  unite 
dorsally  to  form  the  dorsal  aorta  (Fig.  5&E).  This  surviving  pair  of  arches  represents  the  fourth 
pair,  and  is  usually  referred  to  as  the  aortic  arch,  since  there  is  but  one.  The  third  pair  of  arches 
is  quite  disconnected  from  the  dorsal  aorta  and  is  represented  by  the  bases  of  the  carotids.  The 
fifth  pair  is  absent.  The  bases  of  the  sixth  pair  persist  as  the  bases  of  the  pulmonary  arteries 
(Fig.  sSE).  The  connection  of  the  sixth  arches  with  the  dorsal  aorta  is  present  in  vertebrate 
embryos  as  the  duct  of  Botallus  or  arterial  duct,  but  after  birth  or  hatching  this  closes  up  and 
degenerates  into  a  band  of  connective  tissue,  the  arterial  ligament  noted  in  the  dissection 
(Fig.  58).  The  branches  of  the  dorsal  aorta  are  similar  to  those  of  the  animals  already  con- 
sidered. The  splitting  of  the  ventral  aorta  into  three  trunks  has  been  emphasized  during  the 
dissection,  and  the  relation  of  this  to  the  double  circulation  through  the  heart  explained  above. 

In  the  venous  system,  likewise,  changes  have  occurred.  The  precaval  vein  is  the  anterior 
cardinal  vein  of  lower  forms;  its  base  connecting  with  the  sinus  venosus  is  the  common  cardinal 
vein.  The  renal  portal  veins  are  the  persistent  posterior  parts  of  the  posterior  cardinal  veins 
(Figs.  55,  p.  205,  and  57,  p.  224).  The  anterior  part  of  the  posterior  cardinal  vein  is  missing  (Fig. 
57!))  and  is  replaced  functionally  by  the  vertebral  vein,  which  is  formed  by  a  longitudinal  anas- 
tomosis between  the  segmental  branches  of  the  embryonic  posterior  cardinal.  It  is  important  to 
note  that  the  posterior  cardinal  (renal  portal)  has  extended  its  posterior  connections,  as  in  A  tn- 
phibia.  Whereas  in  the  elasmobranch  the  renal  portal  vein  collects  only  from  the  tail,  in  Amphibia 
and  reptiles  it  collects  from  the  hind  limb  as  well  (Fig.  57).  This  is  due  to  a  union  between 


COMPARATIVE  ANATOMY  OF  THE  CIRCULATORY  SYSTEM  241 

the  renal  portal  and  abdominal  veins  as  shown  in  Figure  57-6.  We  note  further  that  the  ventral 
abdominal  vein  (which  is  homologous  with  the  lateral  abdominal  vein  of  elasmobranchs)  has 
shifted  its  anterior  connections;  in  elasmobranchs  it  returns  to  the  common  cardinal  vein 
as  in  Figure  57^!,  but  in  Amphibia  and  reptiles  it  passes  to  the  liver  where  it  joins  the  hepatic 
portal  vein,  as  in  Figure  57^.  It  thus  happens  that  in  Amphibia  and  reptiles  the  blood  from 
the  hind  limbs  and  tail  can  pass  either  into  the  renal  portal  system  or  into  the  ventral  abdominal 
veins.  This  arrangement  appears  to  be  an  attempt  to  prevent  the  stagnation  of  the  blood 
from  the  posterior  part  of  the  body  in  the  kidneys.  It  will  be  noted  that  the  ventral  abdomi- 
nal veins  act  as  connections  between  the  renal  portal  and  hepatic  portal  systems.  The  renal 
portal  vein  passes  into  a  capillary  system  in  the  kidneys  from  which  the  blood  is  re-collected 
into  the  postcaval.  There  is,  however,  some  evidence  that  already  in  the  turtle  there  exist 
direct  channels  through  kidneys  by  which  the  renal  portal  veins  empty  directly  into  the  post- 
caval. This  is  the  beginning  of  the  retrogression  of  the  renal  portal  system  as  a  portal  system. 
The  postcaval  vein  with  the  loss  of  the  anterior  parts  of  the  posterior  cardinal  veins 
becomes  the  chief  vein  of  the  posterior  part  of  the  body.  Its  mode  of  origin  in  Amphibia  and 
reptiles  is  shown  in  Figure  57$  and  C.  Its  anterior  part  arises  from  the  hepatic  veins  which 
are  the  proximal  portions  of  the  vitelline  veins.  Its  posterior  part  between  the  kidneys  con- 
sists of  the  two  subcardinal  veins,  which  in  elasmobranchs  are  continuous  with  the  posterior 
cardinals.  The  middle  part  of  the  postcaval  between  these  two  regions  is  formed  by  an  out- 
growth from  the  hepatic  portion.  In  some  urodeles  the  posterior  cardinals  persist,  and  they, 
together  with  the  postcaval,  connect  with  the  subcardinals,  as  in  Figure  57C,  but  in  reptiles 
these  parts  of  the  posterior  cardinals  vanish,  leaving  only  the  postcaval  to  collect  from  the 
kidneys  (Fig.  57 D). 

E.   THE  CIRCULATORY  SYSTEM  OF  THE  PIGEON 

In  case  a  fresh  specimen  is  provided  for  this  work  it  should  be  opened  as 
before  by  deflecting  the  pectoral  muscles  from  either  side  of  the  keel  of  the 
sternum,  then  cutting  through  the  sternum  on  each  side  of  the  keel  and  removing 
a  median  portion  of  the  sternum  including  the  keel.  The  peritoneal  cavity  is 
to  be  opened  as  before  by  a  longitudinal  incision.  The  specimen  should  have 
been  injected  through  the  pectoral  artery. 

i.  The  chambers  of  the  heart. — The  heart  is  relatively  large  and  more  com- 
pact than  in  the  forms  previously  studied.  The  chambers  are  more  closely  knit 
together  than  in  the  lower  vertebrates.  The  major  portion  of  the  heart  is  formed 
of  the  right  and  left  ventricles,  which  together  constitute  a  muscular  thick-walled 
cone,  having  a  pointed  apex  directed  posteriorly  and  a  broad  base  directed 
anteriorly.  The  two  ventricles  are  completely  separated  from  each  other,  but 
the  division  between  them  is  indistinct  externally.  This  division  passes  obliquely 
from  the  left  side  of  the  base  to  about  the  middle  of  the  right  side  of  the  heart; 
the  left  ventricle  is  therefore  much  the  larger  of  the  two  and  includes  the  whole 
of  the  apex  of  the  heart.  Anterior  to  the  ventricles  are  the  two  much  smaller 
auricles,  thin-walled  chambers.  The  division  between  auricles  and  ventricles 
is  generally  concealed  by  a  line  of  fat  which  should  be  removed.  From  the 
anterior  end  of  the  heart  between  the  auricles  the  great  arteries  spring  without 
the  intervention  of  a  conus  arteriosus.  On  raising  the  ventricles  the  dorsal 
oortions  of  the  auricles  become  visible.  There  is  no  sinus  venosus,  the  great 


242       LABORATORY  MANUAL  FOR  VERTEBRATE  ANATOMY 

veins  opening  directly  into  the  right  auricle.  There  are  three  of  these  veins,  the 
two  precavals  and  the  postcavaL  The  postcaval  enters  the  right  auricle  from 
behind,  emerging  from  the  liver.  The  precavals  come  from  the  anterior  part 
of  the  body,  one  on  each  side,  and  curving  toward  the  heart  at  the  level  of  the 
auricles  enter  the  right  auricle.  The  pulmonary  veins  may  be  noticed  opening 
into  the  left  auricle. 

2.  The  hepatic  portal  system. — Turn  to  the  peritoneal  cavity.     Cut  across 
the  falciform  ligament  of  the  liver  near  the  gizzard,  noting  first  the  small  vein  pass- 
ing from  the  ventral  ligament  of  the  gizzard  in  the  falciform  ligament  to  the 
liver.     The  lobes  of  the  liver  may  now  be  turned  forward.     Running  along  the 
dorsal  surface  of  the  liver  and  branching  into  its  substance  is  the  large  hepatic 
portal  vein.     The  main  part  of  the  vein  enters  the  right  lobe  of  the  liver,  coursing 
between  the  two  bile  ducts.     The  remainder  of  it  lies  along  the  dorsal  surface 
of  the  left  lobe  of  the  liver,  sending  branches  into  the  liver,  and  at  the  left  receives 
the  left  and  median  gastric  veins  from  the  margin  and  left  side  of  the  gizzard  and 
from  the  proventriculus.     Follow  posteriorly  that  part  of  the  hepatic  portal 
which  lies  between  the  two  bile  ducts.     It  is  soon  seen  to  be  formed  by  the  union 
of  three  veins,  a  superior  mesenteric,  a  gastroduodenal,  and  an  inferior  mesenteric. 
The  superior  mesenteric  collects  from  the  greater  part  of  the  small  intestine.     The 
gastroduodenal  receives  the  right  gastric  vein  from  the  right  side  of  the  gizzard; 
the  pancreatico-duodenal  vein,  which  runs  along  the  duodenal  loop  collecting  from 
duodenum  and  pancreas;  and  the  mesenteric  vein  from  the  last  loop  of  the  small 
intestine.     The  inferior  mesenteric  vein  runs  along  the  large  intestine  from  which 
it  collects  many  branches.     At  its  posterior  end  it  turns  dorsally  and  joins  the 
renal  portal  system,  where  it  will  be  followed  later. 

Draw  the  branches  of  the  hepatic  portal  system. 

3.  The  systemic  veins. — As  already  stated,  these  consist  of  two  precavals 
and  one  postcaval. 

a)  The  branches  of  the  precaval  veins:  As  both  veins  have  identical  branches, 
only  one  need  be  followed.  Find  the  vein  on  each  side  lateral  to  the  auricle  and 
trace  each  into  the  right  auricle,  lifting  the  heart.  The  left  precaval  passes 
around  the  left  auricle  to  enter  the  right  auricle.  The  right  precaval  is  much 
shorter  and  enters  the  right  auricle  directly. 

Follow  one  precaval  forward.  It  lies  just  posterior  to  a  large  artery  and  is 
there  seen  to  be  formed  by  the  union  of  three  large  veins,  laterally  the  pectoral 
vein,  slightly  anterior  and  dorsal  to  this  the  subclavian  vein,  and  anteriorly  the 
jugular  vein.  Each  of  these  veins  should  be  followed.  The  pectoral  vein  at  its 
union  with  the  others  receives  the  internal  mammary  vein,  ascending  from  the 
inner  surface  of  the  ribs,  and  has  also  a  tributary  from  the  sternum  and  coracoid. 
The  main  vein  is  formed  laterally  by  the  union  of  two  veins  emerging  from  the 
pectoral  muscles.  These  may  be  followed  into  the  muscles  from  which  they  are 
seen  to  collect  many  branches.  The  subclavian  vein  passes  deep  dorsally  ventral 


COMPARATIVE  ANATOMY  OF  THE  CIRCULATORY  SYSTEM  243 

to  a  group  of  nerves  (brachial  plexus)  and  somewhat  concealed  by  arteries  which 
should  not  be  injured.  As  the  brachial  vein  it  emerges  from  the  wing  and  then 
receives  a  branch  from  the  shoulder  muscles.  The  jugular  vein  passes  anteriorly 
on  the  dorsal  side  of  the  large  arteries.  On  tracing  the  jugular  forward  it  will 
be  found  to  receive  the  following  veins,  named  in  order  from  the  heart  forward: 
on  the  medial  side  some  small  and  then  a  large  branch  from  the  crop  (at  the  point 
of  entrance  of  these  into  the  jugular  is  situated  a  small  reddish  body,  the  cervical 
lymph  gland)-,  on  the  lateral  side  a  vein  from  the  ^shoulder,  and  at  the  same 
level,  the  vertebral  vein  from  the  vertebral  column;  medially,  another  branch 
from  the  crop;  laterally  a  large  vein  from  a  plexus  of  blood  vessels  in  the  skin  of 
the  neck  r  then  small  veins  from  the  esophagus  and  trachea.  On  freeing  the 
anterior  end  of  the  esophagus  (also  trachea)  and  cutting  across  it,  the  jugular 
vein  can  be  followed  to  the  soft  palate,  where  it  joins  its  fellow  of  the  opposite 
side.  Posterior  to  this  union  each  receives  a  plexus  of  veins  from  the  skin  of 
the  face.  On  dissecting  away  the  soft  palate  from  the  anastomosis  of  the  two 
jugular  veins,  branches  from  the  skull  will  be  found  passing  into  the  anastomosis. 

Draw  the  branches  of  the  precaval  vein  as  far  as  found. 

b)  The  postcaval  vein:  Raise  the  ventricles  of  the  heart  and  note  once  more 
the  large  postcaval  vein  emerging  from  the  liver  and  entering  the  right  auricle 
between  the  two  precaval  veins.  Note  the  large  hepatic  veins  which  it  receives 
from  the  liver.  The  left  one  of  these  hepatic  veins  receives  the  small  vein  of 
the  falciform  ligament  mentioned  previously.  Follow  the  postcaval  vein  into 
the  peritoneal  cavity,  turning  all  of  the  viscera  to  the  left.  The  postcaval  will 
be  picked  up  again  at  the  posterior  margin  of  the  right  lobe  of  the  liver  in  contact 
with  the  dorsal  body  wall.  The  postcaval  is  here  seen  to  be  formed  by  the  union 
of  two  veins,  the  iliac  veins.  In  males  the  two  oval  testes  will  be  noted  at  this 
point  of  junction.  In  females  the  single  ovary  and  oviduct  will  be  noted  to  the 
left,  concealing  the  left  iliac  vein.  Each  iliac  runs  along  the  ventral  face  of  a 
three-lobed  organ,  the  kidney,  which  is  set  close  against  the  dorsal  body  wall. 
Follow  the  right  iliac  vein.  From  between  the  first  and  second  lobes  of  the 
kidney  it  receives  the  large  femoral  vein,  emerging  from  the  leg.  The  femoral 
vein  receives  a  small  branch  from  the  body  wall.  Posterior  to  the  entrance  of 
the  femoral  vein  the  iliac  vein  corresponds  to  the  renal  portal  vein  of  reptiles  and 
Amphibia  and  may  be  so  named.  From  between  the  second  and  third  lobes  of 
the  kidney  it  receives  the  sciatic  vein  which  also  comes  from  the  thigh.  At  the 
posterior  end  of  the  kidneys  the  two  renal  portal  veins  have  an  anastomosis  with 
each  other.  From  this  anastomosis  rises  the  inferior  mesenteric  vein,  already 
noted  as  a  branch  of  the  hepatic  portal  system.  It  runs  in  the  mesorectum.  The 
anastomosis  of  the  renal  portals  also  receives  in  the  median  line  a  small  caudal 
vein  from  the  tail  and  on  each  side  an  internal  iliac  vein  from  the  roof  of  the 
pelvic  region.  The  left  iliac  and  renal  portal  veins  are  the  same  as  the  right, 
except  that  in  the  female  the  left  veins  receive  genital  veins  from  the  ovary  and 


244       LABORATORY  MANUAL  FOR  VERTEBRATE  ANATOMY 

oviduct.  These  may  be  seen  by  turning  the  oviduct  to  the  right.  In  their  course 
over  the  kidneys  the  renal  portal  veins  give  off  branches  into  the  kidney  as 
in  lower  forms.  There  is  probably  some  portal  circulation  in  the  kidneys,  but 
most  of  the  blood  from  the  renal  portals  passes  directly  into  the  iliac  veins. 
The  iliac  vein  receives  renal  veins  from  the  kidney  of  which  there  is  one  chief 
renal  vein,  which  runs  along  the  medial  side  of  the  kidney  but  is  so  imbedded 
in  the  kidney  substance  as  to  be  difficult  to  identify.  Between  the  two  renal 
portal  and  iliac  veins  runs  the  dorsal  aorta. 

Turn  the  animal  dorsal  side  up  and  remove  the  skin  over  the  thigh.  By 
separating  the  muscles  pick  up  the  femoral  and  sciatic  veins  and  trace  them  into 
the  leg.  The  sciatic  vein  accompanies  the  large  sciatic  nerve  and  soon  turns 
forward  to  run  parallel  to  the  femoral  vein.  Both  /eins  are  accompanied  by 
arteries  of  the  same  name. 

Draw  the  branches  of  the  postcaval  vein  and  the  renal  portal  system. 

4.  The  pulmonary  veins. — The  pulmonary  veins  emerge  on  each  side  from 
the  lung  and  pass  toward  the  heart  immediately  posterior  to  the  precaval  veins. 
There  is  usually  one  pulmonary  vein  from  each  lung  but  there  may  be  two.     Note 
the  branches  collected  by  each  vein  from  the  lung.     The  veins  pass  to  the  dorsal 
side  of  the  bases  of  the  precavals  and  enter  the  left  auricle.     Their  entrance  into 
the  auricle  is  best  seen  later  when  the  heart  is  dissected. 

5.  The  arterial  system. — It  has  already  been  noted  that  the  great  arteries 
spring  directly  from  the  ventricle.    They  are  situated  between  the  two  auricles. 
Separate  their  bases  from  the  auricles.     It  will  be  then  be  found  that  there  are 
two  arterial  trunks.     The  larger,  medially  located  one  is  the  aorta.     The  smaller 
one,  passing  to  the  left  and  dorsal  to  the  aorta,  is  the  pulmonary  artery.     The 
ventral  aorta  in  birds  is  split  into  these  two  vessels. 

a)  The  anterior  branches  of  the  aorta:  Follow  the  aorta  away  from  the  heart. 
The  aorta  immediately  gives  rise  in  the  median  line  to  two  large  arteries,  the 
brachiocephalic  (innominate)  arteries.  The  aorta  then  turns  to  the  right  and 
disappears  dorsally.  It  will  be  followed  at  a  later  time.  Identify  the  branches 
of  the  brachiocephalic  arteries;  as  both  have  identical  branches  follow  only 
one.  Each  proceeds  laterally  and  slightly  anteriorly  and  forks  into  two  branches, 
an  anterior  common  carotid  artery  and  a  lateral  subclaman  artery.  The  sub- 
clavian  artery  soon  gives  rise  to  a  number  of  branches:  the  small  internal 
mammary  artery  passing  posteriorly  along  the  inner  surface  of  the  ribs,  the  two 
pectoral  arteries  to  the  pectoral  muscles  along  with  the  veins  of  the  same  name, 
and  the  axillary  artery  to  the  wing.  The  axillary  artery  runs  anteriorly  and 
after  giving  off  a  branch  into  the  shoulder  enters  the  wing  as  the  brachial  artery. 
The  two  common  carotid  arteries  pass  forward  and  at  the  level  of  the  cervical 
lymph  gland  each  gives  rise  to  a  vertebral  artery,  which  passes  dorsally  into 
the  vertebrarterial  canal  of  the  vertebral  column.  The  common  carotid  arteries 
then  approach  the  median  line  and  penetrate  the  muscles  on  the  ventral  sur- 


COMPARATIVE  ANATOMY  OF  THE  CIRCULATORY  SYSTEM  245 

face  of  the  vertebral  column.  On  separating  these  muscles  in  the  midventral 
line,  the  two  arteries  may  be  followed  forward.  They  pass  anteriorly  side  by 
side.  Shortly  before  they  reach  the  head,  they  diverge,  and  at  the  angle  of  the 
jaws  each  divides  into  an  external  carotid,  from  which  branches  may  be  traced 
to  the  esophagus,  palate,  and  head  generally,  and  into  a  more  deeply  situated 
internal  carotid  which  passes  through  the  skull  to  the  brain. 
Draw  the  branches  of  the  brachiocephalic  artery. 

b)  The  pulmonary  arteries:  The  pulmonary  artery  passes  to  the  left  side  of 
aorta  and  immediately  forks  into  right  and  left  pulmonary  arteries.     The  left 
artery  goes  directly  to  the  left  lung.     The  right  artery  turns  and  passing  on  the 
dorsal  side  of  the  brachiocephalic  arteries  and  posterior  to  the  turn  of  the  aorta 
enters  the  right  lung.     It  will  be  better  seen  in  the  next  paragraph. 

c)  The  aorta:  The  aorta  turns  to  the  right,  forming  what  is  called  the  arch  of 
the  aorta.     This  may  be  followed  by  cutting  across  the  right  precaval  vein 
and  the  right  brachiocephalic  artery.    The  arch  of  the  aorta  curves  to  the  dorsal 
side  of  the  right  pulmonary  artery,  which  can  now  be  traced  into  the  lung, 
and  turns  caudad.    Follow  it  by  dissecting  away  the  tissue  between  the  heart 
and  the  right  lung  and  by  breaking  through  the  oblique  septum.    Turn  the 
viscera  to  the  left.     Cut  through  the  postcaval  vein.    The  aorta,  now  called 
the  dorsal  aorta,  lies  in  the  median  dorsal  line  between  the  two  lungs.     It  gives 
off  small  branches  to  the  esophagus  and  body  wall  in  its  passage  along  the  pleural 
cavities.     At  the  entrance  to  the  peritoneal  cavity  the  large  coeliac  artery  arises 
from  the  aorta.     This  runs  posteriorly  along  the  proventriculus  to  which  it 
branches.     The  coeliac  artery  then  gives  rise  to  the  relatively  small  left  gastric 
artery,  which  passes  to  the  left  side  of  the  gizzard  branching  to  this  side  and  the 
edge  of  the  gizzard.     The  coeliac  artery  then  passes  by  the  spleen  to  which  it 
gives  small  splenic  arteries  and  just  beyond  the  spleen  gives  rise  to  the  hepatoduo- 
denal  branch.     This  sends  a  hepatic  branch  into  the  liver  and  then  as  the  anterior 
pancreatico-duodenal  artery  runs  along  the  duodenal  loop  supplying  duodenum 
and  pancreas.     The  coeliac  artery  continues  as  the  right  gastric,  which  spreads 
out  over  the  right  surface  of  the  gizzard.     The  right  gastric  sends  a  large  posterior 
pancreatico-duodenal  branch  to  the  duodenal  loop  and  pancreas  and  a  mesenteric 
branch  to  a  loop  of  the  small  intestine. 

Very  shortly  posterior  to  the  origin  of  the  coeliac  artery,  the  superior  mes- 
enteric artery  arises  from  the  dorsal  aorta  and  branches  to  the  small  intestine. 
One  of  these  branches  passes  along  the  large  intestine  and  anastomoses  with  the 
inferior  mesenteric  artery  described  below. 

The  dorsal  aorta  now  passes  between  the  two  kidneys.  It  gives  off  on  each 
side  the  renolumbar  artery  which  supplies  the  anterior  lobe  of  the  kidney  and 
then  passes  to  the  body  wall  and  some  muscles  of  the  thigh.  In  female  specimens 
genital  arteries  are  given  off  to  the  ovary  and  oviduct  from  the  renolumbar  artery 
and  the  renofemoral  artery.  The  renofemoral  artery  is  the  large  vessel  arising 


246       LABORATORY  MANUAL  FOR  VERTEBRATE  ANATOMY 

next  from  the  dorsal  aorta;  it  supplies  the  middle  and  posterior  lobes  of  the 
kidney,  then  proceeds  to  the  lateral  body  wall,  and  as  the  femoral  artery  supplies 
the  leg.  It  may  be  followed  by  turning  the  animal  dorsal  side  up  and  looking 
between  the  muscles  of  the  thigh  along  the  course  of  the  femoral  vein  previously 
identified.  The  femoral  artery  accompanies  the  large  sciatic  nerve  and  branches 
into  the  leg  muscles.  Returning  to  the  peritoneal  cavity,  trace  the  dorsal  aorta 
further.  As  it  passes  between  the  kidneys  it  gives  off  lumbar  arteries  into  the 
dorsal  body  wall.  At  the  posterior  end  of  the  kidneys  it  forks.  At  the  point 
of  forking  arise  the  inferior  mesenteric  artery,  which  runs  anteriorly  in  the 
mesorectum  and  anastomoses  with  a  branch  of  the  superior  mesenteric  artery, 
and  the  caudal  artery,  which  proceeds  straight  posteriorly  in  the  median  line  to 
the  tail.  The  two  forks  of  the  dorsal  aorta  are  named  the  internal  iliac  arteries. 
They  pass  posteriorly  along  the  roof  of  the  pelvic  region.  The  left  one  gives 
off  branches  into  the  oviduct. 

Draw  the  branches  of  the  dorsal  aorta. 

6.  The  structure  of  the  heart. — Free  the  heart  by  cutting  across  the  great 
vessels.  Note  that  all  veins  enter  the  apparently  anterior  end  of  the  heart,  hav- 
ing shifted  forward  out  of  the  transverse  septum.  As  already  stated,  there  is 
neither  sinus  venosus  nor  conus  arteriosus  in  the  bird's  heart.  Only  two  of  the 
original  four  chambers  of  the  heart  have  persisted,  namely,  auricle  and  ventricle, 
but  each  of  these  is  subdivided  into  two  completely  separate  halves,  the  right 
and  left  auricles  and  ventricles.  The  auricles  are  small  thin-walled  chambers 
anterior  to  the  ventricles.  In  the  walls  of  the  right  auricle  identify  the  openings 
of  the  systemic  veins  and  in  the  left  auricle  of  the  pulmonary  veins.  Slit  open 
the  right  auricle  and  looking  within  note  the  thin  inter  auricular  septum  separating 
it  from  the  left  auricle.  A  fold  extends  from  this  septum  to  the  entrance  of  the 
postcaval  vein  partly  concealing  the  entrance.  Note  the  deep  cleft,  the  auriculo- 
ventricular  opening,  through  which  the  right  auricle  opens  into  the  right  ventricle. 
Similarly  open  the  left  auricle  and  find  the  left  auriculo-ventricular  opening. 
Cut  across  the  apex  of  the  ventricles.  Observe  the  crescentic  form  and  relatively 
thin  walls  of  the  right  ventricle  and  circular  section  and  enormously  thickened 
wall  of  the  left  ventricle.  What  appears  to  be  the  internal  wall  of  the  ventricles 
is  the  interventricular  septum,  completely  separating  the  cavities  of  the  two  ven- 
tricles. Open  the  right  ventricle  by  a  slit  extending  from  the  previously  cut 
apex  to  the  base.  Note  the  single  valve  which  guards  the  right  auriculo-ventricular 
opening;  it  is  a  muscular  band  extending  from  the  ventricular  wall  to  the 
auriculo-ventricular  opening.  In  the  left  side  of  the  anterior  end  of  the  right 
ventricle  find  the  opening  of  the  pulmonary  artery,  or  probe  into  the  base  of 
the  pulmonary  artery  and  note  emergence  of  the  probe  into  the  right  ventricle. 
At  the  base  of  the  pulmonary  artery  are  three  pocket-like  semilunar  valves.  Cut 
into  the  left  ventricle  and  note  the  two  thin  membranous  valves  which  guard 
the  auriculo-ventricular  opening.  They  are  called  the  mitral  valve.  Each  is 


COMPARATIVE  ANATOMY  OF  THE  CIRCULATORY  SYSTEM  247 

attached  by  delicate  cords,  the  chordae  tendinae,  to  the  wall  of  the  ventricle. 
The  wall  of  the  ventricle  has  several  muscular  ridges  which  project  into  the 
cavity;  they  are  called  the  columnae  carnae.  Find,  to  the  medial  side  of  the 
mitral  valve,  the  opening  of  the  left  ventricle  into  the  aorta.  Probe  into  this 
and  satisfy  yourself  that  it  leads  into  the  aorta.  Note  the  three  semilunar  valves 
at  the  beginning  of  the  aorta. 

The  removal  of  the  heart  permits  the  tracing  of  the  esophagus  into  the  pro- 
ventriculus  and  of  the  bronchi  into  the  lungs.  The  form  and  extent  of  the  lungs 
can  also  be  observed  to  advantage  at  this  time. 

7.  The  circulation  through  the  heart  and  the  comparison  of  the  circulatory  system  of 
bird  and  reptile. — In  birds  the  heart  is  completely  divided  into  right  and  left  auricles  and 
ventricles.  The  venous  blood  enters  the  right  auricle  from  the  systemic  veins,  passes  into  the 
right  ventricle,  and  out  into  the  pulmonary  arteries  which  convey  it  to  the  lungs.  After 
aeration  in  the  lungs  the  blood  returns  by  way  of  the  pulmonary  veins  to  the  left  auricle,  from 
which  it  flows  into  the  left  ventricle  and  out  of  the  aorta.  Thus,  the  right  side  of  the  heart 
contains  only  venous  blood  and  the  left  side  only  arterial  blood.  There  is  a  perfect  double 
circulation,  both  kinds  of  blood  flowing  simultaneously  through  the  heart,  the  two  streams 
completely  separated  from  each  other.  Since  the  aorta  is  connected  only  with  left  ventricle, 
the  arterial  system  receives  pure  arterial  blood.  The  perfection  of  the  double  circulation  is 
achieved  by  the  splitting  of  the  ventral  aorta  into  two  trunks — aorta  and  pulmonary — which 
are  connected  with  the  left  and  right  sides  of  the  heart,  respectively.  The  sinus  venosus  is 
apparently  absent,  but  in  reality  is  reduced  and  incorporated  into  the  right  auricle.  The 
conus  arteriosus  is  represented  by  the  semilunar  valves  of  the  aorta  and  pulmonary  artery. 

The  aortic  arches  are  further  modified  from  the  condition  seen  in  reptiles.  As  in  the 
turtle,  the  bases  of  the  common  carotid  arteries  represent  the  third  aortic  arches.  The  union 
of  the  subclavians  with  these  is  secondary.  The  arch  of  the  aorta  is  the  right  fourth  aortic 
arch,  the  left  fourth  arch  having  vanished  during  embryonic  development.  There  is  con- 
sequently in  birds  no  complete  aortic  arch  as  in  the  preceding  forms  but  only,  so  to  speak, 
one-half  an  arch,  the  persistent  half  being  the  right  one.  The  pulmonary  arteries  represent 
the  sixth  aortic  arch,  separated  as  in  reptiles  from  the  aorta  (Fig.  $8F,  p.  267). 

The  venous  system  is  reptilian  in  character.  The  two  precaval  veins  are  similar  to  those 
of  the  turtle  and  are  homologous  with  the  anterior  cardinal  veins  of  lower  forms  (the  internal 
jugular  branch  being  the  original  anterior  cardinal).  The  bases  of  the  precavals  entering  the 
sinus  venosus  are  the  common  cardinal  veins.  The  posterior  cardinal  veins  are  as  in  the 
turtle  represented  only  by  their  posterior  portions,  which  are  named  in  the  adult  the  renal 
portal  veins.  These  veins  have,  as  in  reptiles  and  Amphibia,  absorbed  the  veins  of  the  legs 
and  tail.  In  birds  it  is  very  interesting  to  note  the  union  which  is  hi  progress  between  the 
renal  portal  system  (posterior  cardinals)  and  the  postcaval  vein.  As  we  shall  see,  this  union 
is  complete  in  mammals.  In  birds  the  renal  portal  system  is  probably  to  a  slight  degree 
functional  as  a  portal  system  through  the  kidneys,  blood  passing  from  the  renal  portal  vein 
into  the  kidneys  and  re-collecting  into  the  renal  veins  tributary  to  the  postcaval  vein.  Most 
of  the  blood,  however,  passes  directly  from  the  renal  portal  veins  into  the  postcaval. 

The  origin  of  the  postcaval  vein  in  birds  is  the  same  as  that  given  for  reptiles.  That 
part  of  it  which  is  situated  between  the  kidneys  is  formed  of  the  subcardinal  veins,  chiefly  the 
right  one.  The  part  through  and  anterior  to  the  liver  conies  from  the  vitelline  veins.  The 
middle  region  of  the  vein  is  a  new  formation.  The  postcaval  is  seen  to  be  usurping  the  renal 
portal  system  and  thus  extending  itself  posteriorly.  This  process  is  completed  in  mammals. 


248       LABORATORY  MANUAL  FOR  VERTEBRATE  ANATOMY 

The  hepatic  portal  system  is  similar  to  that  in  the  forms  already  discussed.  The  renal 
portal  system  is  identical  with  that  of  reptiles.  The  inferior  mesenteric  vein  which  connects 
the  two  portal  systems  is  probably  homologous  with  the  ventral  abdominal  veins  of  reptiles. 
It  is,  however,  of  decreased  importance  as  a  channel  between  the  two  portal  systems,  owing 
to  the  junction  of  the  renal  portal  system  with  the  postcaval  vein. 

F.      THE   CIRCULATORY   SYSTEM  OF   THE  MAMMAL 

The  specimen  should  have  been  injected  in  the  arterial  system. 

i.  The  chambers  of  the  heart. — The  heart  is  relatively  large  and  compact, 
the  chambers  closely  united  with  each  other.  The  pericardial  sac  should  be 
removed  if  this  has  not  been  done  previously.  In  case  the  thymus  gland  is 
well  developed,  it  will  be  necessary  to  dissect  this  away  from  the  anterior  part 
of  the  heart.  The  greater  portion  of  the  heart  consists  of  the  two  ventricles. 
/These  constitute  a  firm  thick-walled  cone,  having  a  posterior  pointed  apex  and  a 
broad  anterior  base.  This  cone  consists  of  two  completely  separated  ventricles, 
the  right  and  left  ventricles;  the  division  between  them  is  marked  externally 
by  an  indistinct  line  or  groove  extending  from  the  left  side  of  the  base  obliquely 
to  the  right,  and  terminating  to  the  right  of  the  apex.  The  groove  contains 
branches  of  the  coronary  artery  and  vein  which  will  be  found  ramifying  over  the 
surface  of  the  ventricles.  The  left  ventricle  is  much  the  larger  of  the  two  and 
includes  the  apex.  Anterior  to  the  base  of  each  ventricle  is  a  much  smaller, 
thin-walled,  generally  dark-colored  chamber,  the  auricle  or  atrium.  Each 
auricle  in  the  contracted  state  presents  a  lobe,  the  auricular  appendage,  project- 
ing medially  and  slightly  posteriorly  over  the  ventricle;  in  the  cat  (and  man) 
this  lobe  has  a  scalloped  margin  and  is  shaped  something  like  the  human  ear 
(hence  the  name  auricle,  meaning  little  ear).1  Extending  anteriorly  from  the 
middle  of  the  base  of  the  ventricles  forward  between  the  two  auricles  is  a  large 
artery,  the  pulmonary  artery.  This  makes  an  arch  to  the  left  and  disappears. 
Dorsal  to  the  pulmonary  is  another  arterial  trunk,  the  aorta.  These  two  trunks 
are  generally  imbedded  in  fat  which  should  be  removed.  They  represent  the 
split  ventral  aorta.  A  conus  arteriosus  is  lacking,  the  arteries  springing  directly 
from  the  ventricles.  Grasp  the  apex  of  the  heart  and  turn  the  heart  forward 
and  note  the  bases  of  the  great  veins  (pulmonary  and  systemic  veins)  entering 
the  auricles.  There  is  no  sinus  venosus  present  as  a  distinct  chamber;  it  is 
greatly  reduced  and  absorbed  into  the  right  auricle  where  it  may  be  located  by 
physiological  experiments  as  a  small  spot  at  the  point  where  the  systemic  veins 
enter  the  right  auricle.  From  the  morphological  point  of  view,  we  may  say  that 
the  mammalian  heart  consists  of  but  two  different  chambers  (each  of  which  is, 
however,  double)  in  contrast  with  the  fish  and  amphibian  heart,  in  which  there 

1  In  human  anatomy  only  the  ear-shaped  lobe  is  named  the  auricle,  the  whole  chamber  being 
termed  atrium.  In  comparative  anatomy,  however,  the  terms  atrium  and  auricle  are  regarded  as 
synonymous. 


COMPARATIVE  ANATOMY  OF  THE  CIRCULATORY  SYSTEM  249 

are  four  chambers  and  the  reptilian  heart  where  the  number  of  different  chambers 
is  reduced  to  three. 

2.  The  hepatic  portal  system. — Turn  to  the  peritoneal  cavity.  Press  the 
lobes  of  the  liver  forward  and  the  other  viscera  to  the  left.  Put  the  hepatoduo- 
denal  ligament  on  a  stretch  by  widely  separating  the  stomach  and  liver,  without, 
however,  tearing  the  ligament.  In  the  ligament  lying  dorsal  to  the  common  bile 
duct  is  the  large  hepatic  portal  vein  (commonly  called  simply  the  portal  vein  in 
mammals,  since  mammals  have  but  one  portal  system).  Free  it  by  carefully 
cleaning  connective  tissue  from  its  surface.  Follow  it  anteriorly  and  note  how 
it  branches  into  the  liver  substance.  Follow  it  posteriorly,  ripping  away  fat 
and  connective  tissue  from  its  surface  with  the  dull  point  of  a  probe.  Note  the 
large  branch  it  sends  into  the  right  lateral  lobe  of  the  liver.  The  branches 
received  by  the  portal  vein  from  the  digestive  tract  are  slightly  different  in  the 
rabbit  and  cat.  In  preserved  specimens  the  branches  are  not  always  easy  to 
follow,  and  the  student  should  identify  as  many  as  possible.  The  arteries 
accompanying  the  veins  must  not  be  injured. 

Rabbit:  Immediately  posterior  to  the  branch  into  the  right  lateral  lobe  ot 
the  liver,  the  portal  vein  receives  on  the  right  side  the  gastroduodenal  vein.  This 
vein  is  soon  seen  to  be  formed  by  the  union  of  two  veins,  a  larger  anterior 
pancreatico-duodenal  vein,  which  appears  as  a  continuation  of  the  main  vein  and 
the  smaller  right  gastro-epiploic  vein.  The  first-named  vessel  runs  in  the  tissue 
of  the  pancreas  alongside  the  first  part  of  the  duodenal  loop,  collecting  tribu- 
taries from  both  pancreas  and  duodenum.  The  right  gastro-epiploic  vein  comes 
from  the  pyloric  region  of  the  stomach  and  receives  also  branches  from  the  great 
omentum.  Shortly  posterior  to  the  entrance  of  the  gastroduodenal  vein  into 
the  portal,  the  portal  receives  on  the  left  side  the  larger  gastrosplenic  vein.  This 
vein  is  seen  to  be  formed  a  short  distance  from  the  portal  by  the  union  of  the 
splenic  and  coronary  veins.  The  latter  comes  from  the  lesser  curvature  of  the 
stomach  where  it  is  seen  to  be  formed  by  numerous  branches  collecting  from  both 
surfaces  of  the  stomach.  The  splenic  vein  is  a  large  vessel  running  in  the  great 
omentum  past  the  spleen  and  extending  as  far  as  the  left  end  of  the  stomach. 
In  its  course  it  collects  numerous  splenic  branches  from  the  spleen  and  the  left 
gastro-epiploic  veins  from  the  stomach  and  omentum.  Some  distance  posterior 
to  the  entrance  of  the  gastrosplenic  vein,  the  portal  receives  the  posterior 
pancreatico-duodenal  vein,  which  runs  in  the  mesentery  of  the  duodenal  loop, 
collecting  from  pancreas,  and  duodenum,  and  anastomosing  with  the  anterior 
pancreatico-duodenal  vein.  At  the  same  level  as  the  entrance  of  this  vein,  the 
portal  receives  on  the  opposite  side  the  inferior  mesenteric  vein.  This  may  be 
traced  alongside  the  descending  colon  and  rectum,  from  which  it  receives  many 
branches  as  well  as  some  from  part  of  the  transverse  colon.  The  main  trunk 
of  the  hepatic  portal  posterior  to  this  point  is  now  named  the  superior  mesenteric 
vein.  It  collects  from  all  parts  of  the  intestine  not  already  mentioned.  In 


250       LABORATORY  MANUAL  FOR  VERTEBRATE  ANATOMY 

tracing  its  branches,  tear  the  mesenteries  which  bind  together  the  coils  of  the 
intestine  as  far  as  necessary  and  also  strip  off  fat  and  lymph  glands.  The 
intestinal  vein  is  the  large  vessel  collecting  from  the  greater  part  of  the  small 
intestine.  It  runs  in  the  middle  of  the  mesentery,  receiving  many  tributaries 
in  its  course.  The  branches  from  the  jejunum  immediately  beyond  the 
duodenum,  however,  enter  the  posterior  pancreatico-duodenal  vein.  The  very 
large  ileocaecocolic  vein  collects  from  the  ileum,  appendix,  caecum,  and  ascend- 
ing and  transverse  colons.  Chief  among  its  tributaries  are:  the  appendicular 
vein  from  the  appendix;  the  anterior  ileocaecal  vein  from  the  sacculus  rotundus, 
proximal  part  of  the  caecum,  adjacent  ileum,  and  ascending  colon;  and  the 
posterior  ileocaecal  vein  from  the  distal  part  of  the  caecum,  adjacent  ileum,  and 
ascending  colon. 

Draw,  showing  the  portal  system. 

Cat:  The  first  tributary  of  the  portal,  on  following  the  portal  away  from 
the  liver,  is  the  coronary  vein  from  the  stomach.  This  lies  in  the  curve  between 
pylorus  and  stomach  and  at  the  lesser  curvature  is  formed  by  the  union  of  many 
branches  from  both  sides  of  the  stomach.  At  about  the  same  level  as  the  entrance 
of  the  coronary  vein,  the  anterior  pancreatico-duodenal  vein  enters  the  portal;  it 
collects  from  the  pancreas  and  duodenum.  At  the  same  level  as  the  two  preced- 
ing, the  right  gastro-epiploic  vein  enters,  coming  from  the  pyloric  region  and  greater 
curvature  of  the  stomach  and  adjacent  greater  omentum.  The  three  veins  just 
described  may  enter  the  portal  separately  or  may  unite  with  each  other  in  any 
combination  before  entering  the  portal.  Beyond  the  entrance  of  these  small  veins 
the  hepatic  portal  receives  a  large  tributary,  the  gastrosplenk  vein.  This  passes 
to  the  left  in  the  substance  of  the  pancreas  receiving  one  or  more  small  middle 
gastro-epiploic  veins  from  the  stomach  wall  and  omentum  and  a  pancreatic  vein 
from  the  pancreas.  Beyond  these  tributaries  the  gastrosplenic  is  formed  by  the 
union  of  two  main  branches,  the  right  and  left  splenic  veins.  The  left  splenic 
vein  passes  in  the  gastrosplenic  ligament  along  the  spleen,  receiving  branches  from 
the  spleen,  the  greater  omentum,  and  several  left  gastro-epiploic  veins  from  the 
omentum  and  stomach.  The  right  splenic  vein  comes  from  the  right  end  of 
the  spleen,  receiving  also  tributaries  from  the  omentum  and  stomach  wall. 
Beyond  the  entrance  of  the  gastrosplenic  vein  the  portal  is  known  as  the  superior 
mesenteric  vein.  This  soon  receives  a  small  posterior  pancreatico-duodenal  vein 
from  the  pancreas  and  distal  part  of  the  duodenum;  next,  the  inferior  mesenteric 
vein  from  the  descending  colon  and  rectum;  and  then  is  seen  to  be  formed  by 
numerous  converging  intestinal  branches  from  the  small  intestine,  caecum,  and 
ascending  colon.  The  lymph  glands  lying  along  the  superior  mesenteric  vein 
as  well  as  fat,  should  be  removed  in  tracing  the  branches. 

Draw  the  hepatic  portal  system. 

It  will  be  seen  that  the  relations  of  the  hepatic  portal  system  are  the  same  in  mammals 
as  in  all  other  vertebrates.  The  system  conveys  all  of  the  venous  blood  from  the  digestive 


COMPARATIVE  ANATOMY  OF  THE  CIRCULATORY  SYSTEM  251 

tract  into  the  capillaries  of  the  liver.  The  purpose  of  this  arrangement  is  that  the  liver  cells 
may  remove  from  the  blood  the  digested  food  materials.  There  is  no  renal  portal  system  in 
mammals,  it  having  been  completely  usurped  by  the  postcaval  vein. 

3.  The  systemic  veins. — There  are  three  systemic  veins  in  the  rabbit — two 
precavals  and  one  postcaval — and  two  in  the  cat — one  precaval  and  one  post- 
caval. The  condition  in  the  cat  is  due  simply  to  the  union  of  the  two  precavals 
anterior  to  the  heart  (Fig.  59,  p.  268.).  Although  the  branches  are  similar  in 
the  two  animals,  they  will  be  described  separately. 

a)  The  branches  of  the  precaval  vein:  This  vein  is  also  called  the  anterior 
vena  cava  and  descending  vena  cava. 

Rabbit:  Turn  the  apex  of  the  heart  forward  and  examine  the  great  veins 
which  enter  the  right  auricle.  The  left  precaval  vein  comes  from  the  left,  and 
passing  around  the  left  auricle  enters  the  left  side  of  the  right  auricle.  It  receives 
small  coronary  veins  from  the  heart  wall.  The  right  precaval  passes  directly  into 
the  right  anterior  part  of  the  right  auricle.  Note  additional  coronary1  veins 
entering  the  right  auricle  directly. 

Carefully  trace  the  right  precaval  forward,  clearing  away  connective  tissue 
and  muscle  from  about  its  course,  and  follow  it  away  from  the  heart.  At  the  point 
of  entrance  into  the  right  auricle  it  receives  from  behind  the  azygos  vein.  Press 
the  lungs  to  the  left  and  follow  the  azygos  posteriorly  along  the  dorsal  thoracic 
wall  near  the  median  line.  Note  the  intercostal  veins  which  enter  it  at  segmental 
intervals;  they  course  along  the  posterior  margin  of  each  rib.  Entering  the 
precaval  immediately  anterior  to  the  entrance  of  the  azygos  is  the  supreme 
intercostal  vein,  the  first  of  the  series  of  intercostal  veins.  Shortly  anterior  to 
this  the  internal  mammary  vein  enters  the  precaval.  This  vein  ascends  on  the 
internal  surface  of  the  chest  very  near  the  midventral  line.  Trace  it  posteriorly, 
noting  branches  from  the  intercostal  muscles.  It  continues  posteriorly  on  the 
abdominal  wall  as  the  superior  epigastric  vein.  The  next  tributary  of  the  pre- 
caval is  the  vertebral  vein.  It  enters  the  medial  side  of  the  precaval  at  about  the 
same  level  as  the  internal  mammary  joins  the  lateral  side.  It  may  be  traced 
deep  dorsally  to  the  cervical  vertebrae  from  which  it  emerges  receiving  a  costocer- 
vical  tributary  from  the  neck.  Beyond  this  point  the  precaval  receives  the  large 
subclavian  vein  from  the  fore  limb.  Follow  this  laterally.  It  passes  between 
the  first  and  second  ribs  into  the  axilla  and  is  then  known  as  the  axillary  vein. 
Expose  the  axilla  by  cutting  down  through  the  pectoral  muscles  near  the  mid- 
ventral  line  and  at  their  insertion  on  the  humerus.  The  pectoral  muscles  should 
then  be  separated  from  the  underlying  serratus  ventralis  but  should  not  be 
removed.  The  large,  stout  white  cords  seen  crossing  the  axilla  are  the  nerves 
of  the  brachial  plexus  and  are  not  to  be  injured.  Lymph  glands — small  rounded 
masses — will  also  be  noted  in  the  axilla. 

1  The  term  coronary  (meaning  literally  a  crown  or  wreath)  is  in  mammals  unfortunately  applied  to 
two  vessels,  those  of  the  heart  wall  and  a  vessel  of  the  stomach  wall  having  radiating  branches.  To 
avoid  confusion  the  latter  is  referred  to  as  the  coronary  vessel  of  the  stomach  (coronaria  ventriculi). 


252       LABORATORY  MANUAL  FOR  VERTEBRATE  ANATOMY 

In  the  axilla  the  axillary  vein  receives  the  following  branches:  the  long 
thoracic  vein,  the  subscapular  vein,  and  the  cephalic  vein.  The  long  thoracic 
vein  runs  caudad  on  the  thoracic  wall  in  the  serratus  muscle;  it  then  passes  to 
the  inner  surface  of  the  skin  and  extends  the  entire  length  of  the  abdominal  wall, 
being  especially  prominent  in  females,  where  as  the  external  mammary  it  collects 
from  the  mammary  glands.  (The  greater  part  of  this  vein  was  probably  removed 
with  the  skin.)  The  subscapular  vein  enters  the  axillary  dorsal  to  the  preceding. 
It  collects  a  conspicuous  branch  (thoracodorsal  vein)  from  the  latissimus  dorsi  and 
cutaneous  maximus  muscles;  it  then  passes  through  the  teres  major  muscle  to 
the  external  surface  of  the  shoulder  where  it  collects  from  various  muscles.  The 
cephalic1  vein  is  the  chief  superficial  vein  of  the  arm.  It  can  best  be  picked  up 
on  the  outer  surface  of  the  upper  arm;  near  the  distal  end  of  the  upper  arm  it 
penetrates  deep  between  muscles  and  passing  between  the  teres  major  and  sub- 
scapularis  muscles  emerges  on  the  internal  surface  of  the  shoulder  and  enters  the 
axillary  vein  at  the  same  place  as  or  in  common  with  the  subscapular  vein. 
Immediately  beyond  the  entrance  of  these  tributaries,  the  axillary  vein  becomes 
the  brachial  vein  of  the  arm.  This  proceeds  along  the  inner  surface  of  the  upper 
arm  in  company  with  an  artery  and  a  nerve. 

Return  to  the  precaval  vein.  At  the  point  of  entrance  of  the  subclavian 
vein  the  precaval  vein  receives  from  the  neck  the  external  and  internal  jugular 
veins.  The  external  jugular  vein  is  the  large  vein  which  extends  forward  in 
the  depressor  conchae  posterior  muscle  (most  superficial  muscle  of  the  ventral 
surface  of  the  neck).  It  appears  as  the  anterior  portion  of  the  precaval.  The 
internal  jugular  vein  is  a  very  small  vein  which  runs  alongside  the  trachea,  pass- 
ing the  thyroid  gland,  and  accompanying  the  carotid  artery  and  the  vagus  nerve. 
The  place  of  entrance  of  the  internal  jugular  as  well  as  its  general  relations  are 
highly  variable;  it  may  enter  the  precaval  after  the  latter  has  received  the  sub- 
clavian, but  it  usually  enters  with  the  external  jugular.  The  precaval  vein  may 
thus  be  said  to  be  formed  by  the  union  of  the  subclavian,  external  jugular,  and 
internal  jugular  veins.  Follow  the  external  jugular.  Shortly  anterior  to  its 
union  with  the  subclavian  it  receives  the  transverse  scapular  vein  from  the  ventral 
end  of  the  shoulder  and  near  the  same  level  has  a  cross-connection  (transverse 
jugular  vein)  with  its  fellow  of  the  opposite  side  (this  union  was  probably 
destroyed  in  the  previous  dissection).  Along  the  neck  it  receives  various  small 
tributaries  from  muscles  and  about  one  inch  posterior  to  the  angle  of  the  jaws  is 
seen  to  be  formed  by  the  union  of  two  veins,  the  anterior  and  posterior  facial  veins. 
The  anterior  facial  vein  proceeds  to  the  angle  of  the  jaws  where  it  is  seen  to  be 
formed  by  the  union  of  veins  from  the  anterior  part  of  the  face  and  jaws.  Its  main 
tributaries  are  the  angular  vein,  which  passes  over  the  ventral  part  of  the  masseter 
muscle  and  then  turns  to  the  region  in  front  of  the  eye,  and  the  deep  facial  vein 
which  emerges  between  the  masseter  and  digastric  muscles  and  passes  along  the 

1  So  named  because  the  corresponding  vein  in  man  was  formerly  thought  to  connect  with  the  head. 


COMPARATIVE  ANATOMY  OF  THE  CIRCULATORY  SYSTEM  253 

surface  of  the  masseter.  Other  tributaries  of  the  anterior  facial  vein  come  from 
the  nearby  lymph  and  salivary  glands.  The  posterior  facial  vein  may  next  be 
followed.  It  passes  to  the  parotid  gland  where  it  receives  a  superficial  vein,  the 
posterior  auricular  vein,  from  the  back  of  the  ear  and  head.  The  main  vein 
beyond  the  entrance  of  this  branch  lies  imbedded  in  the  parotid  gland  which 
may  be  dissected  from  it.  The  vein  is  accompanied  by  the  facial  nerve.  At  the 
base  of  the  ear  it  is  formed  by  the  inferior  ophthalmic  vein  from  the  orbit,  the 
temporal  veins  from  the  temporal  region,  and  the  anterior  auricular  vein  from 
the  region  in  front  of  the  ear. 

The  internal  jugular  vein  extends  the  length  of  the  neck,  receiving  but  few 
small  branches,  of  which  the  chief  ones  are  those  from  the  thyroid  gland.  It 
may  be  traced  to  the  occipital  region  of  the  skull,  from  which  it  emerges  by  way 
of  the  jugular  foramen;  it  collects  part  of  the  blood  from  the  brain.  As  already 
stated,  its  size  and  place  of  junction  with  the  external  jugular  are  highly  variable. 

The  left  precaval  vein  is  identical  in  its  tributaries  with  the  right,  except 
that  there  is  no  azygos  vein  on  the  left  side. 

Draw  the  branches  of  the  precaval  as  far  as  found. 

Cat:  Turn  the  apex  of  the  heart  to  the  left  and  note  the  large  vein  which 
enters  the  anterior  margin  of  the  right  auricle.  This  is  the  precaval  vein.  Note 
that  there  is  no  such  vein  on  the  left  side.  Instead  there  is  a  vein  called  the 
coronary  sinus,  which  runs  along  the  dorsal  surface  of  the  heart  in  the  groove 
between  the  auricles  and  ventricles.  The  coronary  sinus  will  be  found  by  clean- 
ing out  the  fat  from  this  groove.  Note  the  numerous  coronary  veins  which  come 
from  the  heart  wall  and  enter  the  sinus.  The  sinus  itself  opens  into  the  left 
posterior  corner  of  the  right  auricle.  It  represents  the  reduced  proximal  part 
of  a  former  left  precaval  vein.  The  distal  part  of  this  vein  is  still  present  and, 
as  we  shall  see  shortly,  is  united  with  the  right  precaval.  Again  pressing  the 
heart  to  the  left,  clean  the  base  of  the  precaval  and  note  the  large  vein  which 
passes  in  front  of  the  root  of  the  right  lung  and  joins  the  precaval  as  the  latter 
enters  the  auricle.  This  tributary  of  the  precaval  is  the  azygos  vein.  Trace  it 
posteriorly,  pressing  the  right  lung  to  the  left.  It  passes  along  the  dorsal  thoracic 
wall  near  the  mid-dorsal  line  and  receives  at  regular  intervals  the  intercostal 
veins.  These  course  along  the  posterior  borders  of  the  ribs.  The  most  anterior 
of  the  intercostal  veins  join  into  a  common  trunk  which  enters  the  azygos  shortly 
caudad  to  the  entrance  of  the  latter  into  the  precaval.  The  azygos  also  receives 
small  branches  from  the  esophagus  and  bronchi. 

Trace  the  precaval  anteriorly.  It  receives  small  branches  from  the  thymus 
gland  and  then  receives  a  tributary  of  moderate  size,  the  common  stem  of  the 
internal  mammary  veins,  which  comes  from  the  midventral  wall  of  the  chest.  On 
following  this  posteriorly  it  is  soon  seen  to  be  formed  by  the  union  of  two  veins, 
the  internal  mammary  veins,  which  run  posteriorly  in  the  chest  wall  one  to  each 
side  of  the  midventral  line,  and  are  extended  onto  the  abdomen  as  the  superior 


254       LABORATORY  MANUAL  FOR  VERTEBRATE  ANATOMY 

epigastric  veins.  In  their  course  the  two  internal  mammary  veins  receive 
branches  from  the  diaphragm,  chest  wall,  pericardium,  etc.  The  precaval  vein 
next  receives  small  branches  from  the  thymus  glands  and  adjacent  muscles,  and 
at  a  level  between  the  first  and  second  ribs  is  seen  to  be  formed  by  the  union  of 
two  large  veins.  These  are  the  brachiocephalic  or  innominate  veins.  They  are 
the  two  precaval  veins  of  embryonic  stages  which  later  unite  to  form  the  single 
precaval  vein  of  adult  anatomy  by  the  crossing  over  of  the  left  vein  to  join  the 
right  one  (Fig.  59,  p.  268).  The  branches  of  the  two  brachiocephalic  veins  are 
identical,  and  only  one  need  be  followed,  preferably  the  right  one,  since  the  right 
side  has  not  been  touched  in  the  previous  dissection.  The  places  of  entrance 
of  the  various  tributaries  are,  however,  somewhat  variable. 

Immediately  anterior  to  the  junction  of  the  two  brachiocephalics,  opposite 
the  first  rib,  each  of  them  receives  on  the  dorsal  side  a  large  tributary.  This  is 
located  by  dissecting  on  the  dorsal  side  of  the  vein  and  lifting  the  vein.  The  main 
part  of  the  tributary  can  be  traced  into  the  cervical  vertebrae;  it  is  the  vertebral 
vein  and  courses  in  the  vertebrarterial  canal,  collecting  from  the  brain  and  spinal 
cord.  Before  it  enters  the  brachiocephalic  the  vertebral  is  joined  by  the  costocer- 
vical  vein,  which  comes  from  the  muscles  of  the  back,  and  receives  branches  also 
from  the  chest  wall  on  the  inner  surface  of  the  first  two  ribs.  The  costocervical 
vein  may  be  picked  up  by  turning  the  animal  dorsal  side  up  and,  on  the  side 
where  the  muscles  were  dissected,  dissecting  in  the  serratus  ventralis  and  the 
epaxial  muscles.  The  communication  of  the  vertebral  and  costocervical  veins 
with  the  brachiocephalic  and  with  each  other  is  variable  and  may  not  be  as 
described  here. 

The  brachiocephalic  at  the  same  place  as  the  entrance  of  the  veins  just 
described  is  seen  to  be  formed  by  the  union  of  two  large  veins,  a  lateral  subdavian 
and  an  anterior  external  jugular.  The  subclavian  will  be  followed  first.  It 
passes  laterally  in  front  of  the  first  rib  into  the  axilla,  where  it  is  known  as  the 
axillary  vein.  Expose  the  axilla  by  cutting  through  the  pectoral  muscles  near 
the  midventral  line  and  at  their  insertion  on  the  humerus.  The  pectoral  muscles 
should  then  be  separated  from  the  underlying  serratus  ventralis  but  should  not 
be  removed.  The  stout  white  cords  crossing  the  axilla  are  the  nerves  of  the 
brachial  plexus  and  are  not  to  be  injured.  Lymph  glands  will  also  be  noted  in 
the  axilla.  The  most  medial  tributary  of  the  axillary  vein  is  the  large  sub- 
scapular  vein  which  passes  through  the  proximal  part  of  the  upper  arm  to  the 
dorsal  side  of  the  humerus  and  collects  from  various  muscles  of  the  upper  arm 
and  shoulder,  receiving  also  the  posterior  circumflex  vein  from  the  external  sur- 
face of  the  upper  arm.  The  beginnings  of  the  subscapular  vein  will  be  found  in 
the  trapezius  muscles.  The  axillary  vein  lateral  to  the  entrance  of  the  subscapu- 
lar receives  the  small  ventral  thoracic  vein  from  the  medial  portions  of  the  pector- 
al muscles.  Lateral  to  this  it  receives  the  long  thoracic  vein,  which  runs  caudad 
along  the  inner  surface  of  the  pectoral  muscles;  and  the  thoracodorsal  vein ,  which 


COMPARATIVE  ANATOMY  OF  THE  CIRCULATORY  SYSTEM  255 

courses  parallel  to  the  preceding  but  dorsal  to  it  and  collects  chiefly  from  the 
latissimus  dorsi  muscle.  There  is  a  broad  connection  between  the  thoracodorsal 
and  subscapular  veins.  Lateral  to  these  branches  the  axillary  vein  is  known  as 
the  brachial  vein.  It  runs  along  the  inner  surface  of  the  upper  arm  in  company 
with  nerves  and  the  brachial  artery.  These  structures  will  be  found  by  separat- 
ing the  muscles  on  this  surface  of  the  upper  arm. 

Return  now  to  the  external  jugular  vein.  It  soons  receives  on  its  medial 
side  the  very  small  internal  jugular  vein  which  passes  forward  in  the  neck  along- 
side the  trachea  in  company  with  the  carotid  artery  and  vagus  nerve.  The 
much  larger  external  jugular  vein  assumes  a  more  superficial  position  and  in 
addition  to  small  branches  from  adjacent  muscles  receives  the  large  transverse 
scapular  vein  from  the  shoulder.  This  passes  laterally  in  front  of  the  shoulder 
and  anastomoses  with  the  cephalic  vein  of  the  arm.  The  cephalic  vein  is  the 
superficial  vein  of  the  fore  limb  and  will  be  found  on  the  external  or  lateral 
surface  of  the  upper  arm.  It  also  connects  with  the  posterior  circumflex  vein 
described  above.  The  external  jugular  anterior  to  the  entrance  of  the  transverse 
scapular  vein  is  situated  in  the  sternomastoid  muscle.  On  following  it  forward 
it  is  seen  to  be  formed  at  the  angle  of  the  jaw  by  the  union  of  the  anterior  and 
posterior  facial  veins.  At  their  point  of  union  they  are  connected  across  the 
ventral  side  of  the  throat  by  the  transverse  vein  which  has  probably  been 
destroyed.  The  anterior  facial  vein  collects  from  the  face  and  jaws  and  sub- 
maxillary  and  lymph  glands,  its  main  tributary  being  the  angular  vein  from  the 
region  of  the  eye.  The  posterior  facial  vein  emerges  from  the  parotid  gland  and 
at  the  place  of  emergence  receives  the  posterior  auricular  vein  from  the  pinna 
and  back  of  the  head.  The  main  vein  then  lies  imbedded  in  the  parotid  gland  and 
may  be  followed  by  dissecting  away  the  gland.  It  is  then  seen  to  be  formed  by 
the  union  of  veins  from  the  temporal  region  and  region  anterior  to  the  ear. 

Draw  the  branches  of  the  precaval  as  far  as  found. 

b)  The  branches  of  the  postcaval:  The  following  description  applies  to  both 
the  rabbit  and  the  cat.  Turn  the  apex  of  the  heart  forward  and  note  the  large 
vein  which  enters  the  right  auricle  from  behind.  This  is  the  postcaval  vein 
(also  called  vena  cava  posterior  or  inferior  and  ascending  vena  cava).  It  passes 
posteriorly  in  the  thorax,  lying  slightly  to  the  right  of  the  median  line,  inclosed 
in  the  free  dorsal  border  of  the  caval  fold  of  the  pleura.  Follow  it  caudad.  It 
passes  through  the  diaphragm  from  which  it  receives  several  phrenic  veins.  In 
the  rabbit  it  then  lies  against  the  dorsal  wall  of  the  peritoneal  cavity  slightly 
to  the  right  of  the  median  line,  dorsal  to  the  right  median  lobe  of  the  liver  and 
in  contact  with  the  hepatic  portal  vein.  It  then  passes  into  the  right  lateral 
lobe  of  the  liver  from  which  it  emerges  near  the  right  kidney.  In  the  cat  the 
postcaval  vein  passes  into  the  right  median  lobe  of  the  liver  and  inclosed  in  the 
liver  substance  traverses  the  length  of  the  liver  emerging  from  the  posterior 
lobule  of  the  right  lateral  lobe.  Note  the  large  hepatic  veins  which  flow  from  the 


256       LABORATORY  MANUAL  FOR  VERTEBRATE  ANATOMY 

liver  into  the  postcaval  vein.  These  are  best  seen  by  dissecting  in  the  substance 
of  the  liver.  Follow  the  postcaval  posteriorly  carefully  cleaning  away  connec- 
tive tissue  and  fat  from  it  and  its  tributaries.  It  runs  slightly  to  the  right  of 
the  mid-dorsal  line  of  the  peritoneal  cavity  alongside  the  dorsal  aorta  which 
must  not  be  injured.  The  first  tributary  of  the  postcaval  is  the  right  adreno- 
lumbar  vein.  This  passes  along  the  posterior  surface  of  a  small  gland,  the  adrenal 
gland,  which  lies  anterior  to  the  kidney  in  contact  with  the  postcaval  vein  and 
will  be  found  by  dissecting  in  the  fat  in  this  location.  The  adrenolumbar  vein 
receives  branches  for  the  adrenal  gland  (which  is  one  of  the  glands  of  internal 
secretion)  and  also  collects  from  the  adjacent  body  wall.  Immediately  posterior 
to  this  vein  the  large  right  renal  vein  passes  from  the  kidney  into  the  postcaval. 
Next,  by  turning  the  viscera  to  the  right  locate  the  left  adrenal  gland  and  kidney 
and  find  the  left  adrenolumbar  and  renal  veins.  They  are  situated  posterior  to 
the  right  ones.  The  left  adrenolumbar  and  renal  veins  generally  unite  to  a 
common  stem  before  they  enter  the  postcaval.  Into  the  left  renal  vein  opens 
the  vein  of  the  left  gonad.  In  male  specimens  this  is  the  left  internal  spermatic 
vein;  it  may  be  traced  posteriorly  (in  contact  with  the  postcaval  in  the  rabbit) 
to  the  scrotum.  In  female  specimens  it  is  the  left  ovarian  vein  which  comes  from 
the  ovary,  a  small  oval  body  lying  about  the  middle  of  the  lateral  wall  of  the 
peritoneal  cavity.  The  right  internal  spermatic  or  ovarian  vein  enters  the  post- 
caval directly,  in  the  cat  shortly  posterior  to  the  right  kidney,  in  the  rabbit  much 
farther  caudad  The  postcaval  vein  in  its  course  along  the  body  wall  receives 
at  regular  intervals  the  paired  lumbar  veins  from  the  wall;  these  are  seen  by 
loosening  the  vein,  raising  it  slightly,  and  looking  on  its  dorsal  surface.  The 
lumbar  veins  are  then  seen  passing  ventrally  in  the  median  groove  between  muscle 
masses.  Near  the  posterior  end  of  the  peritoneal  cavity  the  postcaval  receives 
a  pair  of  iliolumbar  veins.  Each  of  these  in  company  with  an  artery  extends 
laterally  along  the  body  wall  and  receives  an  anterior  branch  from  the  neighbor- 
hood of  the  kidney.  Sometimes  the  left  ovarian  vein  enters  the  left  iliolumbar. 
Posterior  to  this  point  the  dorsal  aorta  comes  to  lie  ventral  to  the  postcaval, 
concealing  the  latter.  The  dissection  of  the  remainder  of  the  postcaval  will 
therefore  be  deferred  until  the  aorta  is  studied. 

It  is  very  common  for  the  postcaval  and  its  branches  to  vary  considerably 
from  the  foregoing  account.  An  apparent  splitting  of  the  postcaval  into  two 
main  trunks  posterior  to  the  kidneys  is  a  common  variation;  others  are  mentioned 
in  R  and  J,  page  328.1 

Draw  the  postcaval  and  its  branches  as  far  as  followed. 

4.  The  pulmonary  veins. — Examine  the  roots  of  the  lungs  and  note  numerous 
veins,  several  on  each  side,  entering  the  left  auricle  from  the  lungs.  These  are 
the  pulmonary  veins.  They  lie  to  either  side  of  the  postcaval  vein,  those  of  the 

1  For  a  more  detailed  account  of  the  occurrence  and  origin  of  these  variations  of  the  postcaval  in 
the  cat  and  man  see  Huntington  and  McClure,  Anatomical  Record,  December,  1920. 


COMPARATIVE  ANATOMY  OF  THE  CIRCULATORY  SYSTEM  257 

right  side  passing  dorsal  to  the  postcaval,  and  in  the  rabbit  those  of  the  left 
side  to  the  dorsal  side  of  the  left  precaval.  They  convey  the  aerated  blood  from 
the  lungs  into  the  left  auricle. 

5.  The  pulmonary  artery. — The  pulmonary  artery  is  the  conspicuous  vessel 
extending  from  the  base  of  the  right  ventricle  forward  between  the  auricles;  it 
soon  curves  to  the  left.     Its  base  is  generally  surrounded  by  fat,  which  should 
be  cleaned  away.     It  divides  in  two  at  the  turn  into  right  and  left  pulmonary 
arteries.     The  division  may  be  found  by  dissecting  along  the  pulmonary  artery 
immediately  in  front  of  the  left  auricle.     Press  the  heart  to  the  right  and  follow 
the  left  pulmonary  artery  into  the  left  lung.     In  the  rabbit  it  passes  to  the  dorsal 
side  of  the  left  precaval  vein  which  may  now  be  severed.     The  left  pulmonary 
artery  courses  parallel  to  and  anterior  to  the  most  anterior  of  the  pulmonary 
veins.     Now  turn  the  heart  to  the  left  and  similarly  find  the  right  pulmo- 
nary artery  proceeding  to  the  right  lung;  to  trace  it  sever  the  precaval  vein.     It 
lies  immediately  anterior  to  the  foremost  pulmonary  vein.     Dorsal  to  the  right 
pulmonary  artery  lies  the  trachea. 

6.  The  aorta  and  its  branches. — Springing  from  the  base  of  the  left  ventricle 
to  the  left  of  and  dorsal  to  the  pulmonary  artery  is  a  very  large  trunk,  the  aorta. 
Right  and  left  coronary  arteries  spring  from  the  base  of  the  aorta  where  it  leaves 
the  ventricle.    The  left  coronary  artery  lies  between  the  pulmonary  artery  and 
the  left  auricle,  and  branches  over  the  ventral  and  left  side  of  the  heart.     The 
right  coronary  artery  lies  along  the  groove  between  the  right  auricle  and  right 
ventricle  and  branches  to  the  right  and  dorsal  surfaces  of  the  heart. 

Follow  the  aorta  forward,  cleaning  away  tissue  from  its  surface.  It  soon 
describes  a  curve,  known  as  the  arch  of  the  aorta,  to  the  left.  From  the  arch  of 
the  aorta  spring  the  large  arteries  of  the  neck,  head,  and  fore  limbs.  These  are 
two  in  number  in  the  cat,  three  in  the  rabbit.  Beginning  at  the  right  they  are 
in  the  rabbit:  the  brachiocephalic  or  innominate  artery;  the  left  common  carotid; 
and  the  left  subclavian.  In  the  cat  the  branches  are  the  brachiocephalic  or 
innominate  artery  to  the  right  and  the  left  subclavian  to  the  left.  The  difference 
is  due  to  the  fact  that  in  the  cat,  not  in  man,  the  left  common  carotid  branches 
from  the  brachiocephalic,  and  this  may  also  occur  in  the  rabbit  as  a  variation. 

Trace  the  brachiocephalic  artery  forward.  The  precaval  vein  and  its 
branches  may  be  removed.  The  artery  gives  off  small  branches  into  the  thymus 
gland  and  trachea  lying  dorsal  to  it  and  then  divides  into  two  branches  in  the 
rabbit  and  three  in  the  cat.  These  are :  right  subclavian  and  right  common  carotid 
in  the  rabbit,  and  right  subclavian,  right  and  left  common  carotids  in  the  cat. 
Each  of  these  will  be  traced  separately. 

a)  Subclavian  artery:  Trace  the  right  subclavian;  both  have  identical 
branches. 

Rabbit:  From  the  posterior  surface  of  the  subclavian  arises  the  internal 
mammary  artery  which  follows  the  vein  previously  described  along  the  ventral 


258       LABORATORY  MANUAL  FOR  VERTEBRATE  ANATOMY 

chest  wall  and  continues  on  the  abdomen  as  the  superior  epigastric  artery.  At 
the  same  level  from  the  posterior  surface  of  the  subclavian,  practically  in  common 
with  the  preceding,  the  supreme  intercostal  artery  arises.  It  runs  posteriorly  on 
the  dorsal  wall  of  the  thorax  and  receives  the  first  intercostal  arteries.  On  its 
anterior  surface  at  about  the  same  level  as  these  the  subclavian  artery  gives  rise 
to  the  vertebral  artery  which  passes  immediately  dorsad  toward  the  cervical  ver- 
tebrae where  it  enters  the  vertebrarterial  canal;  and  to  the  superficial  cervical 
artery  which  ascends  in  the  lateral  part  of  the  neck,  supplying  various  muscles, 
its  main  branch  (ascending  cervical)  accompanying  the  external  jugular  vein. 
The  transverse  artery  of  the  neck  leaves  the  subclavian  at  the  same  place  or  in 
common  with  the  supreme  intercostal  artery.  It  passes  dorsally  in  front  of  the 
first  rib  through  a  loop  formed  by  two  nerves,  and  emerges  on  the  medial  side 
of  the  serratus  ventralis  muscle.  It  is  best  found  by  looking  on  this  muscle  and 
then  tracing  the  artery  toward  the  subclavian.  After  giving  off  the  foregoing 
branches  the  subclavian  passes  in  front  of  the  first  rib  into  the  axilla  where  it 
is  named  the  axillary  artery.  This  lies  between  two  of  the  stout  nerves  belong- 
ing to  the  brachial  plexus.  Its  branches  are  similar  to  those  of  the  axillary  vein 
and  accompany  the  veins.  After  giving  rise  to  the  small  thoracoacromial  artery 
to  the  pectoral  and  deltoid  muscles,  the  axillary  gives  off  the  long  thoracic  and 
subscapular  arteries,  accompanying  the  veins  previously  described.  The  former 
runs  posteriorly  along  the  serratus  muscle  and  then  as  the  external  mammary 
artery  passes  to  the  under  surface  of  the  skin  of  the  lateral  abdominal  wall,  being 
especially  conspicuous  in  females.  (Most  of  this  vessel  was  destroyed  in  remov- 
ing the  skin.)  The  subscapular  has  a  conspicuous  branch  (thoracodorsal  artery) 
passing  caudad  to  the  latissimus  dorsi  and  cutaneous  maximus  muscles;  it 
then  turns  dorsally  and  perforating  the  teres  major  emerges  on  the  outer  surface 
of  the  shoulder,  supplying  various  muscles.  Near  the  point  of  origin  of  the 
subscapular  the  deep  artery  of  the  arm  arises,  and  after  giving  off  branches  into 
the  subscapular  muscle  passes  between  this  muscle  and  the  teres  major  to  the 
dorsal  part  of  the  arm  where  it  runs  in  company  with  one  branch  of  the  cephalic 
vein  and  a  nerve,  all  three  situated  internal  to  the  lateral  head  of  the  triceps  which 
should  be  deflected.  The  axillary  artery  now  passes  to  the  upper  arm,  where  as 
the  brachial  artery  it  courses  along  the  inner  surface  of  the  limb  in  company 
with  the  brachial  vein  and  nerves. 

Draw  the  branches  of  the  subclavian. 

Cat:  At  the  level  of  the  first  rib  the  subclavian  has  four  branches:  internal 
mammary,  vertebral,  costocervical  axis,  and  thyrocervical  axis.  The  internal  mam- 
mary springs  from  the  ventral  surface  of  the  subclavian,  accompanies  the  cor- 
responding vein  along  the  chest  wall,  and  passes  on  to  the  abdominal  wall  as 
the  superior  epigastric  artery.  The  vertebral  artery  arises  from  the  dorsal  sur- 
face of  the  subclavian  and  passes  dorsally  into  the  vertebraterial  canal,  giving 
off  small  branches  into  the  neck  muscles.  The  costocervical  axis  divides  in  two 
almost  at  once.  One  branch,  the  supreme  intercostal  artery,  passes  posteriorly 


COMPARATIVE  ANATOMY  OF  THE  CIRCULATORY  SYSTEM         259 

near  the  mid-dorsal  line  of  the  thorax,  giving  off  intercostal  branches  and  then 
supplying  the  deep  muscles  of  the  back.  The  other  branch  of  the  costocervical 
axis  leaves  the  thoracic  cavity,  passing  deep  dorsally  in  front  of  the  first  rib, 
and  divides  into  the  transverse  artery  of  the  neck,  supplying  the  serratus  ventralis 
and  rhomboideus  muscles,  and  the  deep  cervical  artery  to  the  epaxial  muscles 
of  the  neck.  These  branches  are  best  found  by  looking  among  the  muscles  in 
question  and  tracing  the  vessels  toward  the  subclavian.  The  thyrocervical  axis 
generally  arises  anterior  to  the  other  branches.  It  passes  forward  near  the 
carotid  artery  and,  after  branching  to  the  muscles  of  the  dorsal  side  of  the  neck, 
turns  laterally  in  front  of  the  shoulder,  being  then  named  the  transverse  scapular 
artery;  it  accompanies  the  external  jugular  vein  for  a  short  distance  and  supplies 
many  muscles  of  the  shoulder  and  neck. 

The  subclavian  artery  now  passes  in  front  of  the  first  rib  into  the  axilla, 
where  it  is  named  the  axillary  artery.  This  gives  off :  the  ventral  thoracic  artery, 
passing  medially  to  the  medial  ends  of  the  pectoral  muscles;  the  long  thoracic 
artery,  passing  posteriorly  along  the  middle  region  of  the  pectoral  muscles  and 
then  to  the  latissimus  dorsi;  and,  near  the  arm,  the  large  subscapular  artery. 
This  gives  off  the  thoracodorsal  artery,  lying  parallel  but  more  dorsal  to  the  long 
thoracic  artery  and  supplying  the  latissimus  dorsi;  the  subscapular  then  turns 
dorsally,  passes  through  the  proximal  part  of  the  upper  arm  dorsal  to  the  humerus, 
and  branches  to  the  muscles  of  the  upper  arm  and  muscles  of  the  back  and 
shoulder. 

The  axillary  artery  then  proceeds  as  the  brachial  to  the  medial  surface 
of  the  fore  limb,  where  it  accompanies  the  brachial  vein  and  some  nerves,  and 
branches  into  the  limb. 

Draw  the  branches  of  the  subclavian. 

b)  Common  carotid  artery:  The  two  common  carotid  arteries  arise  in  the 
cat  from  the  brachiocephalic  and  immediately  diverge;  in  the  rabbit  the  right 
one  arises  in  common  with  the  right  subclavian,  while  the  left  usually  springs 
independently  from  the  arch  of  the  aorta.  Trace  the  common  carotids  forward. 
Their  branches  are  similar  in  the  two  animals.  They  pass  anteriorly  in  the 
neck,  one  to  each  side  of  the  trachea,  to  which  they  give  small  branches.  At  the 
level  of  the  anterior  end  of  the  thyroid  gland  each  supplies  a  superior  thyroid 
artery  to  the  gland.  At  the  level  of  the  larynx  there  are  branches  into  the 
larynx  and  adjacent  parts  (probably  destroyed)  and  an  occipital  branch  into  the 
dorsal  muscles  of  the  neck.  The  common  carotid  at  about  this  same  level  gives 
off  the  internal  carotid  artery.  In  the  rabbit  this  artery  arises  at  the  place  where 
the  carotid  passes  to  the  dorsal  side  of  the  shining  ligament  of  the  digastric.  In 
the  cat  it  is  much  smaller  and  arises  at  the  same  level  as  the  occipital  artery. 
In  both  animals  the  internal  carotid  passes  dorsally  in  company  with  nerves  and 
enters  the  skull  by  a  foramen  through  the  tympanic  bulla.  It  need  not  be 
followed.  The  artery  beyond  this  point  is  called  the  external  carotid  artery. 


2<5o  LABORATORY  MANUAL  FOR  VERTEBRATE  ANATOMY 

At  the  angle  of  the  jaw  it  branches  to  all  parts  of  the  head.  Its  chief  branches 
are :  the  lingual  artery  into  the  tongue  and  the  external  maxillary  running  along 
the  ventral  border  of  the  masseter  muscle  and  branching  to  the  upper  and  lower 
lips  and  jaws.  The  main  artery  then  passes  along  the  posterior  border  of  the 
masseter  muscle.  It  receives  auricular  and  temporal  branches  from  the  pinna 
and  temporal  regions  and  then,  as  the  internal  maxillary  artery,  turns  internal 
to  the  masseter  muscle  and  is  lost  to  view.  It  need  not  be  followed  farther. 
Draw  the  branches  of  the  common  carotid  artery. 

c)  The  thoracic  aorta:    After  having  given  rise  to  the  subclavians  and  the 
carotids,  the  aorta  arches  to  the  left.     Note  as  it  passes  the  left  pulmonary,  the 
strong  fibrous  band  which  connects  the  two  vessels.     This  is  the  arterial  liga- 
ment or  ligament  of  Botallus  and  is  the  remnant  of  the  embryonic  connection 
between  the  aorta  and  the  pulmonary  (Fig.  58,  p.  267).     Follow  the  aorta  pos- 
teriorly, pressing  the  left  lung  to  the  right.     It  descends  posteriorly  lying  against 
the  dorsal  wall  of  the  thorax  to  the  left  of  the  median  line.     It  is  situated  within 
the  mediastinum;  the  mediastinal  wall  may  be  cleared  away.     The  aorta  in  its 
course  along  the  thorax  is  named  the  thoracic  aorta.     Its  chief  branches  are  the 
paired  intercostal  arteries  which  arise  from  the  aorta  at  segmental  intervals  and 
run  along  the  thoracic  wall  along  the  posterior  margin  of  the  ribs.     The  aorta 
also  has  small  bronchial  arteries  to  the  bronchi  and  esophageal  arteries  to  the 
esophagus.     Along  the  dorsal  surface  of  the  aorta  on  its  left  side  runs  a  delicate 
tube,  resembling  a  streak  of  fat.     This  is  the  thoracic  duct,  the  main  lymphatic 
channel  for  the  posterior  part  of  the  body.     Trace  it  forward;   its  connection 
with  the  jugular  vein,  generally  at  the  point  of  union  with  the  subclavian,  may 
be  found. 

The  aorta  penetrates  the  diaphragm  to  which  in  the  rabbit  it  gives  superior 
phrenic  arteries  and  passes  into  the  peritoneal  cavity  where  it  is  known  as 
the  abdominal  aorta. 

d)  The  abdominal  aorta:   Turn  the  digestive  tract  to  the  right  and  locate 
the  dorsal  aorta  after  it  has  passed  the  diaphragm.     It  will  be  found  against  the 
dorsal  wall  in  the  median  dorsal  line.     Clear  away  the  mesogaster  and  clean  the 
surface  of  the  aorta.     Follow  it  identifying  its  branches.     These  branches  con- 
sist of  unpaired  median  visceral  branches  to  the  digestive  tract,  paired  lateral 
visceral  branches  to  the  kidneys  and  reproductive  organs,  and  paired  somatic 
branches  to  the  body  wall. 

Shortly  posterior  to  the  diaphragm  the  aorta  gives  rise  to  two  large  unpaired 
visceral  arteries,  the  coeliac  and  the  superior  mesenteric  arteries.  In  the  cat  the 
second  is  shortly  posterior  to  the  first,  while  in  the  rabbit  the  superior  mesenteric 
artery  lies  one-half  inch  posterior  to  the  coeliac.  As  the  branches  of  these  two 
vessels  are  different  in  the  two  animals  owing  to  the  differences  in  their  diges- 
tive tracts,  it  will  be  necessary  to  describe  them  separately. 

Rabbit:  The  coeliac  artery  near  its  origin  from  the  aorta  gives  rise  to  the 
small  inferior  phrenic  arteries  to  the  diaphragm.  Beyond  this  point  the  splenic 


COMPARATIVE  ANATOMY  OF  THE  CIRCULATORY  SYSTEM  261 

artery  arises  from  its  posterior  surface.  This  vessel  passes  in  the  mesogaster 
to  the  spleen,  where  it  runs  in  the  gastrosplenic  ligament.  In  its  course  to  the 
spleen  it  provides  the  short  gastric  arteries  to  the  left  end  of  the  stomach;  along 
the  spleen  it  supplies  splenic  branches  to  the  spleen;  beyond  the  spleen  it 
branches  into  the  omentum;  at  about  the  middle  of  the  spleen  a  large  branch, 
the  left  gastro-epiploic  artery,  arises  from  the  splenic  and  passes  to  the  greater 
curvature  of  the  stomach.  The  coeliac  artery  beyond  the  splenic  passes  to  the 
lesser  curvature  of  the  stomach  where  it  may  best  be  followed  by  turning  the 
stomach  forward.  Here  it  gives  off  a  group  of  vessels,  the  left  gastric  (or  coronary) 
arteries  which  radiate  to  the  stomach  wall  on  both  sides  of  the  lesser  curvature 
and  also  send  small  branches  to  the  esophagus.  Shortly  beyond  this  point  the 
coeliac  artery  is  known  as  the  hepatic  artery,  which  passes  along  the  right  end 
of  the  lesser  curvature,  very  shortly  giving  rise  to  the  gastroduodenal  artery.  This 
runs  to  the  pyloric  region  and  there  branches  into  the  anterior  pancreatico- 
duodenal  artery  to  the  pancreas  and  first  part  of  the  duodenum  and  the  right 
gastro-epiploic  artery  which  returns  to  the  stomach  wall  by  way  of  the  great 
omentum.  The  hepatic  artery  now  passes  to  the  dorsal  side  of  the  pylorus  and 
enters  the  hepatoduodenal  ligament.  After  giving  off  the  small  right  gastric 
artery  to  the  pylorus  it  proceeds  to  the  liver,  lying  to  the  right  of  the  bile  duct. 
The  superior  mesenteric  artery  is  the  chief  artery  of  the  intestine  and  has 
many  and  complicated  branches  in  the  rabbit,  these  branches  following  for  the 
most  part  the  branches  of  the  hepatic  portal  vein.  Clean  the  surface  of  the 
vessel  and  follow  it;  it  runs  alongside  the  superior  mesenteric  vein.  The  first 
branch  is  the  small  middle  colic  artery,  arising  from  the  ventral  wall  of  the  superior 
mesenteric  and  passing  to  the  transverse  colon  and  beginning  of  the  descending 
colon.  At  the  same  level  but  from  the  dorsal  side  arises  the  posterior  pancreatico- 
duodenal  artery  which  passes  to  the  duodenal  loop  and  pancreas.  The  superior 
mesenteric  artery  then  forks  into  the  intestinal  artery,  which  runs  in  the  mesentery 
of  the  small  intestine  and  gives  off  numerous  branches  ventrally  into  the  intestine, 
and  into  the  large  ileocaecocolic  artery.  This  last  has  many  branches  to  the 
ileum,  the  caecum,  the  appendix,  and  the  ascending  colon.  Its  branches  are: 
small  arteries  to  the  terminal  part  of  the  ascending  colon;  the  anterior  right 
colic  artery  which  forks  several  times,  supplying  the  greater  part  of  that  portion 
of  the  ascending  colon  which  bears  the  haustra;  the  posterior  right  colic  artery 
arising  near  the  preceding  and  supplying  the  remainder  of  the  haustra-bearing 
region  of  the  ascending  colon,  its  end  joining  one  end  of  the  preceding;  the  appen- 
dicular  artery  arising  with  the  preceding  and  running  along  the  appendix  and  that 
part  of  the  ileum  adjacent  to  the  appendix;  the  large  posterior  ileocaecal  artery 
passing  to  the  greater  part  of  the  caecum  and  to  that  portion  of  the  ileum  lying 
between  the  caecum  and  the  ascending  colon;  the  much  smaller  anterior  ileocaecal 
artery  to  the  more  distal  part  of  the  caecum  and  adjacent  ileum,  and  running 
toward  the  preceding  with  which  it  anastomoses ;  and  the  caecal  artery  or  arteries 
to  that  portion  of  the  caecum  which  adjoins  the  appendix. 


262       LABORATORY  MANUAL  FOR  VERTEBRATE  ANATOMY 

Cat:  The  coeliac  artery  passes  toward  the  stomach  and  very  soon  divides, 
at  about  the  same  level,  into  three  branches.  The  most  cranial  one  is  the  hepatic, 
the  next  one,  the  left  gastric,  and  most  caudal  and  largest  is  the  splenic.  Trace 
the  splenic  artery.  It  courses  in  the  great  omentum  toward  the  spleen  and  forks. 
One  branch  goes  to  the  left  end  of  the  spleen  and  sends  also  branches  into  the 
pancreas  and  the  short  gastric  arteries  to  the  stomach.  The  other  branch  passes 
to  the  right  end  of  the  spleen  and  supplies  also  branches  to  the  pancreas,  the 
omentum,  and  the  left  gastro-epiploic  arteries  to  the  greater  curvature.  The 
left  gastric  or  coronary  artery  passes  to  the  lesser  curvature  where  it  splits  into 
many  branches,  supplying  both  sides  of  the  stomach.  The  hepatic  artery  passes 
along  the  border  of  the  left  end  of  the  pancreas  and  to  the  dorsal  side  of  the  lesser 
curvature  and  enters  the  hepatoduodenal  ligament.  It  is  best  found  by  separat- 
ing the  stomach  and  liver.  It  lies  to  the  left  side  of  the  hepatic  portal  vein.  As 
it  passes  the  pylorus  it  gives  off  the  large  gastroduodenal  branch.  This  branches 
into  the  anterior  pancreatico-duodenal  artery  descending  along  the  beginning  of 
the  duodenum  and  supplying  also  the  pancreas;  the  right  gastro-epiploic  passing 
from  the  pylorus  along  the  greater  curvature  of  the  stomach  to  the  left;  and  the 
small  pyloric  artery  to  the  pyloric  region  (this  may  also  arise  independently  from 
the  hepatic).  The  hepatic  artery  proceeds  into  the  liver  sending  a  cystic  artery 
to  the  gall  bladder. 

The  superior  mesenteric  artery  supplies  the  greater  part  of  the  intestine.  It 
passes  toward  the  intestine.  Follow  it,  cleaning  away  fat  and  lymph  glands  from 
its  surface.  It  first  gives  rise  to  the  middle  colic  artery  which  passes  to  the  trans- 
verse and  descending  parts  of  the  colon.  A  little  farther  on  the  superior  mesen- 
teric gives  rise  simultaneously  to  the  posterior  pancreatico-duodenal  artery  which 
ascends  along  the  duodenum,  supplying  it  and  the  pancreas  and  anastomosing 
with  the  anterior  pancreatico-duodenal;  and  to  the  ileocolic  artery  to  the  caecum 
and  terminal  portion  of  the  ileum  and  sending  also  a  right  colic  branch  to  the 
ascending  colon  (this  last  may  arise  independently  from  the  superior  mesenteric) . 
The  superior  mesenteric  then  divides  into  numerous  intestinal  branches  to  the 
small  intestine. 

Draw  the  branches  of  the  coeliac  and  superior  mesenteric. 

Return  now  to  the  dorsal  aorta.  Its  next  branches  are  the  paired  adreno- 
lumbar  and  renal  arteries.  In  the  rabbit  the  adrenolumbars  are  branches  of 
the  renals,  but  in  the  cat  they  arise  independently.  They  pass  close  to  the 
adrenal  gland  to  which  they  give  an  adrenal  branch  and  then  course  along  the 
dorsal  body  wall.  In  the  cat  each  sends  a  phrenic  artery  anteriorly  to  the  dia- 
phragm. The  renal  arteries  are  large  vessels  passing  into  the  kidneys.  The 
aorta  posterior  to  the  kidneys  gives  rise  to  the  paired  arteries  to  the  gonads  (these 
may,  however,  branch  from  the  renals) .  They  are  the  internal  spermatic  arteries 
in  the  case  of  the  male  and  run  posteriorly  on  the  dorsal  wall  to  the  scrotum. 
In  the  female  the  corresponding  ovarian  arteries  are  larger,  and  in  the  cat  con- 


COMPARATIVE  ANATOMY  OF  THE  CIRCULATORY  SYSTEM  263 

voluted,  and  pass  laterally  to  the  ovaries.  In  its  passage  along  the  mid-dorsal 
line  the  aorta  gives  off  paired  lumbar  arteries  at  segmental  intervals.  These 
are  found  by  loosening  the  aorta  and  looking  on  its  dorsal  surface.  Posterior 
to  the  genital  arteries  the  inferior  mesenteric  arises  as  an  unpaired  visceral  branch 
and  passes  to  the  descending  colon  and  rectum,  running  in  the  mesocolon.  In 
the  mesocolon  it  forks  into  the  left  colic  artery  passing  craniad  along  the  descend- 
ing colon  and  the  superior  hemorrhoidal  artery  passing  caudad  to  the  posterior 
part  of  the  descending  colon  and  the  rectum. 

Add  these  vessels  to  the  drawing  of  the  aorta. 

The  digestive  tract  may  now  be  removed  and  discarded,  leaving  the  end  of 
the  large  intestine  in  place.  Hold  the  stump  of  the  colon  together  with  the 
urinary  bladder  and  in  female  specimens  the  uterus  (the  forked  coiled  tube  at 
the  posterior  end  of  the  peritoneal  cavity)  back  against  the  pubes  and  follow  the 
aorta  farther.  Near  the  end  of  the  peritoneal  cavity  it  forks  into  the  two  common 
iliac  arteries  in  the  rabbit;  in  the  cat  it  gives  off  a  pair  of  external  iliac  arteries 
followed  shortly  by  a  pair  of  internal  iliac  arteries.  Anterior  to  this  place  in 
the  cat,  or  in  the  rabbit  at  the  level  of  the  fork  or  from  the  common  iliac  arteries, 
a  pair  of  iliolumbar  arteries  arises  and  passes  laterally  along  the  body  wall. 
The  iliolumbar  artery  divides  into  an  anterior  branch,  which  passes  forward 
toward  the  kidney,  and  a  posterior  branch,  which  extends  to  the  thigh. 

The  two  common  iliac  arteries  in  the  rabbit  soon  fork  into  an  anterior  external 
iliac  and  a  posterior  internal  iliac.  In  the  cat  the  external  and  internal  iliacs 
arise  separately  from  the  aorta,  the  latter  immediately  posterior  to  the  former. 
After  giving  rise  to  the  iliacs  the  aorta  continues  in  the  mid-dorsal  line  as  the 
small  median  sacral  or  caudal  artery,  lying  halfway  between  the  two  internal 
iliacs.  In  the  cat  this  vessel  arises  from  the  fork  of  the  internal  iliacs.  In  the 
rabbit  it  springs  anterior  to  the  forking  of  the  aorta,  from  the  dorsal  surface  of 
the  latter;  its  origin  is  concealed  by  the  postcaval  vein  and  will  be  seen  later. 
The  sacral  artery  supplies  the  sacral  region  and  the  tail. 

Follow  the  external  iliac.  It  passes  laterocaudad  out  of  the  peritoneal 
cavity,  in  the  rabbit  to  the  dorsal  side  of  the  inguinal  ligament.  As  it  passes 
through  the  abdominal  wall  or  shortly  beyond  the  wall  it  gives  rise  to  the  deep 
femoral  artery  (cat)  or  the  inferior  epigastric  (rabbit).  In  the  cat  this  vessel 
gives  off  branches  into  the  thigh,  while  these  are  lacking  in  the  rabbit.  In  both 
animals  the  following  branches  are  present :  branches  into  the  mass  of  fat  between 
the  thighs  and  into  the  external  genital  organs,  of  which  one  branch,  in  male 
specimens,  constitutes  the  external  spermatic  artery;  and  the  main  vessel  then, 
as  the  inferior  epigastric  artery,  turns  craniad  and  ascends  in  the  abdominal 
wall,  running  along  the  inner  surface  of  the  rectus  abdominis  muscle.  It  anas- 
tomoses with  the  superior  epigastric  artery.  In  the  rabbit  there  arises,  either 
from  the  inferior  epigastric  at  the  origin  of  the  latter  from  the  external  iliac  or 
from  the  external  iliac  itself  near  by,  the  superficial  epigastric  artery  which  extends 


264       LABORATORY  MANUAL  FOR  VERTEBRATE  ANATOMY 

forward  on  the  inner  surface  of  the  skin  of  the  abdominal  wall  and  anastomoses 
with  the  external  mammary,  a  branch  of  the  long  thoracic.  These  vessels  are 
particularly  prominent  in  females,  but  the  greater  part  of  their  course  is 
destroyed  in  removing  the  skin.  The  external  iliac,  now  named  the  femoral, 
proceeds  along  the  center  of  the  medial  surface  of  the  thigh,  giving  branches  into 
the  leg  muscles.  For  these  branches  consult  R  and  J,  and  B. 

Follow  the  internal  iliacs,  being  careful  not  to  injure  the  end  of  the  post- 
caval  vein  lying  in  contact  with  them  nor  any  parts  of  the  urogenital  system 
(in  males  do  not  injure  the  male  ducts  curving  around  the  base  of  the  urinary 
bladder).  The  internal  iliacs  lie  against  the  dorsal  wall.  At  their  origin  from 
the  common  iliac  (rabbit)  or  posterior  to  their  origin  from  the  dorsal  aorta  (cat) 
each  gives  rise  to  an  umbilical  artery  which  passes  to  the  bladder  or  in  female 
rabbits  to  the  uterus  first  with  a  branch  to  the  bladder.  The  internal  iliacs  then 
pass  to  the  dorsal  side  of  the  postcaval  vein.  To  follow  them  dissect  as  deeply 
as  possible  between  the  rectum  and  the  base  of  the  thigh.  The  internal  iliacs 
give  some  branches  to  the  pelvis  and  then  each  gives  off  a  middle  haemorrhoidal 
artery  to  the  rectum.  This  accompanies  the  rectum  to  the  anus  but  cannot  be 
followed  at  this  time.  In  female  cats  the  uterine  artery  arises  from  the  middle 
haemorrhoidal  and  passes  anteriorly  again  to  the  uterus.  The  internal  iliacs 
cannot  be  followed  farther  conveniently.  They  give  branches  to  the  tail  and 
thigh. 

Draw  the  branches  of  the  iliacs  adding  them  to  the  drawing  of  the  dorsal 
aorta  already  made. 

7.  The  posterior  portion  of  the  postcaval  vein. — The  postcaval  may  now  be 
followed  caudad  from  the  point  where  it  was  previously  left  by  removing  the 
arteries  which  cover  it.  Its  tributaries  should  be  traced  as  far  as  practicable  to 
the  posterior  end  of  the  peritoneal  cavity,  dissecting  deeply  dorsally  as  before. 

Rabbit:  The  postcaval  receives,  at  the  same  level  as  the  forking  of  the  dorsal 
aorta,  the  two  large  external  iliac  veins.  It  then  continues  in  the  mid-dorsal 
line  for  a  short  distance  caudad  to  this  point,  this  portion  often  receiving  the 
name  of  common  internal  iliac  vein,  and  is  then  seen  to  be  formed  by  the  union 
of  the  two  internal  iliac  or  hypogastric  veins.  Trace  the  external  iliac;  its 
branches  are  similar  to  and  accompany  those  of  the  artery  of  the  same  name. 
It  soon  receives  the  vesical  vein  from  the  bladder :  this  vein  in  females  also  collects 
from  the  uterus.  At  the  place  where  it  passes  through  the  abdominal  wall,  the 
external  iliac  receives  the  inferior  epigastric  vein.  The  main  part  of  this  runs 
forward  along  the  internal  surface  of  the  rectus  abdominis  muscle  and  anasto- 
moses anteriorly  with  the  superior  epigastric.  The  inferior  epigastric  near  its 
place  of  entrance  into  the  external  iliac  also  receives  tributaries  from  the  fat 
between  the  bases  of  the  thighs  and  the  external  genital  region,  and  sends  a 
superficial  epigastric  vein  along  the  inner  surface  of  the  skin  of  the  lateral  abdomi- 
nal wall.  This  last-named  vessel  is  particularly  conspicuous  in  females,  but  is 


COMPARATIVE  ANATOMY  OF  THE  CIRCULATORY  SYSTEM  265 

destroyed  in  removing  the  skin;  it  anastomoses  with  the  external  mammary,  a 
tributary  of  the  long  thoracic.  The  external  iliac  passes  to  the  dorsal  side  of 
the  inguinal  ligament,  and  as  the  femoral  vein  continues  along  the  medial  side 
of  the  leg  in  company  with  the  femoral  artery.  Follow  the  internal  iliacs. 
After  a  short  distance  the  sacral  or  caudal  vein  enters  one  of  them,  usually  the 
left  one;  it  accompanies  the  caudal  artery.  Caudad  of  this,  each  internal  iliac 
receives  the  middle  haemorrhoidal  vein  which  ascends  from  the  anus  and  lies 
along  the  side  of  the  rectum.  The  tributaries  of  the  internal  iliac  cannot  be 
followed  farther  conveniently;  they  come  chiefly  from  the  gluteal  region. 

Draw  the  branches  of  the  postcaval,  adding  them  to  the  drawing  previously 
made.  The  iliacs  and  postcaval  may  then  be  removed  and  the  origin  of  the 
caudal  artery  from  the  aorta  traced. 

Cat:  The  postcaval  is  formed  dorsal  to  the  forking  of  the  aorta  by  the  union 
of  the  two  large  common  iliac  veins.  One  of  them,  usually  the  left  one,  receives 
the  small  sacral  or  caudal  vein  which  lies  parallel  to  the  artery  of  the  same  name. 
About  one  inch  posterior  to  its  junction  with  the  postcaval  each  common  iliac 
is  formed  by  the  union  of  the  internal  iliac  (hypogastric)  and  the  external  iliac. 
The  former  receives  branches  from  the  gluteal  region  and  receives  the  middle 
haemorrhoidal  vein,  which  runs  along  the  sides  of  the  rectum  from  the  anus 
forward,  and  also  collects  from  the  bladder.  The  external  iliac  passes  out  of 
the  abdominal  cavity.  At  the  place  of  exit  it  receives  on  its  medial  side  the 
deep  femoral  vein,  which  collects  from  the  thigh,  from  the  fat  between  the  thighs, 
from  the  external  genital  region  (receiving  in  males  the  external  spermatic  vein 
from  the  testes),  and  also  receives  the  inferior  epigastric  vein  from  the  inner 
surface  of  the  rectus  abdominis  muscle.  The  branches  from  the  thigh  may 
enter  the  external  iliac  separately.  The  external  iliac,  now  known  as  the  femoral 
vein,  passes  along  the  thigh,  receiving  branches  from  the  leg  muscles.  For  these 
consult  R  and  J. 

Draw  these  branches  as  far  as  found,  adding  them  to  the  drawing  of  the  post- 
caval vein  previously  made. 

8.  The  structure  of  the  heart. — Remove  the  heart  from  the  body,  cutting 
across  the  bases  of  the  great  vessels.  Identify  the  systemic  veins  entering  the 
right  auricle  (three  in  the  rabbit,  two  in  the  cat)  and  the  pulmonary  veins  enter- 
ing the  left  auricle.  Cut  into  the  wall  of  each  auricle  by  a  transverse  slit  and 
wash  out  the  clotted  blood  which  generally  fills  the  interior.  Note  the  thick 
ridged  walls  of  the  auricular  appendages  and  the  thinner  smoother  walls  of  the 
remainder  of  the  auricle.  Note  the  inter  auricular  septum  extending  dorsally 
between  the  two  auricles  and  completely  separating  them.  Find  the  large 
auricula-ventricular  openings  in  the  floor  of  the  auricles.  In  the  cat  find  near 
the  dorsal  edge  of  the  interauricular  septum  the  opening  of  the  coronary  sinus 
into  the  right  auricle,  noting  the  valve — the  valve  of  the  coronary  sinus — which 
guards  the  opening.  The  coronary  sinus  is  the  remnant  of  the  original  left 


266       LABORATORY  MANUAL  FOR  VERTEBRATE  ANATOMY 

precaval  vein.  Cut  off  the  apex  of  the  heart  and  note  the  thick  walls  and  rounded 
form  of  the  left  ventricle,  and  smaller  size,  thinner  walls,  and  crescentic  form  of 
the  right  ventricle.  Cut  open  the  right  ventricle  by  an  oblique  cut  beginning 
at  the  cut  surface  already  made  and  extending  out  through  the  pulmonary  artery, 
slitting  open  this  artery.  Wash  out  the  right  ventricle.  Its  cavity  is  rather 
small,  the  walls  being  deeply  cleft  by  muscular  ridges,  the  trabeculae  carnae. 
From  the  walls  project  a  number  of  pointed  finger-like  muscles,  the  papillary 
muscles,  which  are  connected  by  slender  fibers,  the  chordae  tendinae,  to  a  thin 
membrane.  This  membrane  consists  of  three  flaps  and  is  called  the  tricuspid 
valve.  Two  of  the  flaps  can  be  stretched  by  pulling  on  the  cut  surfaces  of  the 
ventricle  while  the  third  lies  collapsed  against  the  interventricular  septum,  to 
which  it  is  fastened  without  the  intervention  of  the  papillary  muscles.  The 
tricuspid  valve  guards  the  right  auriculo-ventricular  opening  and  prevents  the 
blood  from  flowing  back  from  the  ventricle  into  the  auricle.  In  the  base  of 
the  pulmonary  artery  note  the  three  pocket-shaped  semilunar  valves.  The  pul- 
monary artery  is  the  sole  exit  from  the  right  ventricle.  Similarly  cut  open  the  left 
ventricle  by  a  longitudinal  slit  from  apex  to  base.  Wash  out  the  interior. 
The  cavity  of  the  left  ventricle  is  considerably  larger  than  that  of  the  right,  and 
its  walls  thicker.  The  two  ventricles  are  completely  separated  by  the  inter- 
ventricular  septum,  which  appears  as  the  common  internal  wall  of  both  ventricles. 
Note  in  the  left  ventricle  the  trabeculae  carnae,  the  papillary  muscles,  and  the 
chordae  tendinae.  The  latter  are  attached  to  the  membranous  bicuspid  or 
mitral  valve,  which  consist  of  but  two  flaps.  This  guards  the  left  auriculo- 
ventricular  opening  and  prevents  the  regurgitation  of  the  blood  from  the  ven- 
tricle back  into  the  auricle.  By  probing,  find  the  sole  exit  of  the  left  ventricle, 
its  opening  into  the  aorta.  Follow  the  probe  by  a  cut  and  note  the  three  semilu- 
nar valves  at  the  base  of  the  aorta. 

Make  drawings  to  illustrate  the  structure  of  the  heart. 

The  removal  of  the  heart  permits  a  clearer  view  of  some  of  the  structures  of 
the  pleural  cavity.  The  student  should  examine  carefully  the  forking  of  the 
trachea  into  the  bronchi,  the  form  of  the  lungs  and  their  relation  to  the  pleural 
cavity,  and  the  pulmonary  arteries  and  veins. 

9.  Comparison  of  the  mammalian  heart  and  circulatory  system  with  those  of  the  preced- 
ing animals. — The  chambers  of  the  mammalian  heart,  like  those  of  birds,  are  but  of  two  kinds 
namely,  auricles  and  ventricles,  in  contrast  to  the  four  different  chambers  of  the  fish  and  amphib- 
ian heart  and  the  three  of  the  reptilian  heart.  Each  is,  however,  completely  divided  into  two 
compartments,  right  and  left,  by  the  formation  of  septa  in  the  center  of  the  originally  single 
chamber.  The  division  of  the  auricle  into  two  chambers  begins  with  the  Amphibia,  while 
that  of  the  ventricle  begins  in  reptiles  and  is  completed  in  the  crocodiles,  birds,  and  mammals. 
In  mammals,  as  in  reptiles  and  birds,  the  conus  arteriosus  has  vanished,  leaving  as  remnants 
the  semilunar  valves  at  the  bases  of  the  great  arteries.  The  sinus  venosus  still  persistent  in 
reptiles  has  disappeared  in  birds  and  mammals,  or,  more  correctly  speaking,  is  represented 
by  a  small  spot  in  the  wall  of  the  right  auricle;  this  spot  is  not  detectable  by  gross  examination. 


COMPARATIVE  ANATOMY  OF  THE  CIRCULATORY  SYSTEM 


267 


The  ventral  aorta  in  mammals  as  in  birds  is  split  into  two  trunks,  in  contrast  with  the 
three  trunks  common  to  reptiles.  These  two  are  the  pulmonary  artery  and  the  aorta.  The 
pulmonary  artery  leaves  the  right  ventricle,  the  aorta  the  left  ventricle.  All  of  the  systemic 
veins  open  into  the  right  auricle  and  the  pulmonary  veins  into  the  left  auricle.  The  right 
half  of  the  heart  is  venous,  and  the  left  arterial,  and  owing  to  the  completion  of  the  interven- 
tricular  septum  there  is  no  mixing  of  arterial  and  venous  blood  in  the  heart,  but  a  complete  and 
perfect  double  circulation  is  maintained  through  the  heart.  The  venous  blood  passes  from  the 

6 


FIG.  58. — Diagrams  to  show  the  evolution  of  the  aortic  arches.  A ,  primitive  condition  with  six 
aortic  arches.  B,  fishes,  the  first  aortic  arch  missing.  C,  some  urodeles,  the  first,  second,  and  fifth 
arches  missing.  D,  anurans,  with  the  connection  k  between  the  pulmonary  arteries  d  and  the  aorta 
obliterated.  E,  reptiles,  showing  the  ventral  aorta  split  into  three  trunks  and  the  fourth  aortic  arch 
h  and  i,  persistent  on  both  sides.  F,  birds,  the  ventral  aorta  split  into  two  trunks,  and  the  fourth 
aortic  arch  //  persistent  on  the  right  side  only.  G,  mammals,  the  ventral  aorta  split  into  two  trunks  and 
the  fourth  aortic  arch  i  persistent  on  the  left  side  only,  a,  interruption  of  the  aortic  arches  by  the  gill 
capillaries  in  fishes;  b,  internal  carotid;  c,  external  carotid;  d,  pulmonary,  developed  from  the  sixth 
arch;  e,  ventral  aorta;  /,  dorsal  aorta;  g,  aortic  arch;  h,  right  fourth  aortic  arch,  called  the  right  aorta 
above  urodeles;  i,  left  fourth  aortic  arch,  called  the  left  aorta  above  urodeles;  j,  common  carotid;  k, 
arterial  ligament  or  obliterated  vessel  originally  connecting  pulmonary  and  aorta;  I,  subclavian. 
(Slightly  modified  from  Wilder's  History  of  the  Human  Body,  courtesy  of  Henry  Holt  and  Company.) 

right  auricle  into  the  right  ventricle  and  out  through  the  pulmonary  artery  to  the  lungs,  where 
it  is  aerated.  The  arterial  blood  returns  by  way  of  the  pulmonary  veins  to  the  left  auricle, 
passes  into  the  left  ventricle,  and  out  of  the  aorta. 

Little  trace  is  left  in  the  adult  mammal  of  the  original  system  of  aortic  arches  passing 
around  the  pharynx.  As  in  all  of  the  land  vertebrates,  the  bases  of  the  common  carotid  arteries 
represent  the  remains  of  the  third  aortic  arches;  the  arch  of  the  aorta  is  the  left  fourth  aortic 


268 


LABORATORY  MANUAL  FOR  VERTEBRATE  ANATOMY 


B 


FIG.  59. — Diagrams  to  show  the  development  of  the  postcaval  vein  in  the  cat.  The  cardinal 
system  of  veins  is  crosshatched,  the  subcardinal  veins  closely  stippled,  the  hepatic  veins  indicated  by 
cross,  vertical,  and  oblique  hatching  combined,  the  supracardinal  veins  by  open  stippling,  and  the 

[Continued  on  opposite  page] 


COMPARATIVE  ANATOMY  OF  THE  CIRCULATORY  SYSTEM  269 

arch,  the  right  side  having  disappeared;  the  fifth  aortic  arches  are  absent;  the  bases  of  the 
sixth  arches  are  the  right  and  left  pulmonary  arteries  (Fig.  586).  The  embryonic  connection 
of  the  aorta  and  the  pulmonary  (Fig.  58)  persists  in  the  adult  as  the  arterial  ligament.  The 
branches  of  the  dorsal  aorta  are  similar  to  those  of  the  other  animals  studied. 

The  venous  system  is  not  markedly  different  from  that  of  reptiles.  There  are  two  precaval 
veins  as  in  the  turtle,  or  in  some  mammals,  the  left  precaval  joins  the  right  precaval  by  a  cross- 
connection  in  front  of  the  heart  forming  a  single  precaval  (Fig.  59).  The  proximal  end  of  the 
left  precaval  then  becomes  the  coronary  sinus  of  the  heart.  The  coronary  sinus  and  bases  of 
the  precavals  are  the  common  cardinal  veins  of  the  dogfish  and  of  the  embryo.  The  internal 
jugular  branch  is  the  original  anterior  cardinal  vein,  but  in  the  adult  mammal  is  often  exceeded 
in  importance  by  its  branch,  the  external  jugular.  The  proximal  portions  of  the  posterior 
cardinal  veins  are  absent  as  in  the  turtle,  except  that  the  azygos  vein,  at  least  in  part,  is  the 
remnant  of  the  right  posterior  cardinal  (Fig.  59).  Usually,  however,  other  veins  also  contribute 
to  the  formation  of  the  azygos.  The  postcaval  vein  has  the  same  relations  as  in  the  turtle,  but 
has  now  extended  itself  posteriorly.  Whereas  in  the  turtle  it  extends  no  farther  caudad  than 
the  posterior  ends  of  the  kidneys,  it  is  now  seen  to  collect  from  the  entire  posterior  part  of  the 
body.  This  change  has  been  accomplished  very  simply;  the  postcaval  vein  unites  with  that 
part  of  the  renal  portal  system  posterior  to  the  kidneys.  The  renal  portal  system  then  passes 
out  of  existence  and  there  is  no  longer  in  the  adult  any  trace  of  a  portal  circulation  through 
the  kidneys.  It  will  be  remembered  that  it  was  explained  in  connection  with  the  turtle  that 


renal  collar  by  vertical  hatching.  A,  early  stage,  showing  the  anterior  and  posterior  cardinal  veins 
a,  b,  c,  the  common  cardinal  vein  dt  the  subcardinal  veins/,  and  the  outgrowth  e  from  the  hepatic  veins 
of  the  liver.  B,  next  stage,  showing  the  union  of  the  hepatic  outgrowth  e  with  the  subcardinal  veins/, 
to  form  the  proximal  part  of  the  postcaval  vein;  the  two  subcardinals  have  united  with  each  other  at  h. 
C,  the  anterior  part  of  the  posterior  cardinal  vein  b  has  separated  from  the  posterior  part  c,  c  now  being 
the  renal  portal  vein;  the  postcaval  vein  is  seen  to  be  formed  of  the  hepatic  vein  e,  the  right  subcardinal/, 
and  to  be  united  by  means  of  the  two  subcardinals  below  h  with  the  renal  portals  c.  D,  the  supracardinal 
system  of  veins  i,  represented  by  open  stippling,  has  appeared  and  has  united  anteriorly  with  the 
anterior  parts  of  the  posterior  cardinals  6,  medially  with  the  subcardinals  by  an  anastomosis  k,  named 
the  renal  collar,  and  posteriorly  with  the  renal  portals  c.  E,  union  of  the  two  anterior  cardinals  by  a  cross- 
connection  p,  and  development  of  the  renal  veins  from  the  renal  collar  k;  the  supracardinal  veins  have 
separated  into  anterior  parts  connected  with  the  posterior  cardinals  b  and  posterior  parts  connected 
with  the  subcardinals  and  renal  portals  c.  F,  continuation  of  E.  G,  adult  stage;  the  left  anterior 
cardinal  joins  the  right  by  means  of  the  cross- vein  p,  which  is  the  left  innominate  vein;  the  common 
stem  a,  which  is  the  right  anterior  cardinal,  enters  the  heart  by  way  of  «,  which  is  the  right  common 
cardinal  vein;  the  left  common  cardinal  vein  persists  as  the  coronary  sinus  o;  the  right  anterior  parts 
of  the  posterior  cardinal  vein  and  supracardinal  form  the  azygos  vein,  b  and  i,  while  on  the  left  side 
these  are  obliterated  at  v;  the  postcaval  vein  is  now  complete  and  is  seen  to  be  composed  of  the  hepatic 
vein  e,  the  right  subcardinal,  the  anastomosis  between  the  two  subcardinals  at  h,  the  right  renal  collar  &, 
the  posterior  part  of  the  supracardinal  vein  *,  and  the  posterior  parts  of  the  renal  portals  (posterior 
cardinals)  c:  the  left  subcardinal  and  posterior  cardinal  contribute  to  the  vein  of  the  left  gonad,  hence 
the  asymmetrical  arrangement  of  the  genital  veins  of  mammals,  a,  anterior  cardinal;  6,  anterior  part 
of  the  posterior  cardinal;  c,  posterior  part  of  posterior  cardinal  or  renal  portal;  d,  common  cardinal; 
e,  hepatic  portion  of  the  postcaval  (this  is  partly  removed  in  Figs.  D-G)\  /,  subcardinal;  g,  gonad; 
A,  union  between  the  two  subcardinals;  *,  supracardinal;  j,  kidney  (metanephros) ;  k,  renal  collar  or 
union  between  subcardinals  and  supracardinals;  /,  adrenal  gland;  mt  vein  to  adrenal  gland;  n,  base 
of  the  precaval  vein  or  right  common  cardinal;  o,  coronary  sinus  or  left  common  cardinal;  p,  left 
innominate  or  connection  between  the  two  anterior  cardinals;  q,  internal  jugular;  r,  subclavian;  s, 
external  jugular;  t,  external  iliac;  u,  internal  iliac.  (After  Huntington  and  McClure  in  the  Anatomical 
Record,  Vol.  XX.) 


270       LABORATORY  MANUAL  FOR  VERTEBRATE  ANATOMY 

the  renal  portal  veins  are  the  posterior  part  of  the  posterior  cardinal  veins,  and  that  the  post- 
caval  vein  between  the  kidneys  is  formed  by  the  subcardinal  veins,  chiefly  the  right  subcardinal. 
Similarly  in  mammals  the  postcaval  vein  is  formed  of  the  distal  ends  of  the  posterior  cardinals, 
of  the  right  subcardinal,  of  the  hepatic  veins  (vitelline)  in  the  region  of  and  anterior  to  the 
liver,  and  in  the  region  between  the  kidneys  and  the  hind  limbs  of  other  subordinate  veins.  This 
will  be  clearer  by  reference  to  Figure  59.  The  manner  of  origin  of  the  postcaval  from  so  many 
different  sources  explains  the  numerous  variations  common  to  this  vein,  such  as  the  frequent 
finding  of  a  double  postcaval,  and  further  explains  the  asymmetrical  origin  of  the  veins  to  the 
reproductive  organs. 

In  conclusion  it  may  be  stated  that  the  embryology  of  the  mammalian  circulatory  system 
furnishes  a  beautiful  and  striking  example  of  the  repetition  of  evolutionary  stages.  In  its 
development  the  mammalian  circulatory  system  passes  successively  through  each  of  the  stages 
which  we  have  found  to  persist  as  the  adult  condition  in  the  types  we  have  studied,  and  the 
evolution  of  this  system  can  be  determined  equally  well  either  by  studying  its  development  in 
the  mammal  or  by  studying  and  comparing  its  form  in  the  adults  of  the  different  classes  of 
vertebrates,  which  were  ancestral  to  the  mammal. 

G.      SUMMARY  OF  THE   CIRCULATORY   SYSTEM 

1.  The  entire  circulatory  system  is  derived  from  the  mesoderm. 

2.  The  first  blood  vessels  are  the  vitelline  (omphalomesenteric)  veins.    These  course  along 
the  intestine  and  are  continued  posteriorly  as  the  subintestinal  vein.    In  forms  with  yolk 
sacs  they  are  the  veins  of  the  yolk  sac. 

3.  As  the  walls  of  the  hypomere  fuse  on  the  ventral  side  of  the  embryo  the  two  vitelline 
veins  unite  to  form  the  heart.    The  heart  lies  in  the  median  ventral  part  of  the  body  inclosed 
in  the  ventral  mesentery. 

4.  The  anterior  end  of  the  heart  continues  forward  as  the  ventral  aorta. 

5.  The  ventral  aorta  forms  a  series  of  loops,  the  aortic  arches,  around  the  pharynx.     These 
unite  dorsally  to  form  the  dorsal  aortae,  at  first  double,  but  subsequently  fusing  to  a  single 
vessel.    In  typical  vertebrates  there  are  six  pairs  of  aortic  arches. 

6.  The  dorsal  aorta  proceeds  posteriorly  along  the  mid-dorsal  line  of  the  coelom,  supply- 
ing branches  to  all  parts  of  the  body  below  the  heart. 

7.  The  chief  somatic  veins  at  first  are  the  anterior  and  posterior  cardinal  veins  uniting  at 
the  level  of  the  heart  to  a  common  cardinal  vein  on  each  side  which  enters  the  sinus  venosus; 
the  subcardinal  veins  extending  along  the  kidneys;  and  the  vein  of  the  lateral  body  wall,  the 
abdominal  or  umbilical  vein,  opening  into  the  common  cardinal. 

8.  Both  arteries  and  veins  are  provided  with  paired  segmental  and  unpaired  non-segmental 
branches.    The  former  are  of  two  kinds:  the  somatic  vessels  to  the  products  of  the  epimere, 
and  the  lateral  visceral  branches  to  the  products  of  the  mesomere.     The  unpaired  branches 
consist  of  visceral  vessels  to  the  digestive  tube.    The  vessels  of  the  appendages — subclavian 
to  the  anterior  appendages,  iliac  to  the  posterior  appendages — consist  simply  of  enlarged 
somatic  vessels. 

9.  In  all  vertebrates  the  vitelline  and  subintestinal  veins  become  converted  into  the  hepatic 
portal  system,  as  explained  in  the  introduction  of  this  section.    The  proximal  portions  between 
the  liver  and  the  heart  form  the  hepatic  veins;   within  the  liver  a  network  of  capillaries  is 
produced;  posterior  to  the  liver  one  of  the  vitellines  with  the  subintestinal  becomes  the  hepatic 
portal  vein. 

10.  The  renal  portal  system  develops  from  the  posterior  parts  of  the  posterior  cardinal 
veins  which  detach  from  the  anterior  portions  and  are  then  known  as  the  renal  portal  veins. 

They  collect  from  the  tail  and  pour  their  blood  into  the  kidneys  from  which  the  blood  passes  into 


COMPARATIVE  ANATOMY  OF  THE  CIRCULATORY  SYSTEM  271 

the  subcardinal  veins  which  establish  connections  with  the  anterior  portions  of  the  posterior 
cardinals. 

11.  The  venous  system  of  fishes  is  in  essentially  the  stage  reached  to  this  point,  with  a 
renal  portal  and  a  hepatic  portal  system,  and  the  cardinal  veins  as  the  chief  somatic  veins.     In 
fishes  there  is  also  an  inferior  jugular  vein  in  the  head  region. 

12.  The  arterial  system  of  fishes  is  also  in  a  primitive  condition.    There  is  a  ventral  aorta 
connected  by  aortic  arches  with  a  dorsal  aorta  which  supplies  the  body  below  the  heart.    Of 
the  original  six  pairs  of  aortic  arches  the  first  is  missing,  the  second  imperfect,  and  only  the 
last  four  are  retained  entire.    In  fishes  the  central  part  of  each  aortic  arch  is  secondarily  broken 
up  into  capillaries  in  the  gill  region;  the  ventral  halves  of  the  arches  then  become  the  afferent 
branchial  arteries,  the  dorsal  halves  the  efferent  branchial  arteries. 

13.  The  primitive  vertebrate  heart  is  a  bent  tube  consisting  of  four  chambers,  named 
from  the  posterior  end  forward:  sinus  venosus,  auricle,  ventricle,  and  conus  arteriosus.    There 
is  a  single  circulation,  composed  of  venous  blood,  through  the  heart;   the  blood  enters  the 
sinus  and  passes  through  the  chambers  of  the  heart  in  the  order  named.    This  condition  is 
permanent  in  fishes  (except  Dipnoi). 

14.  The  anterior  cardinal  veins  persist  in  all  vertebrates  as  the  internal  jugular  veins. 
These  unite  with  other  veins  of  the  anterior  part  of  the  body  to  form  large  trunks,  the  precaval 
veins.     The  bases  of  the  precavals  are  the  common  cardinal  veins.    In  some  mammals  the 
left  precaval  joins  the  right  precaval  in  front  of  the  heart,  forming  a  single  trunk.    In  such 
cases  the  reduced  base  of  the  left  precaval  (left  common  cardinal)  remains  as  the  coronary 
sinus,  a  vein  of  the  heart  wall. 

15.  The  anterior  portions  of  the  posterior  cardinal  veins  lose  their  importance  in  land 
vertebrates  and  disappear  above  urodeles,  except  that  in  mammals  the  proximal  portion  of 
the  right  one  contributes  to  the  azygos.    The  posterior  portions  of  the  posterior  cardinals 
become  the  renal  portal  veins  in  all  vertebrates.    In  fishes  they  collect  only  from  the  tail, 
while  above  fishes  they  also  usurp  the  veins  from  the  leg  which  at  first  open  into  the  abdominal 
veins.    The  renal  portal  veins  in  fishes,  Amphibia,  and  reptiles  flow  into  a  capillary  system  in 
the  kidneys,  from  which  the  blood  is  re-collected  into  the  subcardinal  veins  which  run  between 
the  two  kidneys. 

1 6.  Simultaneously  with  these  changes  in  the  posterior  cardinals  there  appears  a  new 
vein,  the  postcaval  vein.    It  is  found  in  a  few  fishes  and  is  present  continuously  from  the 
Amphibia  onward.     It  is  formed  by  the  union  of  a  hepatic  vein  in  the  liver  (which  is  in  turn 
derived  from  the  vitellines)  with  the  subcardinal  veins,  chiefly  the  right  subcardinal.    The 
subcardinals  then  become  parts  of  the  postcaval  vein. 

17.  Simultaneously  with  these  changes  the  abdominal  veins,  which  become  connected 
with  the  renal  portal  veins  when  the  latter  usurp  the  veins  from  the  legs,  change  their  anterior 
connections.    Whereas  they  originally  entered  the  common  cardinal  veins,  they  now  enter 
the  hepatic  portal  vein.    The  abdominal  veins  thus  come  to  constitute  a  connection  between 
the  renal  portal  and  hepatic  portal  systems. 

18.  In  Amphibia  and  reptiles  the  postcaval  vein  extends  only  to  the  posterior  end  of  the 
kidneys. 

19.  In  birds  the  postcaval  vein  establishes  direct  connections  with  the  renal  portal  veins; 
the  renal  portal  circulation  is  thus  greatly  reduced.    In  mammals  the  connection  is  completed 
so  that  the  veins  from  the  legs,  tail,  and  adjacent  regions  pass  directly  into  the  postcaval. 
The  renal  portal  circulation  then  vanishes. 

20.  With  the  changes  outlined  in  paragraph  18  the  abdominal  vein  loses  its  function.     It 
is  probably  present  in  birds  but  has  established  different  connections  and  is  lacking  in  mammals 
except  monotremes.     In  embryonic  stages,  however,  this  vein  (or  veins)  is  of  great  importance 
as  the  veins  of  the  allantois  or  embryonic  respiratory  organ. 


272       LABORATORY  MANUAL  FOR  VERTEBRATE  ANATOMY 

21.  The  hepatic  portal  system  remains  unchanged  throughout. 

22.  The  ventral  aorta  persists  as  a  single  vessel  in  fishes  and  Amphibia.    In  reptiles  it 
splits  into  three  trunks,  right  and  left  aortae  and  pulmonary;  in  birds  and  mammals  into  two 
trunks,  the  aorta  and  the  pulmonary. 

23.  The  aortic  arches  are  more  or  less  modified  in  land  vertebrates.     In  all  of  these  the 
first  two  have  vanished;  and  in  all  but  certain  Amphibia  the  fifth  has  likewise  disappeared. 
In  these  Amphibia  (urodeles  and  Gymnophiona)  the  three  (or  four)  remaining  arches  still 
form  complete  arches,   connecting  with   the  dorsal  aorta.    In  Anura,  reptiles,  birds,  and 
mammals  the  third  and  sixth  arches  lose  their  connection  with  the  dorsal  aorta,  only  the  fourth 
arches  retaining  this  connection.    The  third  arches  persist  as  the  bases  of  the  carotid  arteries, 
the  fourth  arches  form  the  aorta,  and  the  bases  of  the  sixth  arches  become  the  right  and  left 
pulmonary  arteries.     In  Anura  and  reptiles  both  fourth  arches  persist,  forming  right  and 
left  aortic  arches  which  unite  dorsally  to  produce  the  dorsal  aorta;  but  in  birds  the  left  fourth 
arch  and  in  mammals  the  right  fourth  arch  disappear.    The  right  fourth  in  birds  and  the  left 
fourth  in  mammals  then  remain  as  a  single  arch  connecting  the  dorsal  aorta  with  the  left 
ventricle.    The  dorsal  aorta  in  all  vertebrates  is  the  main  artery  of  the  body  posterior  to  the 
heart. 

24.  The  pulmonary  veins  appear  as  new  structures  in  the  air-breathing  vertebrates.    They 
enter  the  left  auricle.     Simultaneously  there  occurs  a  change  in  the  heart  and  the  double  circu- 
lation is  initiated. 

25.  In  Amphibia  the  four  chambers  of  the  heart  are  retained  as  in  fishes,  but  the  auricle 
is  partially  or  completely  divided  into  right  and   left  auricles.    The  sinus  venosus  is  then 
attached  to  the  right  auricle,  while  the  pulmonary  veins  enter  the  left  auricle.    The  right  side 
of  the  heart  consequently  contains  venous  blood  and  the  left  side  arterial  blood.    There  is  then 
a  double  circulation  through  the  heart,  but  the  two  kinds  of  blood  are  imperfectly  separated. 

26.  Above  Amphibia  the  conus  arteriosus  is  reduced  to  valves  (the  semilunar  valves)  al 
the  bases  of  the  great  arteries  which  thereupon  spring  directly  from  the  ventricles. 

27.  In  reptiles  the  sinus  venosus  is  retained,  the  two  auricles  are  completely  separated, 
the  ventricle  is  usually  incompletely  separated  into  right   and  left  chambers.    The  double 
circulation  is  present  but  imperfect  as  in  Amphibia. 

28.  In  birds  and  mammals  the  sinus  venosus  is  reduced  to  a  mere  node  in  the  wall  of  the 
right  auricle,  the  systemic  veins  then  entering  the  right  auricle  directly.    The  ventricle  is 
completely  divided  by  a  partition  into  right  and  left  ventricles;  consequently  the  two  circula- 
tions through  the  heart  are  wholly  separated,    The  right  side  of  the  heart  is  venous,  the  left 
arterial. 


XII.     THE  COMPARATIVE  ANATOMY  OF  THE  UROGENITAL  SYSTEM 

The  excretory  organs  of  vertebrates  consist  of  a  pair  of  kidneys  and  their  ducts  and  the 
reproductive  system  of  a  pair  of  gonads  or  sexual  glands  and  their  ducts.  These  two  systems 
are  generally  closely  united,  since  the  reproductive  organs  use  the  ducts  of  the  kidneys  to 
convey  their  products  to  the  exterior.  Hence  it  is  customary  to  consider  the  two  systems 
together  as  the  urogenital  (urino genital)  system. 

A.      EMBRYONIC   ORIGIN  AND   EVOLUTION   OF   THE   UROGENITAL  SYSTEM 

1.  Development  of  the  kidneys. — The  kidneys  or  excretory  organs  arise  from  the  meso- 
mere;  this  it  will  be  recalled  is  the  small  middle  region  of  the  mesoderm.    Within  the  mesomere 
there  are  first  produced  the  tubules  of  the  kidneys.    These  tubules  arise  through  a  hollowing 
out  of  the  tissue  of  the  mesomere.     The  tubules  of  the  kidney  belong  to  the  category  of  ducts 
known  as  coelomoducts*    Each  excretory  tubule  opens  into  the  coelom  (cavity  of  the  hypomere) 
by  means  of  a  ciliated  funnel-shaped  opening  called  a  nephrostome,  pursues  a  more  or  less 
winding  course  in  the  mesomere  and  finally  terminates  in  the  collecting  duct.     At  first  there 
is  a  pair  of  tubules  to  each  segment,  but  later  they  increase  greatly  in  number  and  become 
compacted  together  into  a  definite  body  which  is  called  a  kidney.     In  connection  with  the 
kidneys  there  are  developed  peculiar  networks  of  capillaries  which  are  known  as  glomeruli.    Each 
glomerulus  consists  of  a  knot  of  capillaries  into  which  blood  is  brought  by  an  artery  and 
conveyed  away  by  a  vein.     This  arrangement  evidently  serves  to  bring  the  blood  into  closer 
contact  with  the  cells  of  the  kidneys.    The  glomeruli  come  into  close  relationship  with  the 
kidney  tubules. 

2.  The  kidneys  and  their  ducts. — We  have  thus  far  spoken  of  the  kidneys  as  if  the  kidneys 
of  all  vertebrates  were  homologous  organs.     This  is  not,  however,  the  case.     On  the  contrary, 
in  the  evolution  of  the  vertebrates  there  have  been  present  three  more  or  less  distinct  excre- 
tory organs,  which  are  similar  in  structure  and  arise  in  a  similar  manner  from  the  mesomere 
but  are  nevertheless  not  identical  (Fig.  60). 

a)  The  pronephros:  The  first  kidneys  to  appear  in  the  chordate  phylum  are  the  paired 
pronephroi  (singular,  pronephros).  The  pronephroi  arise  from  the  anterior  end  of  the  mesomere, 
far  forward  in  the  body  and  extend  over  several  segments  (Fig.  6o4).  They  are  composed  of 
tubules,  each  pronephros  possessing  one  tubule  to  each  segment  of  the  body.  Each  tubule 
consists  of  a  simple  curved  tube,  opening  at  one  end  into  the  coelom  by  a  nephrostome  and  at 
the  other  end  into  a  common  duct,  the  pronephric  duct,  which  collects  from  all  of  the  tubules. 
The  pronephroi  are  present  in  the  adult  stage  of  only  cyclostomes  and  a  very  few  fishes;  they 
appear,  however,  as  transient  structures  during  the  development  of  all  other  vertebrates. 
In  most  fishes  and  Amphibia  they  are  probably  functional  as  excretory  organs  during  embryonic 
stages.  In  reptiles,  birds,  and  mammals  they  appear  during  early  embryonic  development, 
but  do  not  develop  completely,  do  not  function,  and  soon  disintegrate  and  disappear.  Glome- 
ruli are  generally  present  in  connection  with  the  pronephroi  but  may  be  absent  even  in 

1  It  was  formerly  supposed  and  is  commonly  taught  that  the  tubules  are  homologous  with  the 
nephridia  of  the  earthworm  and  other  oligochaetes.  It  has  been  shown,  however,  that  the  nephridia 
are  of  ectodermal  origin,  while  the  tubules  of  the  vertebrate  kidney  are  of  mesodermal  origin.  It  is 
therefore  desirable  to  adopt  the  terminology  of  Goodrich,  designating  as  nephridia  such  excretory 
or  genital  ducts  as  are  homologous  with  the  nephridia  of  the  earthworm;  while  those  genital  or  excre- 
tory ducts  or  tubules  which  arise  in  the  coelomic  wall  are  named  coelomoducts. 


274 


LABORATORY  MANUAL  FOR  VERTEBRATE  ANATOMY 


functional  cases.  The  glomeruli  of  the  pronephros  are  generally  of  the  type  known  as  external 
glomeruli,  that  is,  they  project  into  the  coelom  adjacent  to  the  nephrostome  and  are  not  directly 
associated  with  the  tubule  (Fig.  61).  In  some  cases,  however,  internal  glomeruli  which  project 
into  the  tubule  occur. 


B 


FIG.  60. — Diagrams  to  show  the  development  of  the  three  kidneys  and  their  ducts  and  their 
relation  to  the  male  gonad.  A,  early  stage  showing  the  pronephros  a  developing  from  the  anterior  end 
of  the  mesomere  c  and  the  pronephric  duct  b  which  has  not  yet  reached  the  cloaca  e.  B,  next  stage, 
illustrating  the  degeneration  of  the  pronephros  at  /,  the  development  of  the  mesonephros  h  from  the 
middle  portion  of  the  mesomere,  the  junction  of  the  pronephric  duct,  now  the  mesonephric  duct  g, 
with  the  cloaca  and  the  beginning  of  the  metanephric  evagination  i  from  the  mesonephric  duct.  C,  later 
stage,  showing  connection  between  certain  tubules  of  the  mesonephros  and  the  testis  j  by  means  of 
tubules,  the  vasa  efferentia  p,  which  grow  out  from  the  mesonephros;  and  the  penetration  of  the  meta- 
nephric evagination  into  the  posterior  end  of  the  mesomere  where  it  is  subdividing  to  form  the  collecting 
apparatus  /,  which  becomes  associated  with  secretory  metanephric  tubules  m,  developed  in  the  meso- 
mere. D,  final  stage,  in  which  the  mesonephros  has  disappeared  except  for  the  remnant  q  which  con- 
nects with  the  testis  j  by  means  of  the  vasa  efferentia  p;  the  mesonephric  duct  g  persists  as  the  vas 
deferens;  the  two  parts  of  the  metanephros  shown  hi  C  have  united  to  form  a  single  organ  r.  a,  pro- 
nephros; b,  pronephric  duct;  c,  mesomere  or  nephrotome;  d,  intestine;  e,  cloaca;  /,  degenerating 
pronephros;  g,  mesonephric  or  Wolffian  duct;  h,  mesonephros  or  Wolffian  body;  i,  metanephric  evagina- 
tion from  the  Wolffian  duct  in  B,  ureter  in  C  and  D;  j,  testis;  k,  coiled  portion  of  the  vas  deferens  form- 
ing part  of  the  epididymis;  /,  collecting  part  of  the  metanephros  derived  from  the  Wolffian  duct;  m, 
execretory  tubules  of  the  metanephros  derived  from  the  mesomere;  n,  nephrostome;  0,  renal  corpuscle 
or  Malpighian  body;  p,  vasa  efferentia;  q,  remnant  of  the  mesonephros,  forming  part  of  the  epididymis; 
r,  metanephros. 

6)  Mesonephros:  The  second  vertebrate  kidneys  are  the  paired  mesonephroi  (singular, 
mesonephros),  also  known  as  the  Wolffian  bodies,  after  the  embryologist  Wolff,  who  first 


COMPARATIVE  ANATOMY  OF  THE  UROGENITAL  SYSTEM 


275 


described  them.  These  develop  in  the  mesomere  immediately  posterior  to  the  pronephroi,  even 
overlapping  the  posterior  ends  of  the  pronephroi.  They  extend  over  a  number  of  segments, 
and  are  much  larger  than  the  pronephroi.  As  they  develop,  the  pronephroi  generally  degen- 
erate. The  mesonephros  consists  of  many  tubules,  a  number  to  each  segment ;  these,  like  the 
pronephric  tubules,  open  at  one  end  into  the  coelom  by  nephrostomes  and  at  the  other  end 
terminate  in  the  pronephric  duct  (Fig.  6o#).  Although  the  pronephroi  vanish,  the  pronephric 
ducts  persist;  they  grow  posteriorly  alongside  the  region  where  the  mesonephroi  are  developing, 
and  become  the  ducts  of  the  latter.  They  are  then  known  as  the  mesonephric  or  Wolffian  duds. 
The  mesonephric  duets  grow  posteriorly  until  they  reach  the  caudal  end  of  the  intestine. 
To  this  they  become  connected,  pouring  their  contents  into  the  terminal  chamber  of  the 
intestine,  which  consequently  receives  the  name  of  cloaca  (Latin,  "a  sewer").  The  structure 


neural  tube 


epimere 


cpimere 
notochord 


dorsal  aorU 


mesonephric 
duct 


nephrostome 


mesonephric 
tubule 

Bowman's  capsule 
internal 
glomerulus 


pronephric 
tubule 


pronephric 
duct 


nephrostome 


MESONEPHROS 


intestine 


FIG.  61. — Diagram  to  show  the  structure  of  the  pronephros  and  mesonephros.  Pronephros  on  the 
right,  mesonephros  on  the  left.  The  chief  difference  is  in  the  relation  of  the  glomerulus;  in  the  pro- 
nephros it  projects  into  the  coelom;  in  the  mesonephros  it  projects  into  the  tubule,  which  forms  a 
Bowman's  capsule  about  it.  (From  Wiedersheim's  Comparative  Anatomy  of  Vertebrates,  courtesy  of 
the  Macmillan  Company.) 

of  the  mesonephros  differs  from  that  of  the  pronephros.  The  tubules  are  more  numerous, 
longer,  and  more  convoluted.  The  glomeruli  are  always  of  the  internal  kind  (Fig.  61).  Each 
glomerulus  projects  into  the  mesonephros,  carrying  the  wall  of  the  tubule  before  it  to  form  a 
flask-shaped  capsule  in  which  the  glomerulus  rests.  The  capsule  is  named  Bowman's  capsule, 
and  this,  together  with  the  glomerulus,  forms  a  rounded  body  known  as  the  Malpighian  body 
or  renal  corpuscle.  The  mesonephric  tubules  generally  open  into  the  coelom  by  nephrostomes 
which  are  situated  near  the  renal  corpuscles. 

The  mesonephroi  are  the  kidneys  of  the  adults  of  fishes  and  Amphibia,  and  are  also  the 
chief  functional  kidneys  of  cyclostomes,  although  the  latter  possess  in  addition  persistent 
pronephroi.  The  mesonephric  or  Wolflian  ducts  are  the  excretory  ducts  in  these  vertebrates, 
opening  into  the  cloaca.  The  mesonephroi  appear  in  the  embryos  of  reptiles,  birds,  and 
mammals,  and  are  in  most  cases  functional  in  the  embryos,  often  becoming  of  large  size. 
They  generally,  however,  lack  nephrostomes.  They  degenerate  before  hatching  or  birth,  or 
very  shortly  after. 


276       LABORATORY  MANUAL  FOR  VERTEBRATE  ANATOMY 

c)  The  metanephroi:  The  third  kidneys  of  vertebrates  are  the  paired  metanephroi.  The 
metanephroi  arise  in  part  from  the  remainder  of  the  mesomere,  posterior  to  the  mesonephroi; 
and  in  part  as  an  outgrowth  from  the  Wolffian  ducts.  This  outgrowth  occurs  as  an  evagination 
from  the  Wolffian  duct  near  its  connection  with  the  cloaca;  it  pushes  anteriorly  and  fuses 
with  that  portion  of  the  metanephros  arising  from  the  mesomere  (Fig.  6oB  and  C).  The 
metanephros  is  thus  an  organ  of  double  origin.  That  part  which  comes  from  the  mesomere 
develops  into  the  tubules  of  the  kidney  and  the  renal  corpuscles;  that  part  derived  from  the 
Wolffian  duct  becomes  the  collecting  tubules  and  the  spaces  (calices)  into  which  they  empty. 
The  stalk  of  the  outgrowth  from  the  Wolffian  duct  becomes  the  metanephric  duct  or  ureter* 
which  conveys  the  excretion  of  the  metanephros  to  the  cloaca.  The  structure  of  the  meta- 
nephros is  similar  to  that  of  the  mesonephros,  except  that  nephrostomes  are  wanting.  The 
metanephric  tubules  are  very  long  and  complicated,  with  several  loops  and  convolutions.  The 
metanephroi  are  the  kidneys  of  the  adults  of  all  reptiles,  birds,  and  mammals. 

From  the  foregoing  account  it  will  be  seen  that  the  three  pairs  of  vertebrate  kidneys  arise 
successively  from  the  mesomere,  each  situated  caudad  to  the  preceding  one.  For  this  reason 
it  seems  probable  that  they  represent  persisting  parts  of  a  kidney  which  originally  extended 
the  entire  length  of  the  mesomere.  The  anterior  part  of  this  hypothetical  kidney  has  gradually 
degenerated  so  that  the  kidney  appears  to  descend  posteriorly.  This  disappearance  of  organs 
from  anterior  segments  is  common  in  heteronomously  segmented  animals. 

3.  The  urinary  bladder. — The  urinary  bladder  is  in  all  forms  above  fishes  a  saclike  evagina- 
tion from  the  ventral  wall  of  the  cloaca.     In  fishes,  the  bladder  when  present  is  formed  in  part 
by  the  enlargement  of  the  terminal  portions  of  the  Wolffian  ducts  as  well  as  by  a  cloacal  evagina- 
tion.   In  the   embryos   of   reptiles,  birds,   and   mammals,  there   is   an   enormous   cloacal 
evagination,  the  allantois,  which  may  be  regarded  as  a  greatly  expanded  urinary  bladder. 
The  allantois  serves  respiratory  as  well  as  urinary  functions.   The  adult  bladder  develops  at 
the  base  of  the  allantoic  stalk.    Adult  birds  lack  a  urinary  bladder.    The  excretory  ducts 
do  not  generally  open  into  the  urinary  bladder  directly,  except  in  mammals. 

4.  The  gonads  and  their  ducts. — The  gonads  or  reproductive  organs  consist  of  a  pair  of 
lestes  in  the  male,  ovaries  in  the  female.    These  organs  arise  as  swellings,  the  genital  ridges, 
on  the  ventral  wall  of  the  mesomere,  immediately  to  the  medial  side  of  the  kidneys.    They 
project  into  the  coelom  and  in  the  mature  condition  are  generally  provided  with  mesenteries. 

a)  Male:  The  testis  consists  of  a  mass  of  tubules,  the  seminiferous  tubules,  which  empty 
into  a  network  of  tubules  called  the  rete,  which  is  situated  usually  along  the  attached  part  of 
the  testis.  Tubular  connections  are  established  during  embryonic  stages  between  the  rete 
and  the  Bowman's  capsules  of  the  anterior  part  of  the  adjacent  mesonephros  (Fig.  6oC). 
These  connections  are  called  the  vasa  efferentia,  and  they  traverse  the  mesorchium  into  the 
mesonephros.  The  vasa  efferentia  are  then  continuous  within  the  mesonephros  with  certain 
of  the  mesonephric  tubules  and  through  them  find  exit  into  the  Wolffian  duct.  It  thus  happens 
that  in  nearly  all  vertebrates  the  Wolffian  duct  serves  as  the  male  duct.  In  elasmobranchs  and 
Amphibia,  in  which  the  mesonephros  is  the  functional  adult  kidney,  the  Wolffian  duct  in  males 
serves  as  both  excretory  and  male  duct  (Fig.  62.4).  In  male  reptiles,  birds,  and  mammals, 
where  the  mesonephros  has  disappeared,  the  Wolffian  duct  remains  as  the  male  duct,  and  is 
then  named  the  vas  deferens  (Fig.  62 C).  In  these  groups  the  mesonephros  disappears,  except 
those  mesonephric  tubules  which  have  established  connections  with  the  rete  testis.  These 
tubules  constitute  the  vasa  efferentia  and  a  part  of  the  epididymis.  Epididymis  is  the  name 
given  to  that  part  of  the  male  duct  which  is  convoluted. 

1  A  great  deal  of  confusion  will  be  avoided  by  reserving  the  term  ureter  for  the  duct  of  the  meta- 
nephros. 


COMPARATIVE  ANATOMY  OF  THE  UROGENITAL  SYSTEM 


277 


mesonephroa 


Wolffian  duct 


intestine 


cloaca 


B 


testis 
vasa  efferentia 

epididymis  (vestige 
of  mesonephros) 


Wolffian  duct  _J 
(vas  deferens) 

metanephros 


cloaca 


urinary 
bladder 


cloaca 


FIG.  62. — Diagrams  to  illustrate  the  urogenital  systems  of  male  and  female  anamniotes  and 
amniotes.  A,  male  elasmobranch  or  amphibian;  the  mesonephros  is  differentiated  into  anterior 
genital  and  posterior  excretory  portions;  the  genital  part  is  connected  with  the  testis  by  means  of  the 
vasa  efferentia  which  are  outgrowths  from  the  mesonephros;  the  mesonephric  or  Wolffian  duct  serves  as 
both  genital  and  excretory  duct;  the  oviduct  or  Mullerian  duct  is  vestigial.  B,  female  elasmobranch 
or  amphibian;  the  ovary  is  not  connected  with  the  mesonephros;  the  mesonephros  and  mesonephric 
duct  serve  only  excretory  functions;  the  oviduct  is  well  developed  and  opens  into  the  coelom  by  the 
ostium  near  the  ovary.  C,  male  reptile,  bird,  or  mammal;  the  excretory  part  of  the  mesonephros  has 
disappeared  but  the  genital  part  persists  as  the  epididymis  (in  part)  which  is  connected  as  in  anamniotes 
with  the  testis  by  means  of  the  vasa  efferentia;  the  Wolffian  duct  is  purely  genital  and  is  renamed  the 
vas  deferens;  the  excretory  function  is  served  by  metanephroi  and  ureters.  D,  female  reptile,  bird,  or 
mammal;  the  mesonephros  and  Wolffian  duct  have  entirely  vanished;  the  condition  of  the  ovary  and 
oviduct  is  the  same  as  in  anamniotes;  the  excretory  function  is  served  by  the  metanephroi  and  ureters 
exactly  as  in  the  male.  (The  changes  in  the  relation  of  the  urogenital  ducts  and  cloaca  which  occur  in 
mammals  are  not  indicated  in  these  figures  but  are  shown  in  Figs.  63  and  64.)  (Slightly  modified 
from  Wilder's  History  of  the  Human  Body,  courtesy  of  Henry  Holt  and  Company.) 


278 


LABORATORY  MANUAL  FOR  VERTEBRATE  ANATOMY 


ft)  Female:  The  ovaries  are  masses  of  connective  tissue  containing  the  developing  eggs, 
each  egg  surrounded  by  a  capsule  of  nutritive  cells  forming  a  follicle.  The  ovaries,  unlike  the 
testes,  never  have  any  connection  with  the  kidneys.  The  ducts  of  the  ovaries  are  named  the 
oviducts  or  Mullerian  ducts.  The  origin  of  the  Miillerian  ducts  is  somewhat  problematical. 
In  elasmobranchs  they  arise  by  a  splitting  of  the  pronephric  duct;  half  of  the  pronephric 
duct  then  becomes  the  oviduct,  and  the  other  half  becomes  the  mesonephric  duct.  Al- 
though this  mode  of  origin  is  the  one  commonly  accepted,  it  cannot  be  demonstrated  for 
other  vertebrates;  in  them  the  oviducts  arise  independently  in  the  mesomere.  The  oviducts 
are  never  directly  connected  with  the  ovaries.  They  open  into  the  coelom  near  the  ovaries 


uterine  tube 


oviduct 


cloaca 


rethra 


uterine  tube 


urogenital  sinus 


—  uterine  tube 


uterus 


uterine  tube 


urethra"'! 
urogenital  sinus 

FIG.  63. — Diagrams  to  show  the  various  types  of  mammalian  oviducts.  A,  condition  found  in 
the  majority  of  female  vertebrates;  the  two  oviducts  are  completely  separate  and  open  independently 
into  the  cloaca.  B-E,  various  conditions  found  in  mammals,  showing  differentiation  of  the  oviducts 
into  uterine  tube,  uterus,  and  vagina,  and  progressive  fusion  of  the  lower  parts  of  the  oviducts:  B,  duplex 
type  found  in  rodents,  in  which  the  two  vaginae  are  united  to  one;  C,  bipartite  type  occurring  in  carni- 
vores; not  only  are  the  vaginae  fused  but  the  lower  parts  of  the  two  uteri  are  fused  to  form  a  single 
body,  divided  in  two  by  a  partition  which  represents  the  fused  walls  of  the  two  uteri;  the  upper  parts  of 
the  two  uteri  remain  separate  as  the  horns;  D,  bicornuate  type,  found  in  many  ungulates,  similar  to  C 
except  that  the  partition  has  disappeared;  E,  simplex  type,  occurring  in  man  and  the  apes,  in  which 
both  vaginae  and  uteri  are  fused  along  their  entire  lengths  leaving  only  the  uterine  tubes  separate. 
Note  further  that  in  B-D  the  urethra  joins  the  vagina  to  form  the  urogenital  sinus  which  opens  to 
the  exterior,  while  in  E  the  urethra  and  vagina  are  wholly  separate  and  open  independently  to  the 
exterior.  (From  Wiedersheim's  Comparative  Anatomy  of  Vertebrates,  courtesy  of  the  Macmillan 
Company.) 

by  a  funnel-shaped  opening,  the  ostium,  which  probably  represents  one  or  more  of  the  nephro- 
stomes  of  the  pronephros  (Fig.  62!$  and  D).  The  eggs  escape  from  the  ovary  by  rupture  of 
the  ovarian  wall,  pass  into  the  ostium  of  the  oviducts  by  methods  which  are  not  always  under- 
stood, and  are  conveyed  down  the  oviducts. 

The  oviducts  in  the  majority  of  vertebrates  remain  as  two  separate  tubes  opening  into 
the  cloaca  (Fig.  63^!).  In  mammals  each  oviduct  is  differentiated  into  a  narrower  anterior 
portion  called  the  uterine  or  Fallopian  tube,  which  bears  the  ostium,  and  a  wider  more  muscular 
posterior  portion,  the  uterus.  In  the  monotremes,  or  egg-laying  mammals,  each  uterus  opens 
separately  into  the  cloaca.  In  the  marsupials  the  terminal  portion  of  the  uterus  is  differentiated 


COMPARATIVE  ANATOMY  OF  THE  UROGENITAL  SYSTEM 


279 


as  a  vagina  to  receive  the  penis.  In  mammals  above  marsupials  the  two  vaginae  fuse  to  a 
single  vagina  (hence  the  name  Monodelphia).  There  is  also  generally  more  or  less  fusion  of 
the  two  uteri  (Fig.  63).  When  only  the  posterior  portions  of  uteri  are  fused,  the  fused  portion 
is  called  the  body  of  the  uterus  and  the  separate  portions  the  horns  of  the  uterus.  In  man  and 
other  primates  the  uteri  are  fused  along  their  entire  length  producing  the  single  uterus  or 
womb.  The  young  of  the  placental  mammals  develop  only  in  the  uterine  part  of  the  oviducts; 
in  those  forms  with  partially  fused  uteri,  only  in  the  horns. 

5.  The  evolution  of  the  cloaca. — The  cloaca  is  found  in  all  vertebrates  except  cyclostomes, 
teleostomes,  and  the  placental  mammals.  It  receives  the  termination  of  the  intestine  and  the 
urinary  and  genital  ducts.  From  the  preceding  account  it  will  be  evident  that  in  the  males 
of  elasmobranchs  and  Amphibia  the  cloaca  receives  only  the  Wolffian  ducts,  while  in  the 
females  both  oviducts  and  Wolffian  ducts  enter  it  (Fig.  62^!  and  B).  It  commonly  happens, 
however,  that  in  the  males  of  these  groups  vestiges  of  the  oviducts  are  present.  In  the  males 


oviduct 


rectum 


FIG.  64. — Diagrams  to  illustrate  the  changes  in  the  cloaca  in  mammals  during  development. 
A,  early  embryonic  stage,  showing  the  cloaca  receiving  the  urinary  bladder,  the  rectum,  and  the  Wolffian 
duct,  as  in  the  lower  vertebrates.  B,  later  stage,  showing  the  beginning  of  the  fold  which  divides  the 
cloaca  into  a  ventral  urogenital  sinus  which  receives  the  urinary  bladder,  Wolffian  ducts,  and  ureters, 
and  into  a  dorsal  part  which  receives  the  rectum.  C,  further  progress  of  the  fold,  dividing  the  cloaca 
into  urogenital  sinus  and  rectum;  the  ureter  has  separated  from  the  Wolffian  duct  and  is  shifting 
anteriorly.  D,  completion  of  the  fold,  showing  complete  separation  of  the  cloaca  into  ventral  uro- 
genital sinus  and  dorsal  rectum.  Note  in  D  that  the  ureter  has  shifted  farther  so  that  it  opens  into  the 
urinary  bladder. 

of  reptiles  and  birds  the  cloaca  receives  the  Wolffian  ducts  (vasa  deferentia)  and  the  ureters; 
in  the  females  the  oviducts  and  the  ureters  (Fig.  62C  and  D).  In  addition,  in  many  fishes, 
Amphibia,  reptiles,  and  the  embryos  of  birds  and  mammals  the  urinary  bladder  opens  into 
the  ventral  wall  of  the  cloaca.  Adult  birds  have  no  urinary  bladder;  mammals  have  one, 
but  it  is  no  longer  attached  to  the  digestive  tract. 

In  placental  mammals  marked  changes  occur  in  the  relations  of  the  terminal  portions  of 
the  urogenital  ducts.  In  the  embryo  the  cloaca  becomes  divided  by  a  fold  which  extends 
posteriorly  to  the  body  wall  and  separates  the  cloaca  into  two  parts,  each  with  its  own  opening 
to  the  exterior  (Fig.  64).  The  dorsal  part  includes  the  intestine  only;  this  terminal  portion 
of  the  intestine  is  called  the  rectum  and  opens  to  the  exterior  by  the  anus.  The  ventral  part 
separated  from  the  cloaca  is  called  the  urogenital  sinus.  It  receives  the  stalk  of  the  bladder 
and  the  excretory  and  genital  ducts.  The  excretory  ducts  (either  Wolffian  ducts  or  ureters)  at 
first  open  into  the  urogenital  sinus,  but  subsequently  the  ureters  shift  so  as  to  open  into  the  blad- 
der, in  all  of  the  placental  mammals  (Fig.  64) .  Thus,  the  ureters  pass  into  the  bladder  while  the 
Wolffian  ducts  (vasa  deferentia)  in  males  or  the  vagina  in  females  unite  with  the  duct  of  the 
bladder,  named  the  urethra,  forming  a  common  tube  or  chamber,  the  urogenital  sinus,  which 
opens  externally  in  front  of  the  anus  by  a  urogenital  aperture.  In  the  females  only  of  the 


280       LABORATORY  MANUAL  FOR  VERTEBRATE  ANATOMY 

highest  placenta!  mammals  there  is  no  urogenital  sinus,  but  the  urethra  and  the  vagina  are 
separate  and  open  separately  to  the  exterior,  the  former  anterior  to  the  latter.  In  such  cases 
there  are  three  openings  in  the  perineum:  the  anus,  the  mouth  of  the  vagina,  and  the  mouth 
of  the  urethra. 

For  more  complete  accounts  of  the  comparative  anatomy  of  the  urogenital  system,  the 
appropriate  chapters  of  K,  W,  or  Wd  should  be  consulted. 


B.      THE   UROGENITAL   SYSTEM   OF   ELASMOBRANCHS 

The  following  outline  applies  to  the  smooth  and  spiny  dogfish  and  the  skate, 
the  latter  being  described  separately.  The  majority  of  the  dogfish  used  for  dissec- 
tion are  immature,  and  it  is  therefore  difficult  or  impossible  to  locate  in  them  all 
of  the  parts  of  the  urogenital  system.  At  least  a  few  mature  males  and  females 
will  be  on  hand  for  demonstration.  The  skates  are  sexually  mature  when  still 
relatively  small,  and  most  of  the  specimens  used  have  fully  developed  repro- 
ductive systems. 

Remove  the  digestive  tract  except  cloaca  and  liver. 

i.  The  female  urogenital  system. — 

Dogfish:  The  ovaries  are  a  pair  of  soft  bodies,  situated  dorsally.  In  the 
spiny  dogfish  they  are  oval  in  form  and  located  dorsal  to  the  liver,  each  with  a 
mesentery,  the  mesovarium.  In  the  smooth  species  they  are  long  and  slender, 
extending  the  whole  length  of  the  coelom,  more  or  less  fused,  and  attached 
posteriorly  to  the  mesentery  of  the  rectal  gland.  Their  posterior  ends  are 
toothed.  There  is  single  mesovarium  for  the  two  ovaries.  In  both  species 
when  mature,  the  large  eggs,  consisting  chiefly  of  yolk,  are  readily  noted  in  the 
ovaries. 

Locate  the  kidneys  lying  against  the  dorsal  body  wall,  one  to  each  side  of 
the  dorsal  aorta.  They  are  long,  slender,  brown  bodies.  Their  posterior  portions 
are  broader  and  thicker  than  the  anterior  portions  and  probably  perform  most 
of  the  work  of  excretion.  The  kidneys  are  retroperitoneal.  Free  their  lateral 
borders  by  slitting  the  pleuroperitoneum  and  note  thickness  of  the  organ  at 
different  levels.  The  kidneys  are  mesonephroi;  the  posterior  thicker  part  may 
be  named  the  caudal  mesonephros,  the  anterior  more  slender  part  the  cranial 
mesonephros.1  Between  the  two  kidneys  is  a  tough  shining  ligament  which 
should  not  be  mistaken  for  a  duct. 

The  oviducts  in  immature  females  are  slender  tubes  running  along  the  ventral 
face  of  the  kidneys,  without  mesenteries.  In  mature  females  they  are  very  large 
tubes  which  spring  free  from  the  kidneys  by  means  of  well-developed  mesenteries, 
the  mesotubaria.  Trace  the  oviducts  forward.  They  pass  forward  along  the 
dorsal  coelomic  wall,  curve  around  the  anterior  border  of  the  liver,  and  enter 
the  falciform  ligament.  Here  the  two  oviducts  are  united  to  a  common  opening. 

1  The  caudal  mesonephros  is  often  regarded  as  a  metanephros  and  its  duct  as  a  ureter.  Professor 
Kingsley  has  kindly  expressed  his  opinion  that  this  usage  is  unjustifiable. 


COMPARATIVE  ANATOMY  OF  THE  UROGENITAL  SYSTEM  281 

the  ostium.  This  is  a  wide,  funnel-shaped  opening  lying  in  the  falciform  liga- 
ment, the  opening  directed  posteriorly.  To  find  the  opening  it  is  generally 
necessary  to  separate  the  walls  of  the  ostium,  as  these  tend  to  adhere.  Trace 
the  oviducts  posteriorly.  They  are  narrow  tubes  at  first,  soon  presenting  in 
mature  specimens  a  slight  enlargement,  the  shell  gland.  Posterior  to  this  they 
narrow  again  and  then  in  mature  females  enlarge  greatly  to  form  the  uterus, 
which  swings  free  by  means  of  the  mesotubarium.  In  immature  females  there 
is  no  such  enlargement,  nor  is  there  any  mesotubarium,  but  the  oviducts  widen 
slightly  as  they  proceed  posteriorly  along  the  ventral  faces  of  the  kidneys. 

Trace  the  oviducts  to  the  cloaca.  Cut  open  the  cloaca  by  a  median  ventral 
slit  which  opens  up  the  intestine.  Note  the  opening  of  the  intestine  into  the 
ventral  part  of  the  cloaca  and  the  slight  fold  which  separates  this  from  the  dorsal 
urogenital  region  of  the  cloaca.  Note  the  urinary  papilla  in  the  median  dorsal 
wall  of  the  cloaca;  it  is  a  conical  papilla  in  the  spiny  dogfish,  a  low  elongated 
ridge  in  the  smooth  species.  In  mature  specimens  the  large  openings  of  the 
oviducts  are  readily  seen  to  each  side  of  the  urinary  papilla.  In  immature 
specimens  they  are  in  the  same  position  but  quite  small.  To  find  them  it  is 
best  to  cut  into  the  posterior  ends  of  the  oviducts  and  probe  toward  the  cloaca. 

The  ducts  of  the  kidneys  are  more  difficult  to  locate  in  females.  The  cranial 
mesonephros  is  provided  with  a  Wolffian  duct.  This  lies  along  the  ventral  face 
of  the  kidney  exactly  dorsal  to  the  oviduct  in  immature  specimens,  or  in  mature 
ones  along  the  line  of  attachment  of  the  mesotubarium.  Locate  it  in  immature 
specimens  by  carefully  stripping  off  the  oviduct  and  also  freeing  the  pleuroperi- 
toneum  from  the  ventral  face  of  the  kidney.  The  Wolffian  duct  is  a  slender 
duct  proceeding  posteriorly  to  the  cloaca.  The  caudal  mesonephros  (posterior 
third)  is  also  provided  with  a  duct,  the  accessory  mesonephric  duct.  In  the 
smooth  dogfish  this  lies  alongside  of  the  Wolffian  duct  and  has  probably  been 
noted  already.  In  the  spiny  species  it  is  more  deeply  located  imbedded  in  the 
kidney  tissue;  it  will  be  found  by  dissecting  in  this  tissue  near  the  median  line. 
The  accessory  ducts  are  delicate  tubes  receiving  ducts  from  the  kidney  at  intervals. 
Both  Wolffian  ducts  and  accessory  ducts  proceed  to  the  cloaca  lying  in  contact 
with  and  on  the  dorsal  surface  of  the  oviducts.  To  follow  them  cut  along  one 
side  of  the  cloaca,  separating  the  cloaca  from  the  body  wall.  They  all  terminate 
by  a  single  median  pore  in  the  urinary  papilla. 

Skate :  The  ovaries  are  a  pair  of  elongated  soft  bodies  containing  large  yellow 
eggs;  they  are  situated  dorsally  in  the  anterior  half  of  the  pleuroperitoneal 
cavity.  The  large  oviducts  pass  dorsal  to  them.  Follow  one  oviduct  forward. 
Its  narrow  anterior  portion  passes  along  the  dorsal  coelomic  wall,  curves  around 
the  anterior  margin  of  the  liver,  and  passing  into  the  falciform  ligament  unites 
with  its  fellow  to  a  single  common  opening  or  ostium.  The  ostium  is  a  wide 
funnel-like  aperture  situated  in  the  ligament  and  facing  posteriorly.  Trace  the 
oviducts  caudad.  After  a  short  distance  they  widen  greatly  to  a  uterus,  the 


282       LABORATORY  MANUAL  FOR  VERTEBRATE  ANATOMY 

beginning  of  which  bears  a  conspicuous  bilobed  swelling,  the  oviducal  gland: 
which  secretes  the  horny  case  in  which  the  eggs  are  laid  (N,  p.  164,  Fig.  96). 
The  uteri  proceed  to  the  cloaca  supported  by  the  thickened  mesotubaria.  Cut 
open  the  cloaca  in  the  median  ventral  line,  slitting  open  the  intestine.  Note 
the  opening  of  the  intestine  into  the  ventral  part  of  the  cloaca  and  the  conspicuous 
horizontal  fold  which  separates  this  from  the  dorsal  urogenital  part.  Cut  into 
the  latter  by  cutting  forward  through  this  fold.  The  cloaca  is  greatly  extended 
and  thickened  in  the  anterior  direction.  Find  the  openings  of  the  oviducts, 
one  to  each  side  of  this  thickened  part  of  the  cloaca.  Halfway  between  the 
openings  of  the  oviducts  is  the  urogenital  opening  in  the  median  dorsal  wall. 

The  main  part  of  the  kidneys  in  female  skates  consists  of  a  thick  rounded 
lobe  lying  against  the  dorsal  wall  at  each  side  of  the  cloaca.  These  lobes  are 
revealed  by  stripping  off  the  thick  pleuroperitoneum  which  covers  their  ventral 
surfaces.  These  lobes  may  be  named  the  caudal  mesonephros.  The  anterior 
part  of  the  mesonephros  or  cranial  mesonephros  is  nearly  degenerate  in  females 
but  will  be  found  as  diffuse  brownish  tissue  extending  forward  ventral  to  the 
dorsal  aorta.  From  the  median  surface  of  the  caudal  mesonephros  several  ducts, 
the  accessory  mesonephric  ducts,  pass  anteriorly  and  medially  in  contact  with 
the  posterior  cardinal  vein  and  open  into  a  small  chamber,  the  urinary  sinus, 
situated  on  the  dorsal  surface  of  the  anterior  end  of  the  cloaca.  The  two  urinary 
sinuses  of  the  two  sides  unite  into  a  common  chamber,  which  is  sometimes  called 
the  urinary  bladder.  It  does  not  correspond  to  the  bladder  of  higher  forms, 
since  it  consists  of  the  enlarged  terminations  of  the  mesonephric  ducts.  Cut 
into  this,  note  the  entrance  into  it  of  the  two  urinary  sinuses,  and  find  the  opening 
in  its  mid-dorsal  wall  by  which  it  opens  into  the  cloaca.  The  Wolffian  ducts  or 
ducts  of  the  cranial  mesonephros  are  slender  tubes  extending  anteriorly  from  the 
urinary  bladder,  lying  on  the  dorsal  surface  of  the  strong  white  portions  of  the 
mesotubaria. 

Draw,  showing  kidneys,  gonads,  and  their  ducts,  and  the  opened  cloaca. 

2.  The  male  urogenital  system. — The  testes  are  a  pair  of  soft  bodies  dorsally 
situated.  In  the  spiny  dogfish  they  are  located  in  the  anterior  part  of  the 
pleuroperitoneal  cavity,  dorsal  to  the  liver;  each  has  a  mesorchium.  In 
the  smooth  dogfish  they  are  long  and  slender  bodies  extending  the  length  of  the 
pleuroperitoneal  cavity,  their  toothed  posterior  ends  attached  to  the  mesentery 
of  the  rectal  gland.  They  are  more  or  less  fused  and  are  supported  by  a  single 
mesorchium.  In  the  skate  the  testes  are  broad,  flat  bodies  against  the  dorsal 
wall;  each  is  provided  with  a  mesorchium. 

The  kidneys  are  identical  with  those  of  the  females  and  should  be  next 
examined  according  to  the  directions  given  under  females.  In  the  male  skate, 
however,  the  cranial  part  of  the  kidney  is  very  much  better  developed  than  in 
the  female  and  extends  forward  as  a  firm  cylindrical  body  on  either  side  of  the 
mid-dorsal  line. 


COMPARATIVE  ANATOMY  OF  THE  UROGENITAL  SYSTEM  283 

As  explained  in  the  introduction  the  male  ducts  in  the  majority  of  vertebrates 
are  the  mesonephric  or  Wolffian  ducts.  In  mature  males  these  ducts  are  con- 
sequently much  larger  than  in  females.  The  Wolffian  ducts  run  posteriorly 
along  the  ventral  face  of  the  kidneys.  In  immature  specimens  each  is  a  slender, 
straight  tube,  similar  to  that  of  the  female,  but  in  mature  males,  it  is  greatly 
coiled.  The  testis  is  connected  with  the  cranial  mesonephros  by  means  of 
delicate  ducts,  the  vasa  e/erentia,  which  run  in  the  mesorchium  and  can  sometimes 
be  seen  by  holding  the  mesorchium  up  to  the  light.  The  vasa  efferentia  connect 
with  the  tubules  of  the  mesonephros.  The  greater  part  of  the  cranial  meso- 
nephros apparently  serves  in  male  elasmobranchs  for  transmitting  the  sperm, 
and  is  sometimes  called  the  epididymis,  since  it  corresponds  to  the  head  of  the 
epididymis  of  mammals.  The  sperm-bearing  tubules  of  the  mesonephros  then 
connect  with  the  Wolffian  duct.  Trace  this  duct  posteriorly.  Its  anterior  part 
is  greatly  coiled  in  mature  males,  but  in  dogfishes  (not  in  skate)  straightens  as 
it  approaches  the  caudal  mesonephros  and  in  all  three  animals  enlarges  upon  the 
surface  of  the  latter  to  form  the  seminal  vesicle.  Trace  it  by  removing  the 
pleuroperitoneum  from  the  ventral  face  of  the  caudal  mesonephros.  At  its 
posterior  ends  on  the  sides  of  the  cloaca,  the  seminal  vesicle  terminates  in  a  sac, 
the  sperm  sac,  which  projects  forward  as  a  blind  sac  lying  against  the  ventral 
surface  of  the  seminal  vesicle. 

Cut  open  the  cloaca  as  directed  under  the  female  and  identify  its  parts  as 
directed  there.  There  is  no  difference  in  the  cloaca  of  the  dogfish  between  the 
males  and  females,  but  in  the  male  skate  the  cloaca  is  very  much  smaller  than 
in  the  female  and  is  not  divided  into  intestinal  and  urogenital  parts.  In  the 
median  dorsal  line  there  is  in  the  male  skate  a  urogenital  papilla. 

The  sperm  sacs  should  now  be  cut  open  and  the  papillae,  where  the  seminal 
vesicles  open  into  them,  identified.  The  two  sperm  sacs  unite  at  their  posterior 
ends  to  form  a  urogenital  sinus  which  opens  at  the  tip  of  the  urinary  papilla. 

The  accessory  mesonephric  ducts  are  similar  to  those  of  the  females;  see  the 
description  under  females.  In  male  dogfishes  each  runs  along  the  medial  side 
of  the  seminal  vesicle  and  enters  the  sperm  sac  near  the  opening  of  the  vesicle. 
In  the  male  skate  several  accessory  ducts  pass  from  the  caudal  mesonephros 
into  the  sperm  sac. 

Draw,  showing  gonads,  kidneys,  and  their  ducts,  and  the  opened  cloaca. 

Look  in  male  dogfishes  for  remnants  of  the  ostium  and  oviducts  in  the  region 
of  the  liver. 

C.  THE  UROGENITAL  SYSTEM  OF  NECTURUS 

i.  The  female  urogenital  system. — The  ovaries  have  already  been  noted  as 
elongated  saclike  bodies  bearing  eggs  of  various  sizes.  Note  the  mesovarium. 
Lateral  to  each  ovary  running  along  the  dorsal  body  wall  is  the  oviduct,  a  thick, 
white,  coiled  tube  supported  by  the  mesotubarium.  Follow  it  anteriorly. 


284       LABORATORY  MANUAL  FOR  VERTEBRATE  ANATOMY 

At  the  anterior  end  of  the  pleuroperitoneal  cavity  it  becomes  of  a  more  delicate 
texture  and  is  fastened  to  the  lateral  wall.  Here  it  opens  by  a  funnel-shaped 
opening,  the  ostium;  the  dorsal  rim  of  this  is  fastened  to  the  body  wall,  but  the 
ventral  rim  is  free  and  can  be  lifted  to  expose  the  opening.  Trace  the  oviducts 
posteriorly  to  the  cloaca.  They  enter  this  one  to  each  side  of  the  large  intestine. 
Cut  the  cloaca  open  by  a  lateral  slit  extending  up  into  the  intestine.  Note  the 
papillae  by  which  the  oviducts  open  into  the  cloaca. 

The  kidneys  are  long  slender  organs  extending  from  the  cloaca  forward 
along  the  dorsal  surfaces  of  the  oviducts.  They  are  retroperitoneal.  They  are 
mesonephroi.  The  duct,  the  Wolffian  or  mesonephric  duct,  lies  along  the  lateral 
border  of  each  kidney.  It  is  very  delicate  in  the  female  and  difficult  to  locate. 
It  proceeds  to  the  cloaca  into  which  it  opens  to  the  dorsal  side  of  the  oviduct. 
In  tracing  it  make  a  cut  along  one  side  of  the  cloaca,  freeing  the  cloaca  from  the 
body  wall. 

Note  the  urinary  bladder  extending  from  the  midventral  region  of  the 
cloaca.  Find  its  opening  into  the  cloaca. 

Draw,  showing  ovaries,  kidneys,  their  ducts,  and  the  opened  cloaca. 

2.  The  male  urogenital  system. — The  testes  are  a  pair  of  elongated  bodies 
situated  to  the  sides  of  the  small  intestine.  Each  has  a  mesentery,  the  mesor- 
chium.  Dorsal  and  lateral  to  each  testis  is  the  long  brown  kidney,  larger  than 
in  the  female.  The  kidney  is  a  mesonephros.  Along  the  lateral  border  of  the 
kidney  is  a  conspicuous  coiled  duct,  the  Wolffian  or  mesonephric  duct.  This  also, 
as  in  vertebrates  in  general,  acts  as  the  sperm  duct.  By  holding  up  the  mesor- 
chium  to  the  light  note  the  delicate  ducts,  the  vasa  efferentia,  which  cross  it  into 
the  kidney.  The  sperm  pass  through  the  tubules  of  the  kidney  and  into  the 
Wolffian  duct.  Trace  the  latter  to  the  cloaca.  Open  the  cloaca  by  a  slit  to 
one  side  of  the  median  line  carrying  the  slit  into  the  large  intestine.  The  openings 
of  the  Wolffian  ducts  into  the  cloaca  are  generally  difficult  to  locate,  owing  to 
their  small  size.  Note  the  urinary  bladder  and  its  opening  into  the  ventral 
cloacal  wall. 

Draw,  showing  testes,  kidneys,  Wolffian  ducts,  and  opened  cloaca. 

D.      THE   UROGENITAL   SYSTEM   OF   THE   TURTLE 

Remove  the  digestive  tract,  if  not  already  done,  leaving  the  large  intestine 
in  place. 

i.  The  female  urogenital  system. — This  consists  as  usual  of  a  pair  of  ovaries 
and  a  pair  of  Mutterian  ducts  or  oviducts.  The  ovaries  have  already  been  noted 
as  large  baglike  bodies  in  the  posterior  part  of  the  pleuroperitoneal  cavity. 
They  usually  contain  yellow  eggs  in  various  states  of  development.  Each  ovary 
is  supported  by  a  mesentery,  the  mesovarium.  Along  the  posterior  border  of 
each  ovary  runs  the  oviduct,  a  large  white  coiled  tube,  supported  by  the  mesotu- 
barium.  Trace  the  oviduct  forward  and  find  the  ostium;  this  lies  in  the  mesen- 


COMPARATIVE  ANATOMY  OF  THE  UROGENITAL  SYSTEM  285 

tery  and  has  winglike  borders  which  are  generally  closed  together  and  should  be 
spread  apart  to  see  the  opening.  Trace  each  oviduct  to  the  cloaca.  Each  opens 
into  the  side  of  the  anterior  end  of  the  cloaca,  ventral  to  the  opening  of  the 
intestine.  The  stalk  of  the  large  bilobed  urinary  bladder  joins  the  cloaca  midway 
between  the  two  oviducts. 

The  cloaca  has  already  been  exposed.  (If  not,  do  so  by  cutting  through 
the  pelvic  girdle  on  each  side  and  removing  the  median  portion  of  the  girdle.) 
Clear  away  connective  tissue  from  around  the  cloaca.  Attached  to  each  side 
of  the  cloaca  posterior  to  the  oviducts  are  two  elongated  sacs,  the  accessory 
urinary  bladders.  Their  function  is  unknown  but  is  possibly  respiratory  or 
hydrostatic.  A  dark  structure  visible  through  the  wall  of  the  cloaca  is  the 
clitoris,  homologous  with  the  penis  of  the  male.  It  is  of  no  use  in  the  female. 

Now  cut  open  the  cloaca  to  one  side  of  the  clitoris,  extending  the  cut  in  the 
median  ventral  line  up  to  the  stalk  of  the  bladder.  Look  into  the  cloaca. 
Observe  that  the  clitoris  consists  simply  of  thickenings  in  the  ventral  wall. 
Find  the  large  openings  of  the  accessory  bladders.  Next  note  the  opening  of 
the  large  intestine.  This  is  the  most  dorsal  of  the  openings  and  is  somewhat 
separated  by  a  fold  from  the  urogenital  openings.  Ventral  to  the  opening  of 
the  intestine  are  the  openings  of  the  oviducts  on  thickened  papillae.  They 
are  best  found  by  cutting  into  the  oviduct  and  probing  posteriorly  into  the 
cloaca.  Between  and  ventral  to  the  oviducal  openings  is  the  opening  of  the 
urinary  bladder. 

The  kidneys  of  the  turtle  are  metanephroi.  They  have  already  been  identified 
as  flattened  lobed  organs  fitting  snugly  against  the  posterior  end  of  the  pleuro- 
peritoneal  cavity.  The  renal  portal  vein  and  its  tributary,  the  internal  iliac,  run 
along  the  ventral  face  of  each  kidney.  Dissect  off  this  vein;  directly  dorsal  to 
it  is  a  tube,  the  metanephric  duct  or  ureter y  extending  from  the  middle  of  the 
kidney  to  the  cloaca.  It  enters  the  cloaca  at  the  base  of  the  oviduct.  By 
making  a  slit  in  it  and  passing  a  probe  into  it  its  opening  into  the  cloaca  will  be 
found  just  anterior  to  the  thickening  caused  by  the  oviducal  entrance. 

Draw  the  female  urogenital  system  with  opened  cloaca. 

2.  The  male  urogenital  system. — The  male  system  consists  of  the  paired 
testes  and  their  ducts.  The  ducts  of  the  testes  are  the  Wolffian  ducts,  now  called 
the  vasa  deferentia. 

Expose  the  cloaca  as  directed  in  the  female  and  find  the  two  accessory 
bladders  attached  to  its  lateral  walls.  Note  the  place  of  attachment  of  the 
rectum  to  the  cloaca  and  ventral  to  this  the  attachment  of  the  urinary  bladder. 
The  dark  mass  seen  through  the  ventral  wall  of  the  cloaca  is  the  penis  or  organ 
of  copulation.  A  rounded  mass  projects  from  the  anterior  wall  of  the  cloaca 
to  either  side  of  the  stalk  of  the  bladder;  they  are  parts  of  the  penis  and  are 
called  the  bulbs  of  the  corpora  cavernosa.  Muscles  which  retract  the  penis  will 
be  seen  attached  to  the  ventral  wall  of  the  cloaca. 


286       LABORATORY  MANUAL  FOR  VERTEBRATE  ANATOMY 

The  kidneys  were  previously  identified  as  flattened  lobed  bodies  fitting 
against  the  posterior  wall  of  the  pleuroperitoneal  cavity.  Each  testis  is  a  yellow 
spherical  body  attached  to  the  ventral  face  of  the  kidney  by  the  mesorchium. 
lateral  and  posterior  to  the  testis  is  an  elongated,  dark-colored  coiled  body,  the 
epididymis.  The  testis  is  connected  to  the  anterior  part  of  the  epididymis 
by  the  minute  vasa  e/erentia  which  run  in  the  mesorchium.  The  vasa  efferentia 
and  this  portion  of  the  epididymis  are  the  remnants  of  the  mesonephros.  The 
remainder  of  the  epididymis  constitutes  the  male  duct  or  vas  deferens  (Wolffian 
duct).  Remove  the  peritoneal  covering  of  the  epididymis  and  uncoil  the  vas 
deferens.  Trace  it  to  the  cloaca.  It  enters  anterior  to  and  at  the  base  of  the 
bulb  of  the  penis. 

Next  cut  open  the  cloaca,  inserting  the  blade  of  the  scissors  into  one  corner 
of  the  anus  and  cutting  far  to  one  side  to  avoid  injuring  the  penis.  Spread 
apart  the  cloacal  walls  and  study  the  penis.  It  consists  of  two  spongy  ridges, 
the  corpora  cavernosa  or  cavernous  bodies,  in  the  ventral  wall  of  the  cloaca. 
Between  these  folds  in  the  midventral  line  is  a  deep  groove,  the  urethral 
groove,  which  in  the  natural  condition  is  practically  converted  into  a  tube  by  the 
approximation  of  the  cavernous  bodies.  The  urethral  groove  terminates  caudad 
at  the  base  of  a  heart-shaped  projection,  the  glans  of  the  penis.  The  anterior 
ends  of  the  cavernous  bodies  form  the  bulbs  already  noted,  which  project  forward 
into  the  coelom  at  the  sides  of  the  stalk  of  the  bladder.  The  bulbs  are  filled 
with  blood  which  they  receive  from  the  internal  iliac  vein.  All  parts  of  the  penis 
are  highly  spongy  and  vascular.  In  the  sexual  act  the  blood  from  the  bulbs 
rushes  into  the  spongy  spaces  of  the  cavernous  bodies  and  the  glans,  erecting 
them,  and  causing  the  cavernous  bodies  to  come  in  contact  above  the  urethral 
groove,  converting  the  latter  into  a  canal  for  the  passage  of  the  sperm. 

The  kidneys  are  metanephroi,  and  their  ducts  the  metanephric  ducts  or 
ureters.  The  ureter  will  be  found  immediately  to  the  dorsal  side  of  the  epididymis 
which  should  be  removed.  The  ureter  is  a  short  straight  tube  proceeding  to 
the  cloaca,  into  which  it  opens  just  anterior  to  the  opening  of  the  vas  deferens. 
The  two  openings  will  be  found  at  the  sides  of  the  anterior  beginning  of  the 
urethral  groove. 

Find  the  openings  of  the  accessory  bladders,  the  urinary  bladder,  and  the 
rectum  into  the  cloaca.  The  latter  is  dorsal  to  the  urogenital  openings. 

Draw  the  male  urogenital  system  with  open  cloaca. 

E.      THE  UROGENITAL   SYSTEM  OF  THE   PIGEON 

i.  The  female  urogenital  system. — Remove  the  digestive  tract,  leaving  the 
large  intestine  in  place.  In  adult  birds  there  is  a  single  ovary  and  oviduct  on  the 
left  side.  The  right  ovary  and  duct  are  present  in  the  embryo  but  almost 
entirely  disappear  before  hatching.  The  ovary  is  a  mass  containing  eggs  of 


COMPARATIVE  ANATOMY  OF  THE  UROGENITAL  SYSTEM  287 

various  sizes,  situated  at  the  anterior  end  of  the  left  kidney.  It  is  attached  by 
a  short  mesovarium.  Posterior  to  the  ovary  the  coiled  left  oviduct  proceeds  to 
the  cloaca,  being  supported  by  the  mesotubarium.  The  ostium  is  situated  in 
the  mesotubarium  near  the  ovary;  it  is  a  wide  opening  with  winglike  borders 
fastened  to  the  mesotubarium.  A  small  remnant  of  the  right  oviduct  is  attached 
to  the  right  side  of  the  cloaca. 

The  kidneys  are  metanephroi.  Each  is  a  flattened,  three-lobed  organ  situated 
against  the  dorsal  wall.  The  ureters  or  metanephric  ducts  are  located  just  dorsal 
to  the  renal  portal  veins  which  should  be  stripped  from  the  face  of  the  kidney. 
The  ureter  begins  on  each  side  at  the  groove  between  the  anterior  and  middle 
lobes  of  the  kidney  and  extends  straight  posteriorly  to  the  cloaca.  The  left 
ureter  is  concealed  by  the  oviduct. 

The  cloaca  is  an  expanded  chamber  receiving  the  rectum  on  its  median 
ventral  surface,  the  left  oviduct  to  the  left,  the  very  small  right  oviduct  to  the 
right,  and  the  ureters  dorsal  to  the  oviducts.  Cut  into  the  cloaca  to  the  right 
of  the  rectum.  Note  that  the  cavity  of  the  cloaca  is  subdivided.  There 
is  a  large  ventral  portion  (coprodaeum)  into  which  the  rectum  opens.  Dor- 
sal to  this  and  separated  from  it  by  a  fold  is  the  urodaeum  into  which  open 
the  oviducts  and  ureters.  The  opening  of  the  left  oviduct  is  readily  found  here; 
the  openings  of  the  ureters  are  more  medial  and  smaller.  The  most  dorsal 
compartment  of  the  cloaca  is  the  proctodaeum,  a  small  chamber  with  a  raised 
rim,  which  opens  to  the  anus.  In  the  anterior  wall  of  the  proctodaeum  dorsal 
to  the  rim  an  opening  may  be  noted;  it  leads  into  a  small  pouch  which  seems  to 
be  functional  in  young  birds  but  degenerates  with  maturity.  It  is  called  the 
bursa  of  Fabricius. 

Draw  urogenital  system  and  cloaca. 

2.  The  male  urogenital  system. — The  testes  are  a  pair  of  oval  organs  at  the 
anterior  ends  of  the  kidneys;  their  size  varies  considerably  with  the  season. 
The  kidneys  are  the  same  as  in  the  female,  and  the  description  given  under  the 
female  should  be  read  and  the  ureters  identified.  The  male  ducts,  vasa  deferentia 
(Wolffian  ducts),  spring  from  the  medial  border  near  the  posterior  end  of  the 
testes  and  pass  posteriorly  parallel  to  the  ureters.  They  are  slender  convoluted 
tubes.  Trace  both  ureters  and  vasa  deferentia  to  the  cloaca. 

The  cloaca  is  smaller  than  in  the  female  and  the  lips  of  the  anus  more  protrud- 
ing. The  rectum  enters  medially  and  ventrally,  the  urogenital  ducts  laterally. 
Cut  into  the  cloaca  as  directed  under  female  and  identify  its  chambers  as  there 
described.  They  are  the  same  in  the  two  sexes,  except  that  the  urodaeum  is 
smaller  and  receives  the  two  vasa  deferentia  instead  of  the  oviduct.  Ureters 
and  vasa  deferentia  open  on  small  papillae  in  the  lateral  walls  of  the  urodaeum. 
There  is  no  penis  in  most  birds. 

Draw 


288  LABORATORY  MANUAL  FOR  VERTEBRATE  ANATOMY 

F.      THE   UROGENITAL   SYSTEM   OF   THE   MAMMAL 

Remove  the  digestive  tract,  leaving  the  rectum  in  place. 

i.  The  kidneys  and  ureters. — The  kidneys  of  mammals  are  metanephroi 
and  their  ducts  the  metanepkric  ducts  or  ureters.  The  kidneys  are  large  oval 
organs  situated  on  the  dorsal  body  wall  of  the  peritoneal  cavity.  As  in  all 
vertebrates  they  are  retroperitoneal.  The  right  kidney  is  generally  considerably 
anterior  to  the  left  one.  Clear  away  fat  and  connective  tissue  from  about  the 
kidneys.  The  medial  face  of  each  kidney  is  concave;  this  concavity  is  called 
the  hilus.  From  the  hilus  a  white  tube,  the  ureter,  passes  out  and  turns  poste- 
riorly. Follow  the  ureters  caudad,  clearing  away  the  fat  from  about  them  and 
note  their  entrance  into  the  bladder.  In  females  the  ureters  pass  dorsal  to  the 
horns  of  the  uterus.  In  males  each  passes  dorsal  to  a  white  cord,  the  male  duct 
or  vas  deferens,  which  loops  over  the  ureter  and  disappears  dorsal  to  the  bladder. 

Remove  by  a  cut  the  ventral  half  of  a  kidney.  A  cavity,  the  sinus,  is 
revealed  within  the  hilus;  this  sinus  is  occupied  chiefly  by  the  renal  pelvis,  or 
expanded  beginning  of  the  ureter,  and  also  by  the  renal  artery  and  vein.  Into 
the  pelvis  the  substance  of  the  kidney  projects  as  the  renal  papilla  on  which  are 
situated  the  microscopic  openings  of  the  collecting  tubules.  The  kidney  sub- 
stance is  readily  divided  into  two  areas,  a  peripheral  region,  the  cortex,  and  a 
central  region,  the  medulla.  The  cortex  contains  the  renal  corpuscles  and  the 
convoluted  and  looped  portions  of  the  kidney  tubules.  The  medulla  is  marked 
by  lines  which  converge  to  the  renal  papilla;  these  lines  are  the  collecting  tubules. 
It  will  be  recalled  that  the  collecting  tubules,  the  pelvis,  and  the  ureter  are 
outgrowths  of  the  Wolffian  duct.  The  collecting  tubules  and  renal  papilla 
together  form  a  pyramid,  of  which  there  is  but  one  in  the  rabbit  and  cat  but 
about  twelve  in  man. 

Draw. 

The  urinary  bladder  is  a  pear-shaped  sac  at  the  posterior  end  of  the  peritoneal 
cavity.  It  is  ventral  to  the  rectum  in  the  male,  ventral  to  both  rectum  and 
uterus  in  the  female.  The  free  anterior  end  of  the  bladder  is  named  the  apex 
or  vertex,  the  posterior  portion  the  fundus.  The  fundus  continues  posteriorly 
as  a  narrowed  stalk,  the  urethra1  (also  called  neck  of  the  bladder).  The  bladder 
is  covered  by  the  peritoneum,  which  is  continuous  with  that  of  the  abdominal 
wall  by  means  of  the  median  and  lateral  ligaments  previously  noted.  The 
pouch  between  the  bladder  and  rectum  (male)  or  bladder  and  uterus  (female) 
is  named  the  rectovesical  or  vesicouterine  pouch,  respectively. 

Draw  the  excretory  system. 

1  The  term  urethra  is  in  much  confusion  in  comparative  anatomy,  owing  to  the  differences  between 
the  urogenital  systems  of  various  mammals.  Although  in  the  embryo  the  urethra  is  the  same  as  the 
urogenital  sinus,  this  is  not  the  case  hi  the  adults  of  most  mammals,  and  consequently  the  use  of  urethra 
as  synonymous  with  urogenital  sinus  appears  to  be  inadvisable.  Urethra  is  therefore  here  employed 
hi  the  same  sense  as  in  human  anatomy,  that  is,  as  the  name  of  the  duct  leading  from  the  bladder  to 
the  exterior. 


COMPARATIVE  ANATOMY  OF  THE  UROGENITAL  SYSTEM  289 

2.  The  female  reproductive  system. — This  consists  of  a  pair  of  ovaries  and 
their  ducts.  The  ovaries  are  very  small  oval  bodies  located  at  the  sides  of  the 
peritoneal  cavity  at  the  anterior  end  of  the  coils  of  the  uterus.  Each  will  be 
seen  to  bear  little  clear  vesicles,  the  Graafian  follicles,  each  of  which  contains 
an  egg  or  ovum;  in  pregnant  females  the  ovary  also  bears  little  hard  lumps, 
the  corpora  lutea,  which  represent  follicles  from  which  the  eggs  of  the  pregnancy 
were  discharged.  The  ovary  is  suspended  by  the  mesovarium,  which  extends 
forward  to  the  kidney  and  is  continuous  posteriorly  with  the  ligament  of  the 
uterus. 

The  ducts  of  the  ovaries  are,  as  in  other  vertebrates,  the  Mullerian  ducts  or 
oviducts,  but  they  are  differentiated  into  several  distinct  parts  in  mammals. 
The  uppermost  portion  of  the  oviducts  is  a  slender  convoluted  tube  which 
passes  lateral  to  the  ovary  and  curves  over  its  anterior  end;  its  mesentery,  the 
mesosalpinx,  forms  a  sort  of  hood,  partly  inclosing  the  ovary.  This  portion  of 
the  oviduct  is  the  uterine  or  Fallopian  tube.  It  opens  in  front  of  the  ovary 
(rabbit)  or  to  the  lateral  side  of  it  (cat)  by  the  ostium  having  fringed  borders, 
the  fimbriae.  On  tracing  the  uterine  tube  posteriorly  it  is  found  to  widen 
suddenly  into  a  thick-walled  tube,  the  uterus  (rabbit)  or  horn  of  the  uterus  (cat). 
The  size  of  this  depends  on  whether  the  animal  is  pregnant  or  not;  in  pregnant 
animals  the  uteri  or  horns  are  greatly  enlarged  and  exhibit  a  series  of  swellings, 
each  of  which  contains  an  embryo  (these  will  be  examined  later).  The  strong 
fold  of  peritoneum  supporting  the  uteri  or  horns  is  the  mesometrium.  Mesova- 
rium, mesosalpinx,  and  mesometrium  together  are  called  the  broad  ligament  of 
the  uterus  in  mammals.  The  round  ligament  of  the  uterus  is  the  fold  extending 
from  the  beginning  of  the  uterus  or  horn  posteriorly  to  the  body  wall;  it  is  con- 
tinuous with,  but  at  right  angles  to,  the  broad  ligament.  In  the  cat  the  two 
horns  of  the  uterus  unite  in  the  median  line,  dorsal  to  the  bladder,  to  a  single 
tube,  the  body  of  the  uterus.  Body  and  horns  together  constitute  the  uterus  or 
womb,  but  the  young  develop  only  in  the  horns.  In  the  rabbit  the  two  uteri  are 
separate  along  their  entire  lengths,  and  consequently  there  is  no  division  into 
body  and  horns.  In  the  cat  the  body  of  the  uterus  continues  posteriorly  as  the 
vagina;  in  the  rabbit  the  two  uteri  join  the  vagina;  the  vagina  is  a  tube  situated 
in  the  median  line  between  the  bladder  and  the  rectum.  It  exits  through  the 
ring  formed  by  the  pelvic  girdle  and  vertebral  column. 

The  external  genital  parts  or  external  genitalia  were  described  with  the 
external  anatomy.  Review  this  (p.  29).  Then  in  the  rabbit  make  an  incision 
through  the  skin  forward  from  the  vulva.  In  the  median  line  beneath  the  skin 
is  a  hardened  body,  the  clitoris,  homologous  with  the  penis  of  the  male.  Its 
anterior  end  is  attached  by  ligaments  to  the  ischium  and  pubic  symphysis.  Cut 
across  the  clitoris  and  note  the  two  cavernous  bodies  of  which  it  is  composed. 
In  the  cat  the  clitoris  is  minute. 


290       LABORATORY  MANUAL  FOR  VERTEBRATE  ANATOMY 

Now  cut  through  the  pubic  and  ischial  symphyses  and  spread  the  legs  well 
apart.  Trace  the  urethra,  the  vagina,  and  the  rectum  posteriorly.  The  urethra 
lies  at  first  on  the  ventral  face  of  the  vagina  to  which  it  is  bound  by  tissue;  it 
then  unites  with  the  vagina  to  form  a  common  tube,  the  urogenital  canal  or  sinus. 
Dissect  this  free  and  lift  it  out  and  follow  it  to  the  urogenital  aperture.  Cut 
open  the  urogenital  aperture  and  note  the  free  posterior  end,  or  glans,  of  the 
clitoris,  projecting  into  the  cavity  in  the  rabbit.  Free  the  rectum  from  the  uro- 
genital canal  and  follow  it  to  the  anus.  Along  its  sides  in  the  rabbit  are  a  pair  of 
elongated  glands,  the  anal  glands.  In  the  cat  the  rounded  anal  glands  or  sacs 
are  situated  one  at  each  side  of  the  rectum,  just  internal  to  the  anus. 

Draw  the  female  urogenital  system. 

Cut  open  the  vagina.  In  the  rabbit  note  the  external  uterine  orifice 
with  raised  fringed  lips  by  means  of  which  each  uterus  opens  into  the  vagina. 
The  rabbit  uterus  is  of  the  duplex  type  (Fig.  63  B,  p.  278).  In  the  cat  the  body 
is  divided  into  lateral  halves  by  a  median  partition,  a  horn  opening  to  each  side 
of  the  partition.  The  cat  uterus  is  of  the  bipartite  type  (Fig.  636").  The  lower 
end  of  the  uterus,  called  the  cervix,  projects  into  the  vagina  by  a  fold.  The 
opening  of  the  body  of  the  uterus  into  the  vagina  is  the  external  uterine  orifice. 

3.  The  male  reproductive  system. — The  external  parts  or  external  genitalia 
were  described  with  the  external  anatomy.  Review  this  (p.  29).  The  two  testes 
are  lodged  in  the  scrotal  sac  which  is  divided  into  two  compartments  by  an 
internal  partition.  Cut  through  the  skin  over  and  in  front  of  one  testis,  exposing 
the  testis  as  an  oval  white  body.  Clear  away  the  connective  tissue  anterior 
to  the  testis  and  find  a  white  cord,  the  spermatic  cord,  passing  forward  and  entering 
the  peritoneal  cavity  through  a  canal  called  the  inguinal  canal.  The  external 
end  of  this  canal  is  the  external  inguinal  ring;  the  internal  end,  the  internal 
inguinal  ring.  Trace  the  spermatic  cord  by  cutting  open  the  canal. 

The  spermatic  cord  contains  a  white  duct,  the  male  duct  or  vas  deferens, 
and  the  blood  vessels  and  nerves  of  the  testis.  The  two  vasa  deferentia  turn 
toward  the  median  line,  loop  over  the  ventral  surfaces  of  the  ureters,  and  dis- 
appear on  the  dorsal  surface  of  the  urethra. 

It  is  now  necessary  to  explain  these  relations  of  the  testes  and  their  ducts.  In  the  verte- 
brates previously  studied  the  testes  lie  within  the  peritoneal  cavity,  but  in  most  adult  mammals 
they  are  situated  posterior  to  this  cavity.  In  the  embryos  of  male  mammals,  however,  the 
testes  lie  within  the  peritoneal  cavity;  subsequently  they  descend  caudad.  There  is  first 
formed  a  sac  of  the  body  wall,  the  scrotal  sac,  which  contains  all  of  the  layers  of  the  body  wall — 
skin,  muscles,  and  peritoneum — and  incloses  a  portion  of  the  coelom,  the  vaginal  sac.  The 
testes  then  descend  into  the  scrotal  sac,  as  shown  in  Figure  65,  carrying  with  them  their  ducts, 
the  nerves,  blood  vessels,  etc.  The  canal  along  which  the  descent  occurs  later  narrows  and 
eventually  is  completely  obliterated  in  the  higher  mammals  but  in  some  remains  open.  (See 
K,  p.  373.)  A  new  canal,  the  inguinal  canal,  is  later  secondarily  formed  around  the  spermatic 
cord.  The  descent  of  the  testes  explains  the  peculiar  looping  of  the  vasa  deferentia  over  the 
ureters  and  the  course  of  the  internal  spermatic  vein.  The  scrotum  is  now  believed  to  serve 
as  a  temperature  regulating  mechanism  for  the  testes. 


COMPARATIVE  ANATOMY  OF  THE  UROGENITAL  SYSTEM 


291 


Now  cut  through  the  pubic  and  ischial  symphyses  and  spread  the  legs  apart. 
Trace  the  vasa  deferentia  and  the  urethra  caudad,  separating  them  from  the 
rectum.  The  ureters  may  be  cut  through  and  the  urinary  bladder  held  poste- 
riorly. The  vasa  deferentia  pass  along  the  dorsal  surface  of  the  urethra.  In  the 
rabbit  they  enlarge  and  enter  an  expanded  sac,  the  seminal  vesicle,  which  pouches 
forward  between  the  rectum  and  the  bladder.  Cut  into  the  seminal  vesicle  and 
note  the  openings  into  its  ventral  wall  of  the  two  vasa  deferentia,  and  in  its 
posterior  dorsal  wall  the  thickening  caused  by  the  prostate  gland.  Find  the  union 
of  the  seminal  vesicle  and  urethra  to  form  a  common  tube,  the  urogenital  sinus. 
In  the  cat  the  two  vasa  deferentia  join  the  urethra  without  the  formation  of  a 
seminal  vesicle,  the  point  of  junction  being  surrounded  by  a  slight  enlargement, 


vas  deferens 


peritoneal 
cavity 
/          vas  deferens 


\ 


epididymis 


gubernaculum 


FIG.  65. — Diagrams  to  illustrate  the  descent  of  the  testis  in  the  male  mammal.  The  testis  descends 
into  the  scrotum  which  is  a  sac  of  the  body  wall  containing  a  portion  of  the  peritoneal  cavity  called  the 
vaginal  sac;  in  the  descent  the  gubernaculum  or  ligament  of  the  testis  shortens;  the  passage  x  along 
which  the  descent  occurs  and  which  at  first  forms  a  connection  between  the  vaginal  sac  and  the  peri- 
toneal cavity  is  later  completely  obliterated  in  the  higher  mammals;  it  does  not  correspond  to  the 
inguinal  canal.  (From  Prentiss  and  Arey's  Textbook  of  Embryology,  courtesy  of  the  W.  B.  Saunder 
Company.) 

the  prostate  gland.  The  common  tube  thus  formed  is  the  urogenital  canal  or 
sinus.  The  bulbourethral  glands  or  Cowper's  glands  are  small  swellings  situated 
on  the  urogenital  canal  shortly  posterior  to  the  prostate  gland  in  the  rabbit, 
about  an  inch  posterior  in  the  cat.  The  terminal  inch  of  the  urogenital  canal  is 
inclosed  in  the  penis.  Cut  into  the  prepuce  and  note  the  pointed  projection 
within  it,  called  the  glans  of  the  penis.  Note  that  the  prepuce  is  simply  a  fold 
of  skin  around  the  glans.  At  the  tip  of  the  glans  is  the  urogenital  opening.  The 
glans  in  the  cat  bears  a  number  of  minute  spines.  Dissect  anteriorly  from  the 
glans  exposing  the  remainder  of  the  penis  as  a  hardened  cylindrical  structure. 
Find  where  the  urogenital  canal  enters  its  anterior  end.  Note  also  the  strong 
attachments  of  the  penis  to  the  pelvic  region.  Cut  across  the  middle  of  the 
penis  and  note  that  it  is  composed  of  two  cylindrical  bodies,  the  corpora  cavern- 
osa  or  cavernous  bodies  closely  placed.  The  urogenital  canal,  here  called  the 
cavernous  urethra,  lies  on  the  dorsal  side  of  the  penis  resting  in  a  depression 


292       LABORATORY  MANUAL  FOR  VERTEBRATE  ANATOMY 

between  the  two  cavernous  bodies.  At  the  anterior  end  of  the  penis  the  two 
cavernous  bodies  diverge,  forming  the  crura  of  the  penis,  which  are  attached  to 
the  ischia.  The  cavernous  bodies  are  spongy  structures  and  in  the  sexual  act 
become  distended  with  blood  so  that  the  penis  is  caused  to  project  out  of  its 
sheath,  the  prepuce. 

Draw  the  parts  of  the  male  genital  system. 

Trace  the  rectum  to  the  anus  following  directions  given  for  the  female. 

The  structure  of  the  testis  may  now  be  investigated.  Each  testis  is  inclosed 
in  a  white  fibrous  sac,  the  parietal  portion  of  the  tunica  vaginalis,  which  is  in 
reality  the  peritoneal  pouch  made  by  the  descent  of  the  testis  (Fig.  65).  Cut 
open  this  sac,  exposing  the  cavity  in  which  the  testis  lies,  this  being  a  part  of  the 
peritoneal  cavity.  The  tunica  vaginalis  which  lines  this  cavity  is  reflected  over 
the  surface  of  the  testis  as  the  visceral  portion  of  the  tunica  vaginalis,  which  forms 
the  outer  thin  coat  of  the  testis.  The  point  of  deflection  lies  along  the  mid-dorsal 
line  of  the  scrotal  sac,  and  a  mesentery  is  formed  along  this  line  between  the  sac 
and  the  testis.  This  mesentery  is  the  mesorchium,  corresponding  to  the  broad 
ligament  of  the  uterus.  The  posterior  end  of  the  testis  is  a  ttached  to  the  posterior 
wall  of  the  scrotum  by  a  short  but  stout  ligament,  the  gubernaculum,  continuous 
with  the  mesorchium.  The  gubernaculum  corresponds  to  the  round  ligament 
of  the  uterus.  The  duct  of  the  testis  is  the  vas  deferens  or  Wolffian  duct.  It  lies 
along  the  dorsal  surface  of  the  testis  much  coiled,  the  coiled  portion  being  named 
the  epididymis.  The  epididymis  begins  at  the  anterior  end  of  the  testis  as  a 
coil,  the  head  of  the  epididymis.  It  then  passes  down  the  dorsal  surface  of  the 
testis  as  a  coiled  tube,  the  body  of  the  epididymis.  At  the  posterior  end  of 
the  testis  it  forms  another  coiled  mass,  the  tail  of  the  epididymis,  to  which  the 
gubernaculum  is  attached.  From  this  the  vas  deferens  proceeds  anteriorly, 
much  convoluted,  and  passes  into  the  inguinal  canal  where  it  becomes  a  straight 
tube.  The  head  of  the  epididymis  is  derived  from  the  mesonephros  and  is 
connected  with  the  tubules  of  the  testis  by  minute  vasa  ejferentia.  The  remainder 
of  the  epididymis  and  the  vas  deferens  are  the  Wolffian  duct. 

Draw,  showing  contents  of  the  scrotum. 

G.   THE  EMBRYONIC  MEMBRANES 

i.  General. — There  are  four  embryonic  membranes  in  vertebrates.  They  are:  the  yolk 
sac,  the  allantois,  the  amnion,  and  the  chorion  (Fig.  66). 

The  yolk  sac  is  simply  an  evagination  from  the  ventral  wall  of  the  intestine.  Its  connec- 
tion with  the  intestine  forms  a  narrow  yolk  stalk.  The  yolk  sac  occurs  in  the  embryos  of  all 
vertebrates  having  meroblastic  eggs  and  also  in  mammals,  for  they  are  descended  from  forms 
which  have  meroblastic  eggs.  The  yolk  sac  is  filled  with  yolk,  except  in  mammalian  embryos 
where  it  is  empty. 

The  allantois  is  a  large  evagination  from  the  floor  of  the  cloaca.  It  is  primarily  respira- 
tory in  function,  but  also  probably  serves  to  hold  embryonic  excretory  materials.  The  adult 
bladder  develops  at  the  base  of  the  allantoic  stalk. 


COMPARATIVE  ANATOMY  OF  THE  UROGENITAL  SYSTEM 


293 


The  amnion  and  the  chorion  are  formed  by  a  fold  of  the  body  wall  (somatopleure)  which 
rises  up  around  the  embryo.  The  folds  of  the  two  sides  meet  above  the  embryo  and  fuse 
across.  The  outer  limb  of  the  fold  becomes  the  chorion,  the  inner  limb  the  amnion  (see  Fig. 
66 A  and  B).  The  amnion  forms  a  sac  inclosing  the  embryo.  The  chorion  is  the  outermost 


FIG.  66. — Diagrams  to  illustrate  the  mode  of  formation  of  the  embryonic  membranes  of  amniotes 
and  of  the  placenta.  A,  cross-section  through  an  early  stage  of  the  formation  of  the  amnion  and 
the  chorion;  a  fold  of  the  somatopleure  is  seen  rising  up  at  a;  the  outer  wall  of  the  fold  b  becomes  the 
chorion,  the  inner  wall  c,  the  amnion.  B,  later  stage  after  completion  of  the  process  of  formation  of  the 
amnion  and  chorion;  the  fold  fuses  across  above  the  dorsal  surface  of  the  embryo  forming  two  mem- 
branes, an  outer  chorion  b  which  incloses  embryo  and  yolk  sac,  and  an  inner  amnion  c,  which  incloses 
the  embryo.  C,  sagittal  section  of  a  later  stage  of  the  embryo  to  show  the  origin  of  the  allantois  k  as  an 
evagination  from  the  digestive  tract  m.  D,  later  stage  following  C,  showing  the  spreading  of  the 
allantois  between  the  chorion  and  the  amnion  and  yolk  sac;  note  that  the  outer  wall  of  the  allantois 
is  in  contact  with  the  chorion,  the  two  together  forming  the  chorio-allantoic  membrane  q.  E,  formation 
of  the  placenta  in  mammals  by  the  penetration  of  the  chorio-allantoic  membrane  q  into  the  wall  of  the 
uterus  n;  the  penetration  takes  the  form  of  treelike  ingrowths  which  are  called  the  chorionic  villi  o;  note 
small  size  of  the  mammalian  yolk  sac  d;  the  placenta  consists  of  the  inner  part  of  the  uterine  wall  and 
the  chorionic  villi;  the  latter  are  generally  restricted  to  certain  areas  of  the  chorio-allantoic  membrane. 
In  C,  D,  and  E  the  two  layers  of  which  the  chorion,  amnion,  allantois,  and  yolk  sac  are  each  composed 
are  omitted  for  simplicity,  a,  amniotic  fold  of  the  somatopleure;  b,  chorion;  c,  amnion;  d,  yolk  sac; 
e,  somatic  mesoderm;  /,  splanchnic  mesoderm;  g,  ectoderm;  h,  entoderm;  i,  notochord;  j,  neural 
tube;  k,  allantois;  /,  body  of  embryo,  head  to  the  left;  m,  digestive  tract  of  embryo;  n,  wall  of  uterus; 
o,  chorionic  villi;  p,  placenta;  q,  chorio-allantoic  membrane.  (Suggested  by  figures  in  Hertwig.) 

membrane  of  the  embryo.    The  yolk  sac  and  allantois  are  between  chorion  and  amnion  on 
the  ventral  side  of  the  embryo  (Fig.  66C). 

The  yolk  sac  and  allantois  are  highly  vascular,  their  blood  vessels  being  named  the  vitelline 
and  umbilical  (allantoic)  vessels.  Much  has  already  been  said  of  the  vitelline  veins.  The 


294       LABORATORY  MANUAL  FOR  VERTEBRATE  ANATOMY 

umbilical  veins  are  the  abdominal  veins  of  lower  forms.    The  amnion  and  the  chorion  never 
contain  any  blood  vessels. 

The  yolk  sac  may  occur  in  the  embryos  of  any  group  of  vertebrates.  The  allantois, 
amnion,  and  chorion  occur  only  in  reptiles,  birds,  and  mammals,  which  are  hence  designated 
amniotes.  The  allantois  spreads  out  inside  of  the  chorion  and  becomes  fused  to  the  chorion 
forming  a  chorio-allantoic  membrane  (Fig.  66D).  In  the  placental  mammals  this  chorio- 
allantoic  membrane  comes  into  close  contact  with  the  internal  wall  of  the  uterus  and  hi  the 
highest  mammals  fuses  inseparably  with  the  uterine  wall  (Fig.  66E).  The  compound  structure 
thus  produced  by  approximation  or  fusion  of  the  chorio-allantoic  membrane  with  the  uterine 
wall  is  designated  the  placenta.  It  is  of  various  shapes  in  various  mammals,  the  name  placenta 
being  derived  from  the  disk  shape  of  the  human  placenta. 

2.  Anamniote  embryo  of  the  dogfish. — Cut  open  the  pregnant  uterus  of  a 
dogfish  and  remove  an  embryo  or  examine  embryos  provided.     Note  that  the 
embryo  is  naked.     From  the  middle  of  its  ventral  wall  hangs  the  large  yolk  sac, 
filled  with  yolk  and  attached  to  the  body  by  the  narrowed  yolk  stalk.     The  yolk 
sac  is  covered  externally  by  a  layer  of  the  body  wall  and  internally  consists  of 
the  intestinal  wall  inclosing  the  yolk.     As  the  yolk  is  used  up  the  yolk  sac  is 
gradually  drawn  into  the  body.     Draw. 

3.  Amniote  embryo  of  the  cat. — If  pregnant  females  are  available,  open  one  of 
the  enlargements  in  the  horns  of  the  uterus.     The  enlargement  contains  an 
embryo.     Note  that  the  embryo  is  inclosed  in  a  thin  membrane,  the  amnion. 
On  the  inner  surface  of  the  uterine  wall  at  the  enlargement  note  a  thickened 
vascular  ring  of  tissue.    This  is  the  placenta;  it  will  probably  peel  off  from 
the  uterine  wall,  especially  in  advanced  stages  of  pregnancy.     Cut  into  the 
amnion  and  note  the  umbilical  cord  extending  from  the  ventral  side  of  the 
abdomen  of  the  embryo  to  the  inner  surface  of  the  amnion.     The  umbilical  cord 
is  a  connection  between  the  embryo  and  its  own  membranes  and  not,  as  popularly 
supposed,  a  connection  between  the  embryo  and  the  mother.     There  is  no 
direct  connection  between  embryo  and  mother. 

H.      SUMMARY  OF  THE  UROGENITAL  SYSTEM 

1.  The  urogenital  system  is  derived  from  the  mesomere  of  the  embryo. 

2.  The  urinary  or  excretory  system  consists  of  the  paired  kidneys  and  their  ducts.    The 
kidneys  are  composed  of  tubules  opening  at  one  end  into  the  coelom  by  a  nephrostome  and  at 
the  other  end  into  the  collecting  duct. 

3.  In  the  evolution  of  the  vertebrates  there  have  been  three  successive  kidneys,  each 
situated  more  posteriorly  than  its  predecessor. 

4.  The  first  kidney  of  vertebrates  is  called  the  pronephros.    Its  duct  is  the  pronephric 
duct.    It  appears  hi  the  embryos  of  all  vertebrates  but  functions  in  the  adults  of  only  cyclo- 
stomes  and  a  few  fishes. 

5.  The  second  kidney  is  the  mesonephros  or  Wolffian  body.     Its  duct  is  the  mesonephric 
or  Wolffian  duct;  this  is  a  continuation  of  the  pronephric  duct.    It  enters  the  cloaca.     The 
mesonephros  is  the  functional  kidney  of  most  adult  fishes  and  all  Amphibia. 

6.  The  third  kidney  is  the  metanephros.     Its  duct  is  the  metanephric  duct  or  ureter. 
The  ureter  and  part  of  the  metanephros  arise  by  evagination  from  the  Wolffian  duct.    The 
metanephros  is  the  functional  kidney  of  the  adults  of  all  the  amniotes. 


COMPARATIVE  ANATOMY  OF  THE  UROGENITAL  SYSTEM  295 

7.  A  urinary  bladder  is  generally  present  as  an  evagination  of  the  ventral  wall  of  the 
cloaca.    It  receives  the  ducts  of  the  kidneys  in  mammals  only.     It  opens  into  the  cloaca  in  all 
forms  except  adult  placental  mammals. 

8.  The  ovaries  and  testes  are  paired  bodies  developed  from  the  ventral  surface  of  the 
mesomere  and  projecting  into  the  coelom. 

9.  The  ovaries  are  always  located  within  the  peritoneal  or  pleuroperitoneal  cavity  in  all 
vertebrates.    The  ducts  of  the  ovaries  are  the  Miillerian  ducts  or  oviducts.    They  are  supposed 
to  be  derived  from  the  pronephric  duct  by  splitting,  but  arise  in  this  way  only  in  elasmobranchs. 

10.  The  oviducts  never  have  any  direct  connection  with  the  ovaries.    They  open  into 
the  coelom  near  the  ovaries  by  an  ostium  which  is  believed  to  represent  one  or  more  nephro- 
stomes. 

11.  The  oviducts  enter  the  cloaca  separately  except  in  the  placental  mammals.    In  these 
latter  the  oviducts  are  more  or  less  united  and  differentiated  into  regions.    This  union  proceeds 
from  their  posterior  ends  anteriorly,  forming  in  the  lowest  placental  mammals  first  a  common 
vagina,  in  higher  forms  a  partially  fused  uterus  with  separate  horns,  and  in  the  primates  a 
single  uterus  produced  by  the  fusion  of  two  originally  separate  uteri.    The  upper  portions  of 
the  oviducts  form  narrow  uterine  tubes  which  always  remain  separate. 

12.  The  testes  are  located  internally  except  in  the  higher  mammals,  where  they  descend 
temporarily  or  permanently  into  pouches  of  the  body  wall  located  externally  in  the  inguinal 
region. 

13.  The  ducts  of  the  testes  in  all  vertebrates  (except  cyclostomes  and  teleostomes)  are  the 
Wolfiian  or  mesonephric  ducts,  also  called  the  vasa  deferentia.    In  the  males  of  groups  where 
the  mesonephros  is  functional  the  Wolffian  ducts  have  both  genital  and  excretory  functions. 
In  groups  where  a  metanephros  is  present  the  Wolfiian  ducts  have  only  genital  functions. 

14.  The  Wolffian  duct,  vas  deferens,  is  always  directly  connected  to  the  testis  by  the 
intervention  of  a  portion  of  the  mesonephros.    This  is  differentiated  into  the  vasa  efferentia 
and  the  epididymis  in  part.    The  remainder  of  the  epididymis  is  part  of  the  Wolffian  duct. 
In  male  amniotes  a  portion  of  the  mesonephros  thus  always  persists  as  part  of  the  male  system; 
in  female  amniotes  the  mesonephros  takes  no  part  in  the  functional  female  apparatus. 

15.  A  cloaca  which  receives  the  intestine  and  urogenital  ducts  is  present  in  most  verte- 
brates (cyclostomes,  teleostomes  and  placental  mammals  excepted).     In  placental  mammals 
the  cloaca  splits  into  a  dorsal  portion  which  receives  the  intestine  and  opens  to  the  exterior 
by  the  anus,  and  a  ventral  portion — the  urogenital  canal  or  sinus — which  receives  the  blad- 
der and  the  urogenital  ducts.     The  bladder  in  mammals  develops  a  stalk,  the  urethra,  which 
leads  to  the  exterior.     The  ureters  shift  so  as  to  open  directly  into  the  bladder.    In  all  male 
placental  mammals  the  vasa  deferentia  (Wolfiian  ducts)  join  the  urethra  to  form  a  urogenital 
canal;  in  most  female  placental  mammals  the  vagina  similarly  unites  with  the  urethra  form- 
ing a  urogenital  canal ;  but  in  primates  the  urethra  and  vagina  open  separately  to  the  exterior, 
a  urogenital  canal  being  absent.     The  male  urogenital  canal  is  generally  provided  in  mammals 
with  various  glands,  whose  secretion  is  believed  to  be  of  importance  for  the  sperm;  its  terminal 
portion  traverses  the  penis  or  organ  of  copulation  and  forms  the  glans  of  the  penis.     The 
penis  is  formed  by  the  union  of  two  spongy  bodies,  the  cavernous  bodies.     It  begins  in  reptiles, 
where  the  cavernous  bodies  are  separate. 


Xin.     THE  COMPARATIVE  ANATOMY  OF  THE  NERVOUS  SYSTEM 
AND  THE  SENSE  ORGANS 

A.      GENERAL   CONSIDERATIONS 

In  this  section  of  the  manual  we  shall  include  both  the  nervous  system  proper  and  the 
sense  organs.  The  former  functions  for  the  conduction,  co-ordination,  and  correlation  of 
stimuli,  while  the  latter  is  differentiated  for  the  reception  of  stimuli. 

1.  The  parts  of  the  nervous  system. — The  nervous  system  consists  of  three  parts:   the 
central  nervous  system,  the  peripheral  nervous  system,  and  the  sympathetic  system.     The 
central  nervous  system  is  composed  of  the  brain,  situated  within  the  skull,  and  of  the  spinal 
cord,  situated  within  the  neural  canal  formed  by  the  neural  arches  of  the  vertebrae.     Brain 
and  spinal  cord  are  made  up  of  both  nerve-cell  bodies  (gray  matter)  and  of  nerve-cell  processes 
(white  matter).    The  peripheral  nervous  system  consists  of  the  cranial  nerves  springing  from 
the  brain,  and  the  spinal  nerves,  springing  from  the  spinal  cord.     The  nerves  are  markedly 
metameric  in  arrangement,  a  pair  being  typically  present  for  each  segment.     Nerves  are 
composed  of  nerve-cell  processes  only.    The  sympathetic  nervous  system  controls  and  regulates 
in  general  the  involuntary  activities  and  the  organs  which  subserve  those  functions,  as  the 
heart,  the  digestive  tract,  the  smooth  musculature  in  general,  the  secreting  glands,  blood  ves- 
sels, respiratory  and  reproductive  systems.     It  consists  chiefly  of  a  paired  cord  lying  against 
the  dorsal  wall  of  the  coelom  from  which  branches  and  networks  extend  into  the  viscera.    The 
sympathetic  system  is  connected  with  the  peripheral  nervous  system  of  which  indeed  it  is 
an  outgrowth.    Associated  with  both  of  these  systems  are  numerous  ganglia,  often  segmen- 
tally  arranged.    A  ganglion  is  a  collection  of  nerve-cell  bodies,  situated  outside  of  the  central 
nervous  system.    Within  the  central  nervous  system  a  similar  collection  is  called  a  nucleus 
or  center,  although  a  few  of  these  are  designated  ganglia,  as  retentions  from  an  older  termi- 
nology. 

2.  The  development  of  the  nervous  system. — The  central  nervous  system  is  formed  as 
previously  learned  by  the  infolding  of  the  ectoderm  in  the  median  dorsal  line  of  the  embryo. 
A  tube  extending  the  length  of  the  embryo  is  thus  produced.     That  portion  of  the  tube 
situated  within  the  head  develops  into  the  brain,  while  that  portion  posterior  to  the  head 
becomes  the  spinal  cord.    The  originally  single  layer  of  enrolled  ectoderm  cells  proliferates  to 
form  a  thick  zone  of  cells  around  the  central  cavity.     Most  of  these  become  nerve  cells, 
while  the  remainder  give  rise  to  supporting  cells.     In  the  spinal  cord  of  all  vertebrates  and 
in  the  brains  of  the  lower  ones  the  zone  of  nerve  cells  retains  its  primitive  position  around 
the  central  cavity,  but  in  the  higher  vertebrates  there  is  considerable  migration  of  nerve  cells 
to  the  periphery  of  the  brain.     Axones  and  dendrites  arise  from  the  nerve  cells  by  outgrowth; 
part  of  these  remain  in  the  central  nervous  system  forming  a  peripheral  zone  of  fibers,  called 
the  white  matter.     Part  of  the  processes  of  the  nerve  cells  grow  out  from  the  central  nervous 
system  (or  grow  into  it  from  sense  organs  and  ganglia)  forming  the  nerves  or  peripheral 
nervous  system.     The  ganglia  of  the  peripheral  nervous  system  arise  chiefly  from  the  neural 
crests;  these  are  a  pair  of  longitudinal  cords  of  ectodermal  cells  which  are  left  outside  of  the 
neural  tube  at  the  tune  of  its  closure.     The  sympathetic  ganglia  arise  by  the  migration  of 
cells  from  the  neural  tube  or  from  the  ganglia  of  the  peripheral  nervous  system. 

In  the  development  of  the  brain  the  original  simple  tube  is  first  marked  off  into  three 
vesicles,  the  primary  brain  vesicles,  by  two  constrictions  (Fig.  67).  These  visicles  are  named 

«o6 


COMPARATIVE  ANATOMY  OF  THE  NERVOUS  SYSTEM 


297 


the  forebrain  or  prosencephalon,  the  midbrain  or  mesencephalon,  and  the  hindbrain  or  rhomb- 
encephalon.  Subsequently  the  first  and  third  vesicles  subdivide  into  two.  There  thus 
arise  the  five  principal  lobes  of  the  brain,  arranged  in  a  longitudinal  series  (Fig.  67).  These 
are  named,  beginning  anteriorly:  the  telencephalon,  the  diencephalon  or  thalamencephalon, 
the  mesencephalon  or  midbrain,  the  melencephalon  or  cerebellum,  and  the  myelencephalon  or 
medulla  oblongata.  Each  of  these  five  divisions  becomes  further  complicated  by  additional 
evaginations,  foldings,  thickenings,  etc.,  but  nevertheless  remains  as  a  clearly  marked  region 
of  the  adult  brain.  The  parts  derived  from  each  of  these  divisions  will  be  studied  in  connec- 
tion with  the  specimens.  The  cavity  of  the  brain  becomes  the  ventricles  of  the  adult  brain. 
The  spinal  cord  develops  chiefly  by  thickening  of  the  lateral  walls  of  the  original  neural 
tube.  The  central  cavity  is  reduced  to  a  small  canal,  the  central  canal.  The  cord  consists 
of  an  internal  zone  of  gray  matter  and  external  zone  of  white  matter  formed  as  described  above. 


B 


FIG.  67. — Diagrams  to  show  the  development  of  the  vertebrate  brain.  A,  early  stage,  showing 
the  three  primary  brain  vesicles,  prosencephalon  a,  mesencephalon  b,  and  rhombencephalon  c.  B,  later 
stage,  showing  division  of  the  prosencephalon  into  telencephalon  d  and  diencephalon  e  and  of  the 
rhombencephalon  into  metencephalon  /  and  myelencephalon  g.  C,  adult  stage  of  a  lower  vertebrate, 
showing  enlargement  of  the  telencephalon  to  form  the  cerebral  hemisphere  i,  differentiation  of  the 
olfactory  lobe  h,  and  development  of  outgrowths  j,  k,  I  from  the  roof  of  the  telencephalon  and  dien- 
cephalon and  of  the  infundibulum  n  from  the  floor  of  the  diencephalon.  D,  adult  stage  of  a  higher 
vertebrate,  illustrating  further  enlargement  of  the  cerebral  hemisphere  i.  a,  prosencephalon;  b,  mesen- 
cephalon; c,  rhombencephalon;  d,  telencephalon;  e,  diencephalon;  /,  metencephalon;  g,  myelen- 
cephalon or  medulla  oblongata;  h,  olfactory  lobe  or  bulb;  i,  cerebral  hemisphere;  jt  paraphysis  or 
evagination  of  roof  of  telencephalon;  k,  parietal  eye  and  /,  pineal  body,  evaginations  from  the  roof 
of  the  diencephalon;  m,  cerebellum;  n,  infundibulum;  o,  pituitary  body.  (From  Parker  and  Haswell's 
Textbook  of  Zoology,  courtesy  of  the  Macmillan  Company.) 

The  dorsal  portions  of  the  gray  matter  are  named  the  dorsal  columns,  the  ventral  portions 
the  ventral  columns,  and  in  some  regions  of  the  cord  there  are  distinct  lateral  columns  (Fig.  68). 
3.  The  functional  composition  of  the  nervous  system. — The  nervous  functions  are  divisible 
into  two  great  classes,  sensory  and  motor.  The  term  sensory  applies  to  those  impulses 
which  come  into  the  nervous  system  from  the  sense  organs,  to  the  nerves  which  conduct 
such  impulses,  and  to  the  tracts,  nuclei,  or  areas  of  the  central  nervous  system  which  are 
concerned  with  these  impulses.  The  dorsal  half  of  the  central  nervous  system  is  sensory. 
The  term  motor  is  similarly  applied  to  the  nerves,  tracts,  areas,  etc.,  which  are  concerned 
with  the  initiation  and  conduction  from  the  central  nervous  system  of  impulses  which  excite 
parts  such  as  the  muscles  and  glands  to  activity.  The  ventral  half  of  the  central  nervous 
system  is  motor.  The  sensory  and  motor  functions  are  each  subdivisible  into  somatic  and 


298 


LABORATORY  MANUAL  FOR  VERTEBRATE  ANATOMY 


visceral  components,  the  former  dealing  with  structures  in  the  body  wall,  the  latter  with  those 
situated  in  the  viscera.  The  visceral  impulses  both  motor  and  sensory  are  always  transmitted 
by  way  of  the  sympathetic  system,  except  in  the  case  of  the  visceral  muscles  of  the  gill  region. 
The  spinal  nerves  transmit  all  four  classes  of  impulses,  while  the  cranial  nerves  are  irregular 
in  this  regard.  The  somatic  sensory  impulses  arise  in  the  sense  organs  peripherally  located 
in  the  body  wall  and  pass  along  nerves  whose  cell  bodies  are  nearly  always  situated  in  the 
ganglia  of  the  peripheral  nervous  system;  the  visceral  sensory  impulses  arise  in  nerve  endings 
in  the  viscera;  both  pass  into  the  central  nervous  system.  The  somatic  motor  impulses 
arise  from  motor  cells  within  the  central  nervous  system  and  pass  out  in  the  nerves  to  the 
voluntary  muscles;  the  visceral  motor  impulses  arise  in  the  brain  and  cord,  nearly  always 


dorsal  root 


visceral  sensory 
nerve  cell 


somatic  sensory 
nerve  cells 


spinal  ganglion 


somatic  motor 
nerve  cells 


-skin 
muscle 


dorsal  ramus 

somatic  sensory  fibers 

somatic  motor  fibers 
ventral  ramus 


white  matter 


ventral  root 

communicating 
ramus 

visceral  motor 
nerve  cell 


epithelium  of 
digestive  tract 

muscle  layer 
of  digestive  tract 


•  visceral  motor  fiber 
*  visceral  sensory  fiber 


FIG.  68. — Diagram  of  a  cross-section  through  the  spinal  cord  and  a  spinal  nerve  to  show  the 
functional  components  of  the  spinal  nerve  and  their  relation  to  the  spinal  cord  and  sympathetic  system. 
Somatic  motor  fibers,  heavy  continuous  lines;  visceral  motor  fibers,  light  continuous  lines;  somatic 
sensory  fibers,  broken  lines;  visceral  sensory  fibers,  dotted  lines.  (Slightly  altered  from  Herrick's 
Introduction  to  Neurology,  courtesy  of  the  W.  B.  Saunders  Company.) 

make  a  relay  in  the  sympathetic  ganglia  or  arise  in  those  ganglia,  and  pass  to  the  involun- 
tary muscles,  glands,  etc.  (Fig.  68). 

In  addition  to  the  foregoing,  it  will  naturally  be  understood  that  a  considerable  part 
of  the  central  nervous  system  is  concerned  with  the  correlation  and  co-ordination  of  the 
foregoing  four  classes  of  functions. 

4.  The  peripheral  nervous  system. — The  spinal  nerves  will  serve  as  examples,  as  they  are 
more  typical.  Each  spinal  nerve  is  connected  with  the  spinal  cord  by  two  roots  or  bundles 
of  fibers,  a  dorsal  and  a  ventral.  The  dorsal  root  bears  a  ganglion,  the  dorsal  or  spinal  ganglion, 
which  consists  of  a  collection  of  sensory  nerve  cells.  The  fibers  springing  from  these  cells 
make  up  most  of  the  dorsal  root.  This  root  enters  the  spinal  cord  and  connects  with  the 
dorsal  gray  column.  The  ventral  root  has  no  ganglion;  its  fibers  arise  from  the  somatic 
motor  cells  of  the  ventral  column  and  the  visceral  motor  cells  of  the  lateral  column  of  the 
gray  matter  of  the  spinal  cord  (Fig.  68).  Beyond  the  ganglion  both  roots  unite  to  form  a 


COMPARATIVE  ANATOMY  OF  THE  NERVOUS  SYSTEM  299 

single  spmal  nerve  which  passes  out  through  the  intervertebral  foramen  and  almost  immedi- 
ately divides  into  three  branches  or  rami — the  dorsal,  ventral,  and  visceral  rami.  Although, 
as  intimated  above,  the  dorsal  root  is  almost  purely  sensory,  entirely  so  in  the  higher  verte- 
brates, and  the  ventral  root  purely  motor,  the  rami  are  mixed.  The  dorsal  and  ventral 
rami  both  contain  somatic  motor  and  sensory  fibers  passing  to  and  from  the  body  wall  and 
also  include  a  few  visceral  fibers.  The  somatic  motor  fibers  of  the  dorsal  rami  pass  to  the 
epaxial  muscles,  those  of  the  ventral  rami  to  the  hypaxial  muscles.  The  visceral  ramus 
(also  called  ramus  communicant)  connects  with  a  sympathetic  ganglion.  It  carries  visceral 
motor  fibers  from  the  spinal  cord  into  the  sympathetic  system,  these  constituting  the  white 
ramus;  and  also  carries  visceral  sensory  fibers  from  the  ganglion  into  the  cord,  these  forming 
the  gray  ramus  (Fig.  68).  The  white  and  gray  rami  together  form  the  visceral  or  com- 
municating ramus,  which  serves  typically  to  connect  each  spinal  ganglion  with  the  adjacent 
ganglion  of  the  sympathetic  system. 

The  spinal  nerves  are  paired  and  segmentally  arranged.  There  are  in  general  as  many 
pairs  of  spinal  nerves  as  body  segments  below  the  head.  In  connection  with  the  paired 
appendages  the  ventral  rami  of  several  successive  spinal  nerves  form  a  network  or  plexus 
from  which  the  nerves  to  the  appendage  arise.  This  innervation  shows:  that  the  limb  muscles 
arise  from  the  hypaxial  parts  of  the  myotomes,  since  the  ventral  rami  supply  only  hypaxial 
muscles;  that  the  limb  muscles  are  derived  from  several  myotomes,  since  there  is  but  one 
spinal  nerve  to  each  segment  of  the  body;  and  that  the  muscles  of  the  appendages  have  under- 
gone much  torsion  and  change  of  position,  resulting  in  a  crisscross  arrangement  of  their 
nerves,  since  each  nerve  retains  its  innervation  to  the  muscle  which  it  originally  supplied. 

The  cranial  nerves  are  much  less  typical  in  arrangement  than  the  spinal  nerves.  Most 
of  them  do  not  contain  the  four  classes  of  fibers.  They  are  attached  to  the  brain  by  roots, 
but  these  are  irregularly  arranged.  Those  cranial  nerves  that  contain  somatic  sensory 
fibers  bear  a  ganglion  corresponding  to  a  spinal  ganglion.  The  composition  and  functions 
of  the  cranial  nerves  will  be  described  in  the  dissections.  There  are  ten  cranial  nerves  in 
anamniotes  and  twelve  in  amniotes,  but  the  additional  two  are  not  new  formations.  The 
cranial  nerves  are  not  segmentally  arranged,  although  probably  so  arranged  in  the  be- 
ginning. 

5.  The  segmentation  of  the  head. — It  was  previously  stated  that  the  vertebrate  head  was 
originally  segmented,  but  the  segmentation  is  now  much  obscured.  In  attempting  to  work 
out  the  head  segmentation,  the  brain,  the  cranial  nerves,  the  visceral  arches,  and  the  true 
head  muscles  (not  visceral  muscles)  have  been  studied.  The  brain  is  plainly  segmented, 
particularly  in  its  posterior  part ;  these  segments  are  called  neuromeres.  Probably  the  poste- 
rior neuromeres  are  true  segmental  divisions.  The  cranial  nerves  are  now  much  altered 
from  their  original  condition  but  they  were  formerly  segmental  nerves,  and  a  certain  amount 
of  evidence  has  consequently  been  obtained  from  a  study  of  them.  The  visceral  arches 
are  plainly  segmentally  arranged,  but  the  relation  of  their  segmentation  to  that  of  the  head 
as  a  whole  is  not  entirely  clear.  It  is  generally  believed  that  the  gill  slits  are  intersegmental 
in  position,  i.e.,  occur  at  the  myosepta.  The  best  evidence  has  been  obtained  from  the  study 
of  the  myotomes  of  the  head.  In  cyclostomes  all  of  these  form  muscles  of  the  adult,  but  in 
other  vertebrates  many  of  them  disappear  in  embryonic  stages.  Those  which  persist  form 
the  muscles  of  the  eyeball  and  the  intrinsic  musculature  of  the  tongue.  (The  student  should 
recollect  that  the  apparent  head  muscles  are  visceral  muscles,  derived  from  the  hypomeres, 
and  not  from  the  myotomes.)  The  various  lines  of  evidence  lead  to  the  following  conclusions: 
that  there  are  four  head  segments  in  front  of  the  ear  of  which  the  first  (anterior  head  cavity) 
is  evanescent,  while  the  next  three  (named  i,  2,  and  3)  give  rise  to  the  muscles  of  the  eyeball; 
behind  the  ear  the  number  of  head  segments  appears  to  be  variable,  about  6-8  in  forms  above 
cyclostomes,  more  in  cyclostomes.  These  postotic  segments  develop  no  muscles  above 


3oo 


LABORATORY  MANUAL  FOR  VERTEBRATE  ANATOMY 


cyclostomes  except  that  the  intrinsic  muscles  of  the  tongue  appear  to  be  derived  from  them. 
These  matters  will  be  clearer  after  a  study  of  the  cranial  nerves.  (See  also  Fig.  69.) 

6.  The  development  of  the  sense  organs. — There  are  three  chief  sense  organs  to  be  consid- 
ered here — the  nose,  the  eye,  and  the  ear.  The  nose  arises  as  a  pair  of  invaginations  of  the  ecto- 
derm of  the  anterior  end  of  the  head.  In  fishes  these  persist  as  a  pair  of  simple  olfactoiy  sacs 
not  connected  with  the  mouth.  From  Amphibia  onward  the  olfactory  sacs  become  connected 
with  the  oral  cavity  by  the  nasal  passages  which  are  then  both  olfactory  and  respiratory. 
The  olfactory  function  is  generally  limited  to  the  dorsal  region  of  the  nasal  sac.  In  most 
vertebrates  the  walls  of  the  nasal  passages  project  into  the  passages  as  curiously  rolled  or 
folded  structures,  the  turbinals  or  conchae,  which  serve  to  increase  the  olfactory  surface  and 
to  warm,  strain,  and  moisten  the  air  passing  in. 

The  eyes  arise  in  part  as  evaginations  of  the  diencephalon.  The  tip  of  the  evagination 
then  invaginates  producing  a  double-walled  cup,  the  optic  cup.  (See  K,  Fig.  228,  p.  213.) 


notochord 


boundary  between 
head  and  trunk. 


superficial 
ophthalmic 

deep  ophthalmic 
prechordal 


spinal  nero 


myotome 


maxillary 
mandibular 


gill  slits 


pretrematic  branch 
posttrematic  branch 


FIG.  69. — Diagram  to  illustrate  the  segmentation  of  the  vertebrate  head  and  the  relation  of  the 
cranial  nerves  to  the  segmentation.  The  numbers  above  the  figure  designate  the  cranial  nerves;  the 
numbers  in  the  figure  are  situated  on  the  head  myo tomes;  the  sensory  part  of  the  nerves  is  represented 
by  heavy  continuous  lines;  the  motor  part  by  broken  lines.  The  anterior  head  cavity  is  the  first 
myotome  and  therefore  the  myotome  which  is  numbered  i  is  really  the  second  myotome  and  so  on; 
but  as  the  myotomes  were  numbered  before  the  anterior  head  cavity  was  discovered,  the  old  numbers  are 
generally  retained.  The  myotomes  numbered  I,  2,  and  3  produce  the  eye  muscles;  those  numbered 
4, 5,  and  6  degenerate  in  the  majority  of  vertebrates;  those  from  7  on  probably  contribute  to  the  tongue 
musculature  but  never  form  typical  parietal  muscles  such  as  occur  in  the  trunk.  It  is  seen  from  the 
figure  that  the  third  cranial  nerve  and  the  deep  ophthalmic  branch  of  the  fifth  belong  to  the  first  (really 
second)  head  segment;  the  fourth  and  remainder  of  the  fifth  to  the  second  (third)  segment;  the  sixth 
and  seventh  to  the  third  (fourth)  segment;  the  ninth  to  the  fourth  (fifth)  segment;  and  the  tenth  to  the 
fifth  to  eighth  (sixth  to  ninth)  segments.  The  gill  slits  are  intersegmental  in  location.  The  relation 
of  the  cranial  nerves  to  the  gill  slits  should  also  be  noted.  (After  Goodrich  in  Part  IX  of  Lankester's 
Treatise  of  Zoology,  courtesy  of  the  Macmillan  Company.) 

The  internal  wall  of  the  optic  cup  develops  into  the  retina  or  light-perceiving  layer  of  the 
eye.  The  external  wall  of  the  cup  becomes  the  pigment  layer  of  the  retina.  The  optic 
nerve  arises  in  the  retina  and  passes  down  the  stalk  of  the  cup  into  the  brain.  The  retina 
is  a  part  of  the  brain  wall,  as  shown  by  its  manner  of  formation.  The  lens  of  the  eye  arises 
from  an  invagination  of  the  ectoderm  over  the  optic  cup.  The  surface  ectoderm  then  remains 
as  the  conjunctiva  of  the  eyeball.  The  other  parts  or  coats  of  the  eyeball  (sclera,  cornea, 
chorioid,  and  iris)  are  developed  from  the  mesenchyme  surrounding  the  optic  cup. 

The  vertebrate  ear  in  its  complete  form  consists  of  three  chambers,  the  internal  ear, 
the  middle  ear,  and  the  external  ear.  The  internal  ear  alone  is  present  in  fishes.  It  arises 
as  a  saclike  invagination  of  the  ectoderm  of  the  head  at  the  level  of  the  hindbrain.  This 


COMPARATIVE  ANATOMY  OF  THE  NERVOUS  SYSTEM  301 

sac  sinks  internally  and  by  constrictions  differentiates  into  various  chambers  and  ducts 
which  will  be  seen  in  the  dissection.  The  middle  ear  appears  in  Amphibia.  It  is  a  chamber 
produced  by  an  evagination  from  the  first  visceral  pouch  and  remains  connected  with  the 
pharyngeal  cavity  by  the  stalk  of  the  outgrowth  called  the  auditory  (Eustachian)  tube. 
The  middle  ear  contains  little  bones  for  transmitting  the  sound;  there  are  three  of  these  in 
mammals,  which  were  treated  with  the  skull.  The  external  ear  begins  in  reptiles  and  birds 
and  is  complete  in  mammals.  It  consists  of  a  passage  invaginated  from  the  region  of  the 
first  gill  slit;  this  passage  is  the  external  auditory  meatus.  The  inner  end  of  this  passage 
comes  in  contact  with  the  wall  of  the  middle  ear,  the  two  walls  then  fusing  to  form  a  mem- 
brane of  double  origin,  the  tympanic  membrane,  commonly  called  the  eardrum.  The 
external  orifice  of  the  meatus  is  surrounded  by  an  outgrowth,  the  pinna  (generally  called 
"ear"),  for  catching  sound  waves. 

For  more  complete  accounts  of  the  development,  comparative  anatomy,  and  functions 
of  the  central  nervous  systems  and  sense  organs,  the  appropriate  chapters  in  K,  W,  or  Wd 
should  be  consulted. 

B.      THE   NERVOUS   SYSTEM  AND   SENSE   ORGANS   OF   ELASMOBRANCHS 

i.  The  spinal  nerves  and  fin  plexi. — Remove  all  of  the  viscera  including  the 
kidneys  and  reproductive  organs  from  the  pleuroperitoneal  cavity.  Note 
against  the  dorsal  coelomic  wall  dorsal  to  the  pleuroperitoneum  the  white  nerves 
passing  out  at  segmental  intervals.  These  are  the  ventral  rami  of  the  spinal 
nerves.  They  lie  along  the  myocommata.  In  the  spiny  dogfish  they  are 
buried  in  the  muscle  and  will  be  revealed  by  cutting  along  the  myocommata. 
Farther  laterally  they  emerge  to  the  internal  surface.  Trace  the  ventral  rami 
into  the  hypaxial  muscles. 

In  the  regions  of  the  paired  fins  the  ventral  rami  supply  the  muscles  of  the 
fins  and  are  more  or  less  united  with  each  other  to  form  a  plexus.  The  plexus 
for  the  posterior  appendage  is  the  lumbosacral  plexus,  for  the  anterior  appendage, 
the  cermcobrachial  plexus.  These  plexi  are  as  follows: 

Dogfish:  The  lumbosacral  plexus  to  the  pelvic  fin  is  found  by  cutting 
through  the  skin  on  the  dorsal  side  of  the  base  of  the  fin.  On  carefully  separat- 
ing the  fin  muscles  from  those  of  the  trunk  the  nerves  of  the  plexus  are  seen  as 
white  cords  passing  into  the  base  of  the  fin.  They  are  more  or  less  imbedded 
in  connective  tissue  which  should  be  carefully  cleaned  away.  There  are  ten 
nerves  passing  into  the  fin  of  which,  however,  only  the  last  ones  are  united  by 
cross-branches  to  form  a  plexus.  The  first  of  the  ten  is  called  the  collector  nerve. 
Trace  it  forward  and  note  that  it  is  formed  by  the  union  of  branches  from  the 
ventral  rami  anterior  to  the  fin. 

The  cervicobrachial  plexus  to  the  pectoral  fin  is  located  by  cutting  through 
the  skin  at  the  base  of  the  fin  on  the  ventral  side.  On  separating  the  skin  from 
the  muscles  of  the  trunk  nerves  will  be  seen  passing  in  the  connective  tissue 
to  the  pectoral  fin.  Proceed  carefully  forward,  carrying  your  cut  into  the  coelom 
at  the  side  of  the  esophagus.  The  plexus  is  then  seen  to  consist  of  a  number  of 
nerves  (nine  in  the  smooth  dogfish,  eleven  hi  the  spiny  species)  passing  from  the 


302       LABORATORY  MANUAL  FOR  VERTEBRATE  ANATOMY 

spinal  cord  into  the  fin.  Only  the  first  four  or  five  of  these,  situated  on  the  dorsal 
side  of  the  bag  formed  by  the  posterior  cardinal  sinus,  are  united  by  cross- 
branches  to  form  a  true  plexus,  the  posterior  ones  passing  directly  into  the  fin. 

Skate :  A  large  number  of  ventral  rami  supply  the  pectoral  fin,  the  anterior 
ones  uniting  to  a  plexus.  Strip  off  the  pleuroperitoneum  at  the  level  of  the  sub- 
clavian  artery  and  note  there  the  enormous  nerve  trunk  of  the  brachial  plexus. 
It  is  formed  by  the  union  (within  the  neural  canal)  of  a  large  number  of  ventral 
rami.  This  will  be  seen  later.  Follow  out  the  nerve  trunk  to  the  pectoral  fin. 
It  lies  along  the  posterior  side  of  the  curved  cartilage  (propterygium)  which  is 
situated  in  the  pectoral  fin  about  halfway  from  the  mid-dorsal  line  to  the  margin. 
Cut  through  skin  and  muscles  on  the  dorsal  side  of  the  animal  along  the  posterior 
and  lateral  side  of  this  cartilage  and  expose  the  trunk.  It  supplies  only  the 
anterior  part  of  the  pectoral  fin.  The  posterior  part  as  already  noted  is  supplied 
by  direct  ventral  rami,  not  forming  a  plexus.  The  lumbosacral  plexus  for  the 
pelvic  fin  is  located  as  follows.  Remove  the  skin  from  the  base  of  the  fin  on  the 
dorsal  side.  This  exposes  a  fan-shaped  layer  of  muscles.  Cut  through  this  and 
just  ventral  to  it  will  be  found  a  number  of  nerves  which  diverge  into  the  fin 
muscles. 

2.  The  sense  organs. — For  the  rest  of  the  section  a  large  separate  head  will 
generally  be  provided.  In  that  case  the  specimens  used  up  to  this  point  may  be 
discarded.  A  very  careful  dissection  of  this  head,  on  which  the  student  will  be 
graded,  is  required. 

a)  The  ampullae  of  Lorenzini:  It  has  already  been  noted  that  the  skin  of 
the  head  is  perforated  by  pores,  from  which  mucus  exudes  under  pressure. 
Note  the  distribution  of  the  pores.     Remove  a  piece  of  skin  from  a  region  bear- 
ing pores  (in  the  skate  from  the  ventral  side  of  the  head)  and  note  that  each 
pore  leads  into  a  canal  of  varying  length  lying  beneath  the  skin.     Each  canal, 
named  the  canal  of  Lorenzini,  terminates  in  a  little  bulb,  the  ampulla  of  Lorenzini, 
which  is  supplied  by  a  nerve,  a  delicate  white  fiber  easily  seen  attached  to  the 
ampulla.     The  function  of  this  sensory  apparatus  appears  to  be  the  perception 
of  vibration  and  pressure  in  the  surrounding  water. 

b)  The  lateral  line  system:  In  fishes  and  Amphibia  (larval  stages  only  of  land 
Amphibia]  there  is  present  a  system  of  sense  organs,  called  the  lateral  line  system, 
which  is  related  to  the  aquatic  mode  of  life.     This  system  is  completely  lost, 
together  with  the  nerves  which  supply  it,  in  the  land  vertebrates.     It  consists 
of  the  lateral  line  canals  and  the  lateral  line  nerves.     The  lateral  line  canals  in 
fishes  consist  of  tubes  situated  on  the  inner  surface  of  the  skin  onto  which  they 
open  by  pores.     In  the  canals  are  sensory  cells  which  closely  resemble  the  sen- 
sory cells  of  the  ear,  and,  in  fact,  the  lateral  line  system  appears  to  be  related 
to  the  internal  ear  both  morphologically  and  functionally.     The  function  of  the 
lateral  line  system  is  believed  to  be  the  perception  of  water  vibrations  of  low 
frequency. 


COMPARATIVE  ANATOMY  OF  THE  NERVOUS  SYSTEM  303 

Dogfish:  Along  the  trunk  the  system  consists  of  the  lateral  lines,  which 
mark  the  position  of  a  canal.  Find  the  lateral  line  on  the  head.  Remove  the 
skin  at  this  place,  noting  the  underlying  canal  and  the  pores  connecting  the  canal 
with  the  surface.  Trace  the  lateral  line  forward,  removing  the  skin  as  you  pro- 
ceed. At  the  level  of  the  spiracles  the  canals  of  the  two  lateral  lines  are  connected 
by  the  supratemporal  canal.  Anterior  to  this,  each  forks  into  a  supraorbital 
canal  passing  forward  above  the  eye  and  an  infraorbital  canal  passing  ventrally 
between  the  eye  and  the  spiracle  and  then  forward  below  the  eye.  Trace  the 
supraorbital  canal  to  the  end  of  the  rostrum;  here  it  turns  and  proceeds  pos- 
teriorly again  parallel  to  its  former  course  and  becomes  continuous  with  the 
infraorbital  canal.  The  latter  gives  off  a  hyomandibular  branch  running  poste- 
riorly along  the  sides  of  the  jaws,  and  turns  to  the  ventral  surface  of  the  ros- 
trum, passing  first  posterior  to  the  nostril  and  then  turning  forward  between 
the  two  nostrils.  There  is  also  a  short  mandibular  canal  under  the  skin  just 
behind  the  lower  jaw;  it  is  not  connected  with  the  other  canals. 

Skate:  The  lateral  line  system  is  more  complex  than  in  the  dogfish  and 
more  difficult  to  follow.  The  lateral  line  canal  runs  on  the  dorsal  surface  just 
lateral  to  the  mid-dorsal  spines.  Remove  the  skin  at  this  place  and  identify 
the  canal.  Trace  it  forward,  removing  the  skin  as  you  proceed.  At  the  pos- 
terior end  of  the  cartilage  (propterygium)  of  the  anterior  part  of  the  pectoral 
fin  it  gives  off  two  canals  which  proceed  posteriorly  over  the  surface  of  the  fin. 
It  then  proceeds  above  the  eye  as  the  supraorbital  canal,  apparently  connecting 
with  its  fellow  by  a  cross-union  on  the  posterior  part  of  the  skull.  The  supra- 
orbital  canal  passes  in  front  of  the  eye  and  as  the  infraorbital  canal  below  the 
eye.  In  the  region  of  the  eye  it  gives  off  branches  over  the  rostrum  and  a  long 
branch  which  proceeds  posteriorly  along  the  lateral  margin  of  the  fin.  On  the 
ventral  side  of  the  skate  there  is  a  prominent  canal  passing  just  lateral  to  the 
gill  slits.  Trace  this  forward,  noting  branches  behind  and  in  front  of  the  nostril 
and  on  the  ventral  surface  of  the  rostrum.  On  the  surface  of  the  pectoral  fins 
after  removal  of  the  skin  the  numerous,  very  long  canals  of  Lorenzini  are  no- 
ticeable. 

Draw,  showing  distribution  of  the  canals. 

c)  The  olfactory  organs:  These  consist  of  a  pair  of  olfactory  sacs  on  the  ventral 
side  of  the  rostrum,  opening  externally  by  the  nostril  or  external  naris,  with  which 
are  associated  various  flaps  of  skin.  Dissect  the  skin  away  from  one  olfactory 
sac  and  cut  away  the  flaps  so  that  you  can  look  into  the  sac.  Note  the  numerous 
plates  or  lamellae  arranged  in  rows  inside  of  the  sac;  these  are  covered  with 
olfactory  epithelium,  the  sense  of  smell  being  quite  keen  in  fishes.  Prove  to 
yourself  that  the  olfactory  sac  is  closed  internally,  having  no  communication 
with  the  oral  cavity. 

Draw,  showing  the  lamellae. 


304       LABORATORY  MANUAL  FOR  VERTEBRATE  ANATOMY 

d)  The  eye  muscles:  Remove  the  tissue  from  about  the  eye  on  the  same  side 
of  the  animal  as  under  c)  and  completely  expose  the  eyeball.  In  doing  this, 
first  cut  away  the  upper  eyelid  (or  in  the  skate  the  skin  over  the  eye),  noting  that 
the  inner  lining  of  the  eyelid  is  continuous  with  a  thin  layer  (conjunctiva)  which 
adheres  closely  to  the  external  surface  of  the  eyeball.  Next  cut  away  very  care- 
fully the  cartilage  between  the  eye  and  the  brain,  which  is  seen  as  a  white 
lobed  structure  in  the  median  region,  and  also  the  cartilage  in  front  of  the  eye. 
Do  not  injure  the  brain  and  do  not  cut  into  the  elevation  dorsal  to  the  spiracle. 
The  stout,  white  bands  seen  in  this  dissection  are  cranial  nerves.  In  the  skate 
very  little  cutting  is  required.  The  large,  somewhat  spherical  body  exposed  is 
the  eyeball.  In  the  dogfishes  it  is  imbedded  in  a  gelatinous  material  which  should 
be  carefully  cleaned  out. 

The  eyeball  reposes  in  a  cavity,  the  orbit,  to  the  walls  of  which  it  is  attached 
by  muscular  bands,  the  eye  muscles.  They  are  voluntary  muscles  derived  from 
the  myotomes  of  the  second,  third,  and  fourth  segments  of  the  head.  There  are 
six  of  these  eye  muscles  which  should  be  identified  as  follows.  From  the  dorsal 
view  four  of  them  will  be  seen.  The  one  which  is  attached  to  the  anterior  wall 
of  the  orbit  is  the  superior  oblique.  The  other  three  originate  from  the  postero- 
lateral  angle  of  the  orbit  and  are  named  recti  muscles.  The  most  anterior  one 
is  the  internal  or  medial  rectus;  its  insertion  on  the  eyeball  is  covered  dorsally 
by  the  superior  oblique.  The  next  rectus  muscle  is  the  superior  rectus,  more 
dorsally  situated  than  the  others.  The  third,  the  external  (or  lateral)  rectus,  is 
inserted  on  the  posterior  surface  of  the  eyeball.  Next,  raise  the  eyeball  dor- 
sally and  note  that  the  conjunctiva  or  most  superficial  coat  over  the  external 
surface  of  the  eyeball  is  continuous  with  the  lining  of  the  lower  lid.  Cut  through 
this  and  free  the  eyeball  ventrally,  cleaning  out  the  gelatinous  and  fibrous  tissue 
which  will  be  found  here.  On  lifting  the  eyeball  the  remaining  two  eye  muscles 
will  be  seen.  The  inferior  oblique  originates  from  the  anteromedial  corner  of 
the  orbit,  the  inferior  rectus  from  the  posteromedial  angle  of  the  orbit;  both  are 
inserted  in  contact  with  each  other  on  the  middle  of  the  ventral  surface  of  the 
eyeball.  The  white  cords  seen  among  the  eye  muscles  are  nerves. 

Draw  the  eyeball  and  its  muscles  from  dorsal  view,  showing  as  many  of  the. 
muscles  as  possible. 

The  eye  muscles  originate  from  the  orbit  and  are  inserted  on  the  eyeball. 
Their  action  is  to  turn  the  eyeball  in  various  directions.  As  already  stated  they 
are  derived  from  three  head  segments  and  are  in  most  vertebrates  practically 
the  only  muscles  developed  from  the  head  myotomes.  These  three  muscle- 
forming  myotomes  are  designated  the  first,  second,  and  third,  although  they  are 
in  reality  the  second,  third,  and  fourth  of  the  head  myotomes,  since  the  true 
first  segment  was  not  noticed  until  later.  According  to  the  recent  investiga- 
tions of  Neal,  the  third  myotome  gives  rise  to  part  of  the  external  rectus,  the 
second  to  the  rest  of  the  external  rectus  and  to  the  superior  oblique,  while  the 


COMPARATIVE  ANATOMY  OF  THE  NERVOUS  SYSTEM  305 

first  myotome  produces  the  other  four  muscles.    This  account  differs  slightly 
from  that  given  in  the  textbooks. 

e)  The  structure  of  the  eyeball:  Cut  through  the  eye  muscles  at  the  insertions 
and  remove  the  eyeball.  As  already  noted  the  outermost  coat  covering  the  front 
of  the  eyeball  is  the  conjunctiva  which  is  deflected  onto  the  inner  surface  of  the 
eyelids.  Note  the  free  edge  of  the  conjunctiva  clinging  to  the  eyeball  where 
the  eyelids  were  cut.  The  conjunctiva  is  the  epidermis  of  the  skin  and  not  one 
of  the  true  coats  of  the  eye.  The  outermost  coat  of  the  eyeball  is  the  sclera  or 
sclerotic  coat,  a  very  tough  membrane  composed  of  connective  tissue.  The  front 
part  of  the  sclera  is  transparent  and  is  named  the  cornea;  the  conjunctiva  is 
inseparably  fused  to  the  outer  surface  of  the  cornea.  Through  the  transparent 
cornea  can  be  seen  an  opening,  the  pupil.  Cut  off  the  dorsal  side  of  the  eyeball 
so  that  you  can  look  within  the  cavity.  Place  the  larger  piece  under  water. 
The  large  spherical  body  in  the  interior  is  the  crystalline  lens.  Note  that  internal 
to  the  sclera  is  a  black  coat,  the  chorioid  coat,  and  internal  to  this  a  soft,  often 
collapsed,  greenish  layer,  the  retina.  Follow  the  chorioid  coat  to  the  front  of 
the  eye  and  note  that  there  it  is  separated  from  the  cornea  forming  a  black  curtain, 
the  iris,  in  the  center  of  which  is  an  opening,  the  pupil.  The  iris  divides  the 
cavity  of  the  eyeball  into  an  external  cavity,  the  anterior  chamber  of  the  eye 
between  the  iris  and  the  cornea,  and  an  internal  chamber,  the  cavity  of  the  vitre- 
ous humor,  between  the  lens  and  the  retina.  The  anterior  chamber  contains  a 
fluid,  the  aquaeous  humor;  the  cavity  of  the  vitreous  humor  contains  a  gelatinous 
material,  the  vitreous  humor  or  vitreous  body,  collapsed  in  the  preserved  specimen. 
The  lens  in  life  is  attached  to  the  margins  of  the  pupil  and  also  to  the  margins  of 
the  retina;  and  the  small  space  between  these  two  points  of  attachment  of  the 
lens  forms  the  posterior  chamber  of  the  eye. 

Draw  the  section,  showing  the  structures  of  the  eye. 

The  retina  is  the  nervous  part  of  the  eye  containing  the  sensory  cells  (rods 
and  cones)  which  are  stimulated  by  light.  The  lens  and  the  two  humors  focus 
the  light  upon  the  retina.  The  focus  is  changed  in  fishes  by  moving  the  lens 
back  and  forth.  The  pupil  regulates  the  amount  of  light  admitted.  The  coats 
of  the  eye  serve  for  protection  and  to  darken  the  interior. 

In  the  orbit  after  removal  of  the  eyeball  note  the  origins  of  the  six  eye  muscles, 
the  optic  pedicel,  a  cartilaginous  stalk  situated  among  the  rectus  muscles  and 
helping  support  the  eyeball,  and  the  optic  nerve,  a  stout  white  stalk  located  in 
front  of  the  rectus  muscles.  The  stout  white  band  in  the  floor  of  the  orbit  is 
the  infraorbital  nerve. 

f)  The  internal  ear:  The  ear  in  fishes  consists  only  of  the  internal  ear  or 
membranous  labyrinth.  This  is  imbedded  in  the  otic  region  of  the  skull.  In  the 
dogfishes  and  skate  it  is  situated  between  the  spiracle  and  the  mid-dorsal  line. 
A  pronounced  elevation  of  the  chondrocranium  is  present  at  this  place.  In  the 
median  line  between  the  two  elevations  will  be  found  a  pair  of  small  holes  in 


306       LABORATORY  MANUAL  FOR  VERTEBRATE  ANATOMY 

the  skin.  Upon  removing  the  skin  bearing  these  holes  the  endolymphatic  fossa 
of  the  chondrocranium  will  be  found  beneath  it.  In  this  fossa  are  two  ducts, 
the  endolymphatic  ducts,  which  open  on  the  skin  by  the  two  holes  just  mentioned. 
These  ducts  connect  the  cavity  of  the  internal  ear  with  the  surface.  Very  care- 
fully shave  off  with  a  scalpel  the  cartilage  of  the  elevation  containing  the  ear, 
working  on  the  same  side  as  before.  There  will  soon  be  noticed  a  canal  in  the 
cartilage  containing  a  delicate  curved  tube.  This  tube  is  the  anterior  vertical 
semicircular  duct.  Continue  removing  the  cartilage,  without  injuring  this  duct. 
The  muscles  posterior  to  the  ear  may  also  be  removed.  Another  tube  will  soon 
be  uncovered  posterior  to  the  first  one;  this  is  the  posterior  vertical  semicircular 
duct.  There  will  next  be  revealed  a  thin-walled  chamber,  the  utriculus,  from  which 
these  two  ducts  spring.  Continue  picking  away  the  cartilage  in  small  pieces,  leav- 
ing all  parts  in  place.  A  third  duct,  the  horizontal  semicircular  duct,  lying  below 
and  lateral  to  the  others,  will  next  be  exposed.  When  the  cartilage  has  been  re- 
moved as  far  as  possible,  the  parts  of  the  internal  ear  may  be  identified.  The 
central  chamber  to  which  the  ducts  are  attached  consists  of  a  smaller  dorso- 
anterior  portion,  the  utriculus,  from  whose  dorsal  tip  the  anterior  vertical  and 
horizontal  ducts  spring;  and  a  larger  ventro-posterior  part,  the  sacculus,  from 
which  the  posterior  vertical  duct  takes  origin.  The  sacculus  fits  into  a  rounded 
depression  in  the  cartilage.  The  semicircular  ducts  are  slender  tubes,  curved  in  a 
semicircle  and  each  terminating  in  a  rounded  sac,  the  ampulla.  The  ampullae 
of  the  anterior  vertical  and  horizontal  ducts  are  in  contact  with  and  communicate 
with  each  other  and  also  with  the  anterior  extension  of  the  ventral  part  of  the 
utriculus.  This  extension  is  called  the  recessus  utriculi.  Both  ends  of  the  posterior 
vertical  canal  join  the  sacculus.  In  each  ampulla  will  be  seen  a  white  sensory 
patch  or  crista,  to  which  a  branch  of  the  auditory  nerve  is  attached.  Larger  sen- 
sory patches,  called  maculae,  also  occur  in  the  recessus  utriculi  and  in  the  sacculus. 
Inside  the  sacculus,  a  white  mass  of  sand  grains  or  crystalline  material,  the  otolith, 
is  visible.  The  movements  of  these  grains  may  be  concerned  in  equilibration. 
The  endolymphatic  duct  opens  from  the  medial  side  of  the  sacculus  but  the  con- 
nection is  difficult  to  find. 

Draw,  showing  parts  of  the  internal  ear.  After  the  drawing  has  been  made,  the 
sacculus  may  be  opened  and  the  otolith  examined. 

The  internal  ear  has  two  functions,  that  of  hearing  and  that  of  equilibration. 
The  whole  structure  is  filled  with  a  fluid,  the  endolymph,  while  the  channels  in 
the  cartilage  are  filled  with  perilymph.  Changes  in  pressure  in  the  endolymph  due 
either  to  the  impinging  of  sound  waves  on  the  head  or  to  changes  in  the  position 
of  the  head  appear  to  be  the  stimuli  which  excite  the  sensory  cells  of  the  cristae 
and  maculae,  producing  in  the  first  case  the  sensation  of  hearing  and  in  the 
second  sensations  of  the  animal's  position  in  the  water,  enabling  it  to  keep  in  the 
desired  position.  According  to  the  experiments  of  Maxwell,  both  (or  either)  the 
cristae  and  the  maculae  control  equilibration.  The  capacity  to  perceive  sounds  is 
presumably  limited  to  the  cristae. 


COMPARATIVE  ANATOMY  OF  THE  NERVOUS  SYSTEM  307 

3.  The  dorsal  aspect  of  the  brain. — The  brain  is  now  to  be  exposed  by  care- 
fully picking  away  the  cartilage  in  small  pieces  from  its  roof.  The  cranial  nerves, 
white  strands  passing  through  the  cartilage,  must  not  be  injured.  One  side  of 
the  head  has  thus  far  been  left  intact  for  the  study  of  the  cranial  nerves.  This 
side  is  now  to  be  exposed  along  with  the  brain  as  far  as  necessary.  Remove  the 
upper  eyelid  as  directed  under  the  eye  but  leave  all  structures  intact.  In 
removing  the  cartilage  between  the  brain  and  the  eye  the  following  nerves 
will  be  noted :  the  superficial  ophthalmic  nerve  running  in  the  wall  of  the  orbit 
near  the  dorsal  surface;  the  small  trochlear  nerve  passing  through  the  back  wall 
of  the  orbit  to  the  superior  oblique  eye  muscle;  in  the  skate  the  larger  oculomotor 
nerve  accompanying  the  trochlear.  Dissect  forward  to  the  olfactory  sacs,  expos- 
ing them  dorsally,  leaving  the  ophthalmic  nerve  in  place.  Remove  the  skin 
behind  the  spiracle  and  note  the  hyomandibular  nerve  passing  posterior  to  the 
spiracle;  this  nerve  is  also  to  be  preserved.  To  expose  the  posterior  part  of 
the  brain  the  internal  ears  of  both  sides  may  be  cut  through  and  the  mass  of 
muscles  posterior  to  the  ear  removed  as  much  as  necessary.  Nerves  will  be 
seen  passing  through  the  cartilage  in  the  ventral  part  of  the  ear,  but  are  not  to 
be  dissected  out  for  the  present.  In  short,  the  dorsal  side  of  the  brain  is  to  be 
fully  exposed,  leaving  all  of  the  more  superficial  nerves  intact.  The  dorsal 
aspect  of  the  brain  will  then  be  studied  first,  and  the  cranial  nerves  afterward. 

The  brain  is  situated  in  a  cavity  in  the  chondrocranium,  which  it  only 
partially  fills.  It  is  covered  by  a  delicate  membrane,  the  primitive  meninx,  in 
which  the  blood  vessels  of  the  brain  are  situated.  The  meninx  is  connected  by 
strands  with  the  membrane  lining  the  cartilaginous  walls  of  the  cranial  cavity. 
The  space  between  brain  and  chondrocranium  is  filled  in  life  by  a  fluid. 

The  most  anterior  part  of  the  brain  is  the  large  olfactory  bulb,  a  nervous  mass 
situated  in  contact  with  the  dorsal  walls  of  the  olfactory  sacs.  From  the  olfactory 
sac  a  number  of  very  short  fibers,  which  together  constitute  the  olfactory  nerve, 
pass  into  the  olfactory  bulb.  The  olfactory  bulb  is  spherical  in  the  dogfishes, 
elongated  in  the  skate.  The  olfactory  bulb  is  connected  with  the  next  part  of 
the  brain  by  a  stalk,  the  olfactory  tract,  which  is  short  in  the  smooth  dogfish. 
The  olfactory  tracts  pass  to  enlarged,  rounded  lobes,  the  olfactory  lobes,  at  the 
anterior  (or  lateral  in  the  skate)  end  of  the  main  mass  of  the  brain.  Posterior 
to  (dogfish)  or  medial  to  (skate)  the  olfactory  lobes  is  situated  another  pair  of 
lobes,  the  cerebral  hemispheres.  The  cerebral  hemispheres  and  the  olfactory 
lobes  are  separated  only  by  a  faint  groove.  All  of  the  parts  thus  far  mentioned 
belong  to  the  telencephalon.  The  olfactory  bulbs,  tracts,  and  lobes  together  con- 
stitute the  rhinencephalon  or  smell  portion  of  the  brain.  In  fishes  the  cerebral 
hemispheres  are  also  largely  concerned  with  the  sense  of  smell. 

Posterior  to  the  cerebral  hemispheres  is  a  depressed  region,  the  diencephalon 
or  thalamencephalon,  very  narrow  in  the  smooth  dogfish.  The  roof  of  this  is 
thin  and  discolored,  due  to  the  fact  that  it  consists  entirely  of  a  plexus  of  blood 


308       LABORATORY  MANUAL  FOR  VERTEBRATE  ANATOMY 

vessels,  called  a  chorioid  plexus.  This  particular  plexus  is  named  the  chorioid 
plexus  of  the  third  ventricle.  The  diencephalon  serves  chiefly  as  a  center  of 
co-ordination  of  the  principal  sensations,  such  as  sight,  hearing,  and  skin  sensa- 
tions, its  ventral  portions  being  devoted  to  smell  and  taste.  The  diencephalon 
is  in  lower  vertebrates  the  chief  controlling  region  of  the  brain.  The  optic  nerve 
passes  from  the  orbit  into  the  ventral  surface  of  the  diencephalon.  It  is  quite 
conspicuous  in  the  skate  and  will  be  seen  in  the  dogfishes  by  gently  pressing  the 
diencephalon  to  one  side.  Posterior  to  the  diencephalon  is  the  midbrain  or 
mesencephalon,  consisting  dorsally  of  two  rounded  lobes,  the  optic  lobes  or  corpora 
bigemina.  The  optic  lobes  are  centers  for  the  visual,  auditory,  and  general  skin 
sensations,  and  these  are  connected  with  corresponding  centers  in  the 
diencephalon.  A  nerve,  the  trochlear  or  fourth  cranial  nerve,  arises  from  the 
posterior  borders  of  the  optic  lobe  on  each  side  and  passes  forward  to  an  eye 
muscle.  By  gently  pressing  the  optic  lobes  to  one  side,  a  nerve,  the  oculomotor 
or  third  cranial  nerve,  will  be  seen  emerging  from  the  ventral  surface  of  the  mid- 
brain  and  passing  to  the  orbit. 

Posterior  to  the  optic  lobes  and  somewhat  overhanging  them  is  the  large 
cerebellum  or  metencephalon.  In  the  spiny  dogfish  and  skate  this  is  slightly  divided 
into  four  quadrants  by  faint  longitudinal  and  transverse  grooves;  in  the  smooth 
dogfish  it  is  marked  by  several  transverse  grooves.  The  cerebellum  is  a  center 
for  the  co-ordination  of  motor  impulses,  including  the  maintenance  of  equilib- 
rium. No  nerves  are  attached  to  the  cerebellum.  Posterior  to  the  cerebellum 
the  elongated  remaining  section  of  the  brain  is  the  medulla  oblongata  or  myelen- 
cephalon.  It  is  continuous  posteriorly  with  the  spinal  cord.  The  anterior  part 
of  the  roof  of  the  medulla  is  thin  and  discolored  and  consists  of  a  chorioid  plexus, 
that  of  the  fourth  ventricle.  The  anterior  end  of  the  medulla  extends  forward 
at  the  sides  of  and  below  the  cerebellum  as  two  earlike  projections,  sometimes 
called  the  auricles.  Remove  the  chorioid  plexus  from  the  roof  of  the  auricles 
and  the  medulla.  The  large  cavity  revealed  is  the  cavity  of  the  fourth  ventricle. 
The  auricles  together  with  the  portion  of  the  dorsal  part  of  the  medulla  just 
posterior  to  them  constitute  the  acustico-lateral  areas,  or  primary  centers  of  the 
lateral  line  system  and  of  hearing  and  equilibration.  On  lifting  the  posterior 
end  of  the  cerebellum  it  will  be  seen  that  the  auricles  are  continuous  with  the 
cerebellum  and  with  each  other,  below  the  overhanging  posterior  portion  of  the 
cerebellum.  By  way  of  this  connection  with  the  cerebellum  the  impulses  which 
come  in  from  the  lateral  line  system  and  internal  ear  are  conveyed  to  the  cere- 
bellum after  making  a  relay  in  the  acustico-lateral  area.  The  entire  dorsal 
rim  of  the  medulla,  including  the  acustico-lateral  areas,  forms  an  elongated 
strip  on  each  side  which  is  known  as  the  somatic  sensory  column.  This  column 
as  its  name  implies  contains  centers  and  tracts  associated  with  all  of  the  body 
senses  except  smell,  taste,  and  vision,  which  as  we  have  seen  are  disposed  of 


COMPARATIVE  ANATOMY  OF  THE  NERVOUS  SYSTEM       309 

in  the  more  anterior  portions  of  the  brain.  Near  the  anterior  end  of  the  somatic 
sensory  column,  at  the  middle  of  the  acustico-lateral  area,  will  be  seen,  by  pressing 
the  latter  toward  the  middle,  the  roots  of  a  number  of  nerves.  These  are  the 
roots  of  the  fifth,  seventh,  and  eighth  cranial  nerves,  to  be  studied  in  more  detail 
later.  At  the  posterior  end  of  the  somatic  sensory  column,  just  anterior  to  the 
point  where  the  walls  of  the  medulla  close  together,  will  be  noted  the  stout 
root  of  the  tenth  cranial  nerve.  In  the  lateral  wall  of  the  medulla,  just  ventral 
to  the  somatic  sensory  column,  is  another  longitudinal  area  marked  by  a  row 
of  rounded  elevations;  this  area  is  the  visceral  sensory  column.  As  its  name 
implies,  it  is  associated  with  sensations  from  the  viscera.  In  fishes  the  gills  are 
important  visceral  structures  so  that  a  considerable  portion  of  this  column  is 
connected  with  the  gills ;  in  fact,  each  of  the  little  elevations  is  said  to  be  a  center 
for  one  visceral  arch.  Ventral  to  the  visceral  sensory  column  is  the  very  slender 
visceral  motor  column,  from  which  impulses  go  to  the  visceral  muscles.  We  have 
already  learned  that  the  visceral  muscles  are  the  muscles  of  the  gill  region.  In 
the  floor  of  the  fourth  ventricle  are  the  two  conspicuous  somatic  motor  columns, 
separated  by  a  median  groove.  The  somatic  motor  columns  are  the  places  of 
origin  of  impulses  to  the  somatic  or  parietal  muscles.  As  previously  stated, 
these  in  the  head  consist  chiefly  of  the  six  eye  muscles. 

The  fourth  ventricle  narrows  posteriorly  and  is  finally  roofed  over  by  the 
fusion  of  the  walls  of  the  medulla.  Shortly  beyond  this  point  the  medulla  is 
continuous  with  the  spinal  cord.  The  posterior  end  of  the  medulla  marks  the 
posterior  end  of  the  brain  but  is  not  sharply  defined. 

Draw  the  dorsal  aspect  of  the  brain.  Place  it  in  the  middle  of  the  page  so 
that  the  cranial  nerves  can  be  added  later. 

4.  The  cranial  nerves. — There  are  ten  cranial  nerves  in  fishes.  They  are  to 
be  dissected  with  great  care  and  their  distribution  noted  and  drawn.  This 
distribution  is  in  general  similar  in  all  vertebrates  except  that  certain  nerve 
trunks  present  in  fishes  disappear  in  the  land  vertebrates.  One  of  the  most 
striking  examples  of  homology  is  found  in  this  distribution  of  the  cranial  nerves 
which  still  in  man  continue  to  supply  the  same  parts  as  in  the  fish. 

Add  to  your  drawing  of  the  dorsal  side  of  the  brain  an  outline  of  the  head  of 
the  animal,  putting  in  outline  the  olfactory  sacs,  eyes,  ears,  and  gill  slits.  As  you 
dissect  the  cranial  nerves  according  to  the  directions  to  be  given  immediately, 
add  each  to  this  drawing,  showing  as  accurately  as  possible  the  location  and 
course  of  each  of  these  nerves  and  the  parts  of  the  head  which  they  supply.  It 
is  necessary  to  enter  a  nerve  on  one  side  only,  and  by  using  the  two  sides  of  the 
drawing  for  different  nerves  it  will  be  possible  to  enter  all  of  them. 

a)  The  first  or  olfactory  nerve:  This  nerve  has  already  been  noted.  It  arises 
from  the  olfactory  cells  in  the  lamellae  of  the  olfactory  sac  and  passes  by  very 
short  branches  into  the  olfactory  bulb.  These  branches  are  practically  invisible. 


310       LABORATORY  MANUAL  FOR  VERTEBRATE  ANATOMY 

From  the  olfactory  bulb,  after  a  relay,  the  olfactory  impulses  pass  along  the 
olfactory  tracts  to  certain  parts  of  the  brain  described  above.  The  olfactory 
nerve  is  a  pure  sensory  nerve. 

b)  The  second  or  optic  nerve:  The  optic  nerve  arises  in  the  retina  of  the  eye 
and  passes  through  the  coats  of  the  eye,  emerging  ventral  to  the  internal  rectus 
muscle.     Find  it  there  on  the  intact  eye  by  pressing  this  muscle  against  the  eye- 
ball.    It  is  a  stout,  white  trunk  which  pierces  the  cartilage  of  the  orbit  and  passes 
to  the  ventral  side  of  the  diencephalon.     It  may  be  seen  here  by  gently  raising 
the  diencephalon.     The  optic  nerve  is  not  really  a  nerve,  for  the  retina  in  which 
it  arises  is  a  part  of  the  brain  wall,  and  in  the  retina  there  are  several  relays  of 
nerve  cells  between  the  rods  and  cones  and  the  cells  of  origin  of  the  optic  nerve. 
The  optic  nerve  is  really  a  tract  of  the  brain.     It  carries  sensory  impulses  only, 
visual  impulses,  and  discharges  them  into  certain  parts  of  the  diencephalon  and 
optic  lobes. 

c)  The  fourth  or  trochlear  nerve:  The  trochlear  nerve  arises  in  the  midbrain 
and  emerges  in  the  groove  between  the  optic  lobes  and  the  cerebellum.    Trace 
it  on  the  side  where  the  eye  is  still  intact.     It  passes  forward  in  the  cranial  cavity 
to  about  the  level  of  the  cerebral  hemispheres;  it  then  turns  abruptly  laterally, 
pierces  the  wall  of  the  orbit,  and  is  distributed  to  the  superior  oblique  muscle 
of  the  eyeball.     It  is  the  motor  nerve  of  this  muscle  and  carries  only  spmatic 
motor  impulses.     Although  it  appears  externally  to  emerge  from  the  roof  of  the 
midbrain,  the  motor  cells  from  which  it  originates  are  in  fact  in  the  floor  of  the 
midbrain  in  a  forward  extension  of  the  somatic  motor  column. 

d)  The  third  or  oculomotor  nerve:  The  oculomotor  nerve  arises  from  the  floor 
of  the  midbrain.     It  is  readily  noticed  in  the  skate,  ascending  to  the  orbit  near 
the  preceding  nerve.     In  the  dogfishes  it  is  deeply  situated  and  is  seen  by  pressing 
the  cerebellum  away  from  the  wall  of  the  orbit.     Follow  it  into  the  orbit  on  both 
sides,  getting  its  general  relations  first  on  the  side  where  the  eyeball  was  removed. 
It  emerges  into  the  orbit  very  near  the  insertion  of  the  superior  rectus  muscle 
and  is  situated  ventral  to  the  superficial  ophthalmic  nerve,  already  noted.     It 
should  not  be  confused  with  the  deep  ophthalmic  nerve  which  is  in  contact  with 
it  as  it  enters  the  orbit;   the  deep  ophthalmic  nerve  runs  through  the  orbit  in 
contact  with  the  medial  surface  of  the  eyeball.     This  nerve  will  be  better  seen 
on  the  intact  side.     Observe  the  branches  given  by  the  oculomotor  nerve  immedi- 
ately after  its  entrance  into  the  orbit  to  the  internal  and  superior  recti  muscles. 
On  the  intact  side  now  loosen  the  eyeball  and  cut  the  insertions  of  the  superior 
oblique  and  superior  rectus  close  to  the  eyeball.     Identify  the  deep  ophthalmic 
nerve  passing  in  the  spiny  dogfish  dorsal  to  the  internal  rectus  lying  against  the 
eyeball  and  in  the  smooth  dogfish  and  skate  ventral  to  the  internal  rectus.     Free 
and  preserve  this  nerve.     Cut  through  the  insertion  of  the  inferior  rectus  and 
the  optic  nerve  and,  pressing  the  eyeball  outward,  note  the  branch  of  the  oculo- 
motor nerve,  which  passes  along  the  posterior  side  of  the  inferior  rectus  muscle, 


COMPARATIVE  ANATOMY  OF  THE  NERVOUS  SYSTEM  311 

turns  ventral  to  it,  and  then  runs  forward  in  the  floor  of  the  orbit  to  the  inferior 
oblique.  Note  also  that  the  branch  which  supplies  the  inferior  rectus  gives  off 
a  branch,  one  of  the  ciliary  nerves,  which  passes  into  the  eyeball  in  company 
with  an  artery.  Along  this  nerve  small  brown  masses  can  be  noticed;  they  are 
the  ciliary  ganglia  belonging  to  the  sympathetic  system.  The  deep  ophthal- 
mic nerve  also  supplies  ciliary  branches  into  the  eyeball.  The  function  of  the 
ciliary  nerves  is  to  control  the  smooth  muscles  of  the  iris,  regulating  the  size  of 
the  pupil;  they  are  visceral  motor  nerves,  making  a  relay  in  sympathetic 
ganglia. 

It  will  be  seen  from  the  foregoing  account  that  the  oculomotor  nerve  supplies  four  eye 
muscles,  namely,  those  which  develop  from  the  so-called  first  head  myotome.  It  is  a  somatic 
motor  nerve  originating  from  the  somatic  motor  column  in  the  floor  of  the  midbrain.  It 
also,  however,  carries  with  it  sympathetic  fibers  of  visceral  motor  function. 

e)  The  sixth  or  abducens  nerve:  The  abducens  originates  from  the  somatic 
motor  column  on  the  ventral  surface  of  the  anterior  end  of  the  medulla.  Its 
origin  will  be  seen  later.  It  penetrates  the  orbit  at  the  point  of  origin  of  the 
external  rectus  muscle  and  passes  along  the  ventral  surface  of  this  muscle  to 
which  its  fibers  are  distributed.  It  will  be  seen  as  a  white  ridge  on  the  ventral 
surface  of  the  muscle. 

It  will  now  be  seen  that  the  third,  fourth,  and  sixth  cranial  nerves  are  somatic  motor 
nerves  to  the  muscles  of  the  eyeball.  The  reason  for  the  allotment  of  three  cranial  nerves 
to  six  small  muscles  has  already  been  indicated.  To  every  segment  of  the  vertebrate  body 
there  should  be  theoretically  a  pair  of  motor  and  sensory  nerves.  Since  as  already  ex- 
plained the  six  eye  muscles  come  from  three  head  myotomes,  it  is  readily  understood  that 
a  somatic  motor  nerve  should  be  present  to  supply  the  derivatives  of  each  of  these  myo- 
tomes. The  sensory  nerves  of  these  segments  will  be  indicated  later. 

/)  The  fifth  or  trigeminus  nerve:  The  trigeminus  is  a  very  large  nerve  with 
four  main  branches  in  elasmobranchs  (three  in  land  vertebrates) .  The  trigemi- 
nus is  attached  to  the  medulla  near  the  anterior  end  of  the  somatic  sensory 
column,  just  behind  the  auricles  of  the  medulla.  Its  roots  here  are  inextricably 
mingled  with  the  roots  of  the  seventh  and  eighth  nerves,  the  three  together 
forming  a  conspicuous  mass  at  the  place  stated.  The  trigeminus  passes  through 
the  adjacent  wall  of  the  orbit  and  should  be  followed  into  the  orbit  by  carefully 
picking  away  the  cartilage  around  it.  As  soon  as  it  penetrates  the  orbit  the 
trigeminus  divides  into  four  branches.  The  first  of  these,  the  superficial  oph- 
thalmic branch,  is  part  of  the  superficial  ophthalmic  trunk  which  has  already 
been  mentioned  several  times.  This  large  trunk  passes  forward  in  the  dorsal  part 
of  the  cartilage  of  the  medial  wall  of  the  orbit.  Trace  it  forward.  It  passes  out 
of  the  orbit  through  the  ophthalmic  foramen  in  the  chondrocranium,  and  above 
the  olfactory  bulb.  Only  a  small  part  of  this  trunk  is  trigeminal;  this  is  sensory 
to  the  skin  dorsal  to  the  orbit.  The  second  branch  of  the  trigeminus  is  the  deep 


312       LABORATORY  MANUAL  FOR  VERTEBRATE  ANATOMY 

ophthalmic  nerve.  This  passes  through  the  orbit  ventral  to  the  preceding  and 
leaves  the  orbit  by  the  orbitonasal  canal.  Upon  tracing  it  forward  it  will  be 
seen  to  join  the  superficial  ophthalmic  and  to  be  distributed  in  common  with  it. 
Both  of  these  branches  of  the  trigeminus  are  pure  somatic  sensory  nerves,  arising 
from  sense  organs  of  the  skin. 

The  two  remaining  branches  of  the  trigeminus  lie  in  the  floor  of  the  orbit. 
To  see  them  remove  the  eyeball  or  study  the  side  where  the  eyeball  was  previously 
removed.  A  broad  white  band,  the  infraorbital  trunk,  is  seen  in  the  floor  of 
the  orbit,  passing  obliquely  laterally.  This  trunk  in  the  dogfishes  is  composed  of 
the  mixed  fibers  of  the  maxillary  branch  of  the  trigeminus  and  the  buccal  branch 
of  the  seventh  nerve  (see  below).  In  the  orbit  the  larger  and  more  medial  portion 
of  the  trunk  is  the  maxillary  branch,  but  farther  out  this  becomes  inextricably 
mingled  with  the  buccal  nerve.  In  the  skate  the  infraorbital  trunk  is  divisible 
into  three  trunks,  of  which  the  outer  one  is  the  maxillary  branch  of  the  trigeminus, 
the  middle  one  the  mandibular  branch  of  the  trigeminus,  and  the  inner  one  the 
buccal  branch  of  the  seventh  nerve.  As  before,  however,  it  should  be  remembered 
that  there  is  an  admixture  of  fibers  of  the  fifth  and  seventh  nerves  in  these  trunks. 
Trace  the  maxillary  branch  of  the  trigeminus  and  buccal  branch  of  the  seventh 
out  from  the  orbit,  along  the  ventral  surface  of  the  rostrum.  The  branches  pass 
to  the  region  below  and  in  front  of  the  eye,  to  the  medial  side  of  the  nostril  (in 
the  skate  to  the  lateral  side  of  the  nostril  also)  and  to  the  angle  of  the  jaws.  In 
the  smooth  dogfish  there  is  a  conspicuous  branch  along  the  lower  jaw  which 
appears  to  be  a  part  of  the  mandibular  branch  of  the  trigeminus.  The  maxillary 
branch  is  sensory  to  the  skin  of  the  rostrum,  while  the  buccal  nerve  supplies  the 
infraorbital  lateral  line  canal  and  near-by  ampullae  of  Lorenzini. 

The  fourth  branch  of  the  trigeminus  is  the  mandibular  branch.  In  the  dog- 
fishes it  separates  from  the  infraorbital  trunk  where  the  latter  enters  the  orbit 
from  the  brain  and  passes  along  the  posterior  wall  of  the  orbit.  In  the  smooth 
dogfish  part  of  it  seems  to  accompany  the  infraorbital  trunk  forward  and  then 
curves  to  the  lower  jaw  as  described  above.  The  mandibular  nerve  is  seen  to 
branch  to  various  muscles  in  the  floor  of  the  orbit  (these  are  gill-arch  muscles) 
and  on  following  it  out  of  the  orbit,  will  be  seen  to  be  distributed  to  muscles 
of  the  lower  jaw  and  to  send  a  sensory  branch  to  the  skin  of  the  lower  jaw,  this 
branch  being  situated  just  behind  the  teeth.  In  the  skate  the  position  of  the 
mandibular  nerve  was  described  above  as  between  the  maxillary  and  the  buccal 
nerves.  Follow  it  forward.  It  curves  around  the  angle  of  the  jaw  and  supplies 
muscles  of  the  lower  jaw  and  the  adjacent  skin. 

It  will  be  observed  that  all  of  the  branches  of  the  fifth  nerve  are  somatic  sensory  nerves 
coming  from  various  sensory  organs  of  the  skin,  except  the  mandibular  nerve  which  also 
contains  some  motor  branches  to  muscles.  As  those  muscles  are  visceral  muscles,  this  part 
of  the  fifth  nerve  belongs  to  the  visceral  motor  system.  The  deep  ophthalmic  nerve  appears 


COMPARATIVE  ANATOMY  OF  THE  NERVOUS  SYSTEM  313 

to  have  been  the  sensory  part  of  the  oculomotor  nerve,  but  it  has  been  absorbed  by  the 
trigeminus  which  has  spread  beyond  its  original  distribution.  The  deep  ophthalmic  alone 
persists  in  higher  forms  as  the  ophthalmic  branch  of  the  trigeminus,  the  superficial  ophthalmic 
branch  disappearing.  The  remainder  of  the  trigeminus  is  the  sensory  portion  of  the  troch- 
lear  nerve  (see  Fig.  69,  p.  300).  It  is  important  to  note  further  that  the  trigeminus  is  the  nerve 
of  the  upper  and  lower  jaws.  Since  the  jaws  and  associated  parts  constitute  the  first  visceral 
arch,  the  trigeminus  is  said  to  be  the  nerve  of  the  first  visceral  arch.  It  is  the  sensory 
nerve  of  this  arch  and  also  the  motor  nerve  of  its  visceral  muscles. 

g)  The  seventh  or  facial  nerve:  This  nerve  is  intimately  related  to  the  tri- 
geminus. It  arises  in  common  with  the  latter  from  the  anterior  end  of  the  me- 
dulla and  divides  into  three  main  branches.  Two  of  these  branches  pass  through 
the  orbit  in  common  with  the  trigeminus.  The  superficial  ophthalmic  branch 
of  the  facial  nerve  accompanies  the  same  branch  of  the  trigeminus  and  forms  the 
greater  part  of  the  superficial  ophthalmic  trunk,  supplying  the  supraorbital  lateral 
line  canal  and  adjacent  ampullae  of  Lorenzini.  The  buccal  branch  of  the  facial 
nerve,  as  already  noted,  forms  in  the  orbit  the  outer  half  of  the  infraorbital  trunk 
and  supplies  the  infraorbital  lateral  line  canal  and  near-by  ampullae  of  Lorenzini. 
These  two  branches  of  the  facial  nerve  are  sensory  branches  which  are  lost 
when  the  lateral  line  system  disappears.  The  third  branch  of  the  facial  nerve 
is  the  large  hyomandibular  trunk  which  has  already  been  located  posterior  to 
the  spiracle.  Trace  it  inward  toward  the  brain,  cutting  tissues  in  its  path. 
It  turns  ventrally  and  runs  through  the  anterior  part  of  the  ear  capsule  deep 
down.  Follow  it  by  removing  the  cartilage  of  the  ear  capsule  in  small  pieces. 
The  nerve  passes  ventral  to  some  of  the  branches  of  the  nerve  of  the  ear  and 
joins  the  anterior  end  of  the  medulla  in  common  with  the  trigeminus  root. 
Near  the  brain  it  has  an  enlargement  or  ganglion  (geniculate  ganglion).  From 
this  ganglion  is  given  off  the  palatine  nerve.  It  will  be  found  by  dissecting  care- 
fully around  and  on  the  ventral  surface  of  the  ganglion.  In  the  skate  it  is  easily 
seen.  It  runs  forward  below  the  orbit  along  the  roof  of  the  mouth  where  it 
supplies  the  taste  buds  and  the  lining  epithelium  in  general.  Now  trace  the 
hyomandibular  outward,  past  the  spiracle.  It  turns  ventrally  and  breaks  up 
into  branches  on  the  side  of  the  head.  These  branches  supply  the  hyoman- 
dibular and  mandibular  lateral  line  canals,  ampullae  and  similar  sense  organs, 
the  muscles  of  the  hyoid  arch,  and  the  lining  of  the  floor  of  the  mouth  cavity 
and  tongue. 

The  hyomandibular  is  seen  to  be  a  mixed  nerve  with  lateral  line,  visceral  motor,  and 
visceral  sensory  components.  The  sensory  part  of  the  facial  nerve  is  supposed  to  belong 
to  the  same  segment  of  the  head  as  that  of  which  the  abducens  constitutes  the  motor  nerve. 
The  facial  nerve  is  the  nerve  of  the  first  gill  slit  (spiracle)  and  of  the  second  visceral  arch  (hyoid 
arch)  and  of  the  muscles  belonging  to  this  arch.  The  lateral  line  parts  of  the  facial  nerve 
which  in  fishes  compose  the  greater  part  of  it  are  lost  in  land  vertebrates,  only  the  palatine 
and  certain  parts  of  the  hyomandibular  branch  persisting  as  the  facial  nerve  of  land  verte- 
brates. 


314       LABORATORY  MANUAL  FOR  VERTEBRATE  ANATOMY 

ti)  The  eighth  or  auditory  nerve:  The  auditory  nerve  is  a  pure  somatic  sensory 
nerve  extending  from  the  internal  ear  to  the  brain.  It  enters  the  anterior  end 
of  the  medulla  and  is  there  mingled  with  the  roots  of  the  fifth  and  seventh  nerves. 
Follow  it  into  the  internal  ear  on  the  side  opposite  that  on  which  the  hyoman- 
dibular  was  dissected.  Note  its  branches  to  each  ampulla  and  the  fanlike  arrange- 
ment of  the  branchlets  to  the  crista  of  each  ampulla.  The  auditory  nerve  also 
collects  a  number  of  branches  from  the  walls  of  the  sacculus  and  utriculus.  It 
carries  impulses  for  hearing  and  equilibration  into  the  acustico-lateral  area  of 
the  medulla  to  which  it  will  be  found  to  be  attached.  There  is  no  motor  nerve 
corresponding  to  the  auditory  nerve. 

i)  The  ninth  or  glossopharyngeal  nerve:  This  nerve  passes  through  the  floor 
of  the  middle  of  the  ear  capsule  (where  it  is  likely  to  be  mistaken  for  a  part  of 
the  auditory),  parallel  to  the  hyomandibular  nerve.  Pare  away  as  much  of  the 
ear  capsule  as  is  necessary  to  reveal  it.  Find  its  attachment  to  the  medulla 
posterior  to  the  auditory  nerve.  Trace  it  out  of  the  ear  capsule.  Just  before  it 
exits  from  the  ear  capsule  it  bears  a  swelling,  the  petrosal  ganglion.  Insert  a 
knife  blade  into  the  second  (first  typical)  gill  slit  (in  the  skate  into  the  dorsal 
wall  of  the  corresponding  visceral  pouch)  and  slit  the  gill  cleft  open  dorsally. 
The  petrosal  ganglion  will  now  be  seen  to  be  located  near  the  upper  limits  of  the 
cleft.  Dissect  the  nerve  from  the  ganglion  toward  the  gill  slit.  It  very  soon 
divides  into  three  branches,  two  smaller  anterior  ones,  and  a  larger  posterior 
one.  The  most  anterior  branch  is  the  pretrematic  branch  and  passes  to  the 
anterior  wall  of  the  visceral  pouch  to  which  it  is  a  sensory  nerve.  The  second 
branch  is  posterior  to  the  pretrematic  branch;  it  is  named  the  pharyngeal  branch 
and  is  a  sensory  nerve  to  the  mouth  cavity.  (This  branch  appears  to  be  lacking 
in  the  skate.)  The  third  and  largest  is  the  post-trematic  branch.  It  passes  to 
the  posterior  wall  of  the  visceral  pouch  and  is  both  sensory  and  motor,  its  motor 
components  supplying  the  muscles  of  the  third  visceral  arch.  The  glossopharyn- 
geal nerve  is  the  nerve  of  the  second  visceral  pouch  and  of  the  third  visceral  arch. 

j)  The  tenth  or  vagus  nerve:  The  vagus  nerve  is  the  very  large  trunk  passing 
through  the  posterior  border  of  the  ear  capsule.  It  is  attached  to  the  sides  of 
the  posterior  part  of  the  medulla.  Dissect  it  out  and  follow  its  course.  It 
passes  to  the  anterior  cardinal  sinus,  the  wall  of  which  is  formed  in  the  dogfishes 
of  a  tough  membrane.  Open  up  the  anterior  cardinal  sinus  by  a  deep  cut  through 
the  muscles  medial  to  the  visceral  pouches.  Follow  the  vagus  nerve  into  the 
anterior  cardinal  sinus.  In  the  dogfishes  it  divides  into  two  trunks  at  the  point 
where  it  penetrates  the  tough  wall  of  the  sinus.  The  medially  situated  trunk 
is  the  lateral  branch  of  the  vagus  and  passes  posteriorly  internal  to  the  lateral 
line  whose  canal  it  supplies.  The  lateral  trunk  is  the  visceral  branch  of  the 
vagus,  which  continues  along  the  anterior  cardinal  sinus.  In  the  skate  the 
vagus  runs  for  a  short  distance  in  the  sinus  before  dividing  into  a  more  dorsal 
lateral  branch,  which  passes  posteriorly  internal  to  the  lateral  line  canal  to  which 


COMPARATIVE  ANATOMY  OF  THE  NERVOUS  SYSTEM  315 

it  is  distributed,  and  a  more  ventral  visceral  branch.  In  all  three  forms,  open 
up  the  remaining  visceral  pouches  as  directed  for  the  ninth  nerve  and  determine 
the  distribution  of  the  visceral  branch  of  the  vagus.  With  the  walls  of  the  sinus 
well  spread  open  note  the  four  branches  crossing  the  floor  of  the  sinus  to  the 
visceral  pouches.  Dissect  out  each  of  these  and  observe  that  each  bears  a  gang- 
lion, beyond  which  it  divides  into  three  branches:  an  anterior  pretrematic 
branch,  a  middle  pharyngeal  branch,  and  a  posterior  post-trematic  branch. 
The  pharyngeal  branch  seems  to  be  missing  in  the  skate.  As  in  the  case  of  the 
ninth  nerve,  the  pre-  and  post-trematic  branches  embrace  the  visceral  pouch, 
which  lies  between  them;  all  three  branches  have  the  same  functions  as  described 
for  the  ninth  nerve.  We  thus  see  that  the  vagus  nerve  supplies  the  remaining 
visceral  arches,  beginning  with  the  fourth,  and  the  remaining  visceral  pouches, 
beginning  with  the  third.  After  supplying  the  gill  apparatus  the  visceral  branch 
of  the  vagus  passes  on  into  the  pericardial  and  pleuroperitoneal  cavities,  supply- 
ing the  heart,  digestive  tract,  and  other  viscera. 

Our  study  of  the  cranial  nerves  shows  that  they  bear  a  definite  relation  to  the  original 
segmentation  of  the  head  and  to  the  gill  apparatus.  The  fifth,  seventh,  ninth,  and  tenth 
cranial  nerves  are  gill-arch  nerves,  each  associated  with  one  or  more  particular  visceral 
arches.  In  the  evolution  of  the  vertebrates  each  nerve  continues  to  supply  its  particular 
arch  or  arches  and  all  derivatives  thereof.  Since  the  muscles  of  the  visceral  arches  are 
visceral  muscles,  the  motor  components  of  the  gill-arch  nerves  in  all  vertebrates  are  visceral 
motor  in  nature.  The  somatic  motor  components  for  the  fifth  and  seventh  nerves,  as  already 
stated,  are  separated  into  special  nerves,  the  eye-muscle  nerves.  There  are  no  somatic 
motor  elements  for  the  ninth  and  tenth  cranial  nerves,  since  the  myotomes  which  should 
give  rise  to  voluntary  muscles  for  their  segments  are  degenerate  in  all  vertebrates  above 
cyclostomes  (Fig.  69,  p.  300). 

5.  The  occipital,  hypobranchial,  and  first  spinal  nerves. — Very  carefully 
expose  the  spinal  cord  posterior  to  the  medulla  by  shaving  away  the  cartilage 
of  the  neural  arches  in  thin  slices.  On  the  dorsolateral  surface  of  the  cord  note 
the  little  swellings,  the  dorsal  or  spinal  ganglia,  attached  to  the  cord  by  the 
sensory  or  dorsal  root.  In  the  skate  the  spinal  ganglia  are  elongated.  Between 
the  first  spinal  ganglion  and  the  root  of  the  vagus  note  two  or  three  small  roots 
springing  from  the  side  of  the  medulla.  These  are  the  occipital  nerves.  They 
innervate  some  muscles  of  this  region  and  also  help  to  form  the  hypobranchial 
nerve  described  below,  and  contribute  to  the  cervicobrachial  plexus.  On  press- 
ing the  spinal  cord  to  one  side  the  ventral  or  motor  roots  of  the  spinal  nerves  will 
be  seen  arising  from  the  ventrolateral  region  of  the  cord,  at  the  same  level  as 
the  occipital  nerves.  The  ventral  roots  are  formed  by  the  union  of  several  small 
rootlets  coming  from  the  cord.  The  most  anterior  ventral  roots  are  situated 
anterior  to  the  dorsal  roots  which  belong  to  the  same  segment. 

In  the  dogfish  the  union  of  dorsal  and  ventral  roots  to  form  a  spinal  nerve 
is  not  easy  to  follow.  It  may  usually  be  seen  by  carefully  paring  down  the  carti- 
lage along  the  side  of  the  spinal  cord.  In  the  skate  the  union  is  easily  followed; 


316       LABORATORY  MANUAL  FOR  VERTEBRATE  ANATOMY 

the  roots  pass  through  the  cartilage  at  the  side  of  the  cord  and  unite  as  they 
exit  from  the  cartilage.  A  large  number  of  the  most  anterior  spinal  nerves 
(really  the  ventral  rami  of  the  spinal  nerves,  the  dorsal  rami  being  very  slender 
in  the  skate)  are  then  seen  to  unite  to  form  the  very  large  nerve  of  the  brachial 
plexus  previously  noted. 

The  hypobranchial  nerve  is  a  trunk  formed  by  contributions  from  the  occipital 
nerves  and  the  first  spinal  nerves.  In  the  dogfishes  it  may  be  located  as  follows. 
Insert  one  blade  of  the  scissors  in  the  angle  of  the  jaw  and  cut  back  across  the 
gill  slits  through  to  the  side  of  the  esophagus  as  was  done  in  an  early  stage  of 
the  dissection  (if  the  same  specimen  is  still  being  used,  this  cut  will  already  have 
been  made).  Open  the  flap  thus  formed  and  expose  the  roof  of  the  mouth. 
Make  a  longitudinal  cut  through  the  mucous  membrane  of  the  roof  in  the  median 
dorsal  line.  Strip  the  membrane  laterally  carrying  with  it  the  free  dorsal  ends 
of  the  gill  cartilages  which  will  be  readily  located  just  lateral  to  the  mid-dorsal 
line.  The  visceral  branch  of  the  vagus  nerve,  which  was  already  seen  from  the 
other  side,  is  now  exposed.  It  lies  along  the  thin  ventral  wall  of  the  anterior 
cardinal  sinus.  Emerging  from  the  muscle  now  exposed  in  the  roof  of  the  mouth 
will  be  seen  the  ventral  rami  of  the  spinal  nerves.  In  the  spiny  dogfish  two  of 
these  (they  appear  as  one  but  will  be  found  to  consist  of  two  on  dissecting  them 
toward  the  median  line)  pass  obliquely  toward  the  visceral  branch  of  the  vagus 
and  enter  its  sheath,  thus  appearing  to  join  it.  The  trunk  they  form  can,  how- 
ever, be  readily  separated  from  the  vagus.  This  trunk  is  the  hypobranchial 
nerve.  In  the  smooth  dogfish  the  several  most  anterior  ventral  rami  unite  to 
form  a  large  trunk,  the  hypobranchial  nerve.  This  nerve  lies  just  anterior  to  the 
nerves  of  the  cervicobrachial  plexus  to  which  it  contributes  branches.  It  passes 
to  the  dorsal  side  of  the  visceral  branch  of  the  vagus  (which  may  be  noted 
proceeding  to  the  esophagus)  posterior  to  the  last  visceral  pouch,  and  turning 
ventrally  courses  along  the  floor  of  the  mouth,  supplying  muscles. 

In  the  skate  the  hypobranchial  nerve  leaves  the  spinal  cord  in  common  with 
the  trunk  of  the  brachial  plexus.  Locate  this  trunk  again  in  the  anterior  wall 
of  the  pleuroperitoneal  cavity,  dorsal  to  the  pleuroperitoneum  and  just  behind 
the  cartilage  of  the  pectoral  girdle.  The  entire  trunk  with  the  exception  of 
one  branch  passes  out  dorsal  to  the  cartilage  to  the  pectoral  fin.  This  one 
branch,  the  hypobranchial  nerve,  turns  forward  and  is  distributed  to  the  muscles 
of  the  floor  of  the  mouth. 

The  occipital  and  hypobranchial  nerves  are  probably  the  homologues  of  the 
twelfth  or  hypoglossal  nerve  of  amniotes,  having  been  taken  into  the  cranium 
in  those  forms.  The  muscles  supplied  by  the  hypobranchial  nerve  are  believed 
to  be  derived  from  the  most  posterior  of  the  head  myotomes,  and  in  higher  forms 
become  the  intrinsic  muscles  of  the  tongue. 

6.  The  ventral  aspect  of  the  brain. — Carefully  free  the  brain  from  the  chon- 
drocranium.  First  cut  through  the  olfactory  tracts  and  gently  lift  the  ante- 


COMPARATIVE  ANATOMY  OF  THE  NERVOUS  SYSTEM       317 

rior  end  of  the  brain.  You  will  next  see  the  two  optic  nerves  entering  the 
ventral  surface  of  the  diencephalon.  Cut  through  them  and  lift  the  brain  farther. 
Next,  pare  away  the  wall  of  the  orbit  on  one  side.  It  will  then  be  seen  that 
certain  structures  attached  to  the  ventral  surface  of  the  diencephalon  extend 
ventrally  into  a  deep  pit  (sella  turcica)  in  the  floor  of  the  cranial  cavity.  Take 
especial  care  to  lift  these  out  intact.  Then  cut  through  the  remainder  of  the 
cranial  nerves  and  cut  across  the  spinal  cord  and  lift  the  brain  out  of  the  cranial 
cavity. 

Examine  the  ventral  surface  of  the  brain.  Note  the  forward  continuation 
of  the  internal  carotid  artery  on  the  mid  ventral  line  of  the  brain.  It  forks  around 
the  ventral  part  of  the  diencephalon  (farther  posteriorly  in  the  skate)  and  passes 
forward  to  the  telencephalon,  distributing  many  branches  to  all  parts  of  the 
brain  as  well  as  to  the  orbit.  The  ventral  surface  of  the  brain  presents  nothing 
new  except  as  regards  the  diencephalon,  where  several  additional  structures  are 
visible.  The  two  optic  nerves  are  seen  attached  to  the  anterior  end  of  the 
ventral  surface  of  the  diencephalon;  as  they  enter  the  latter  they  cross,  the 
crossed  region  being  named  the  optic  chiasma.  From  the  chiasma  a  broad  band, 
the  optic  tract,  extends  dorsad  and  caudad  into  the  dorsal  part  of  the  diencephalon 
and  into  the  optic  lobes.  It  is  readily  seen,  especially  in  the  spiny  dogfish,  by 
scraping  off  the  primitive  meninx  at  this  place.  It  is  thus  evident  that  the  visual 
impulses  pass  into  these  two  portions  of  the  brain.  Posterior  to  the  optic  chiasma 
the  floor  of  the  diencephalon  bulges  ventrally  and  posteriorly  as  the  infundibu- 
lum,  consisting  in  large  part  of  two  rounded  lobes,  the  inferior  lobes.  These  are 
devoted  mainly  to  smell  and  taste.  From  between  the  two  inferior  lobes  a 
stalk  projects  caudad  and  widens  into  a  soft  sac.  This  whole  structure  is  the 
hypophysis  or  pituitary  body.  The  dorsal  part  of  the  sac  has  thinner  walls, 
generally  discolored  because  of  contained  blood;  it  is  called  the  vascular  sac  or 
saccus  vasculosus.  The  hypophysis  is  generally  more  or  less  torn  in  removing 
the  brain.  It  is  known  to  be  a  gland  of  internal  secretion  in  mammals  and 
probably  has  the  same  function  in  other  vertebrates.  Dorsal  to  the  vascular 
sac  the  roots  of  the  oculomotor  nerves  will  be  found  springing  from  the  floor  of 
the  midbrain.  The  ventral  surface  of  the  remainder  of  the  brain  presents  nothing 
new.  The  roots  of  the  cranial  nerves  should  be  identified  on  the  medulla.  On 
the  ventral  surface  of  the  medulla  will  be  found  the  roots  of  the  sixth  nerves. 
If  not  identifiable  on  the  brain,  they  will  usually  be  found  adhering  to  the  floor 
of  the  cavity  from  which  the  brain  was  removed. 

Draw  a  profile  view  of  the  brain. 

7.  The  sagittal  section  and  the  ventricles  of  the  brain. — The  brain  like  the 
remainder  of  the  central  nervous  system  is  hollow.  Its  cavities  are  known  as 
ventricles  and  are  continuous  with  each  other  by  means  of  narrow  passages. 
The  fourth  or  last  ventricle  of  the  brain  has  already  been  identified  as  the  cavity 
within  the  medulla  oblongata.  The  ventral  portion  of  this  ventricle  is  named 


318       LABORATORY  MANUAL  FOR  VERTEBRATE  ANATOMY 

from  its  shape  the  fossa  rhomboidea.  Bisect  the  brain  by  a  median  sagittal  cut. 
Examine  the  cut  surface,  best  under  water.  From  the  fourth  ventricle  a  narrow 
passage,  the  aqueduct  of  the  brain,  extends  anteriorly.  It  communicates  with  the 
cavity  of  the  cerebellum,  the  cerebellar  ventricle,  and  the  cavities  of  the  optic 
lobes,  the  optic  ventricles.  The  thick  floor  of  the  midbrain  ventral  to  the  aqueduct 
forms  the  cerebral  peduncles.  The  aqueduct  opens  into  the  cavity  of  the  diencepha- 
lon,  the  third  ventricle.  The  roof  of  the  diencephalon  is  very  thin  and  consists  of  a 
chorioid  (vascular)  plexus  which  is  folded  into  the  cavity.  The  anterior  part  of  this 
roof  in  the  dogfishes  extends  dorsally  into  a  sac,  the  paraphysis,  resting  against  the 
telencephalon.  It  is  regarded  as  part  of  the  latter.  Posterior  to  the  paraphysis, 
a  thin  transverse  partition,  the  velum  transversum,  is  seen  in  the  roof  of  the 
diencephalon,  particularly  in  the  spiny  dogfish.  This  marks  the  dorsal  boundary 
between  diencephalon  and  telencephalon.  The  small  thickened  region  of  the 
diencephalon  just  in  front  of  the  anterior  end  of  the  optic  lobe  is  the  habenula, 
a  smell  center.  From  the  habenula  a  slender  process,  the  pineal  body,  may  be 
seen  in  favorable  specimens,  extending  dorsally,  just  back  of  the  paraphysis. 
The  entire  roof  of  the  diencephalon,  including  chorioid  plexus,  habenula,  and 
pineal  body,  is  named  the  epithalamus  and  is  concerned  chiefly  with  smell.  The 
lateral  walls  of  the  diencephalon  constitute  the  thalamus,  an  important  correla- 
tion center  for  various  body  senses.  The  ventral  part  of  the  diencephalon  is 
named  the  hypothalamus.  It  consists  of  the  infundibulum,  including  the  inferior 
lobes,  the  hypophysis,  and  the  mammillary  bodies.  The  former  should  be  identi- 
fied on  the  section.  The  mammillary  bodies  are  the  thickened  part  of  the  ventral 
wall  above  the  vascular  sac.  The  hypothalamus  is  concerned  with  smell  and 
taste.  Note  that  the  cavity  of  the  third  ventricle  extends  into  all  parts  of  the 
hypothalamus.  The  third  ventricle  connects  by  a  passage,  the  foramen  of  Monro, 
or  interventricular  foramen,  with  the  cavity  in  each  half  of  the  telencephalon. 
(In  the  skate,  the  telencephalon  is  solid.)  These  two  cavities  are  named  the 
first  and  second  or  lateral  ventricles.  They  extend  out  into  the  olfactory  bulbs 
through  the  olfactory  tracts.  Cut  into  the  telencephalon  to  see  its  ventricles. 
Draw  the  sagittal  section. 

C.      THE  NERVOUS  SYSTEM  AND   SENSE   ORGANS   OF  NECTURUS 

i.  The  spinal  nerves. — The  spinal  nerves  are  best  found  as  follows.  Make 
a  longitudinal  cut  along  the  side  of  the  body  below  the  lateral  line.  Cut  through 
the  external  and  internal  oblique  muscles  and  separate  this  mass  of  muscle  from 
the  thin  layer  of  transverse  muscles  lying  next  to  the  coelom.  The  ventral  rami 
of  the  spinal  nerves  will  now  be  seen  running  between  the  oblique  and  transverse 
muscles,  along  the  myosepta,  and  supplying  the  hypaxial  muscles.  Trace  one 
of  these  toward  the  vertebral  column,  cutting  away  muscles  from  its  course. 
It  lies  just  behind  the  rib  which  may  be  cut  away.  The  nerve  may  be  traced 
up  to  the  vertebra  where  it  is  imbedded  in  an  orange-colored  material.  On 


COMPARATIVE  ANATOMY  OF  THE  NERVOUS  SYSTEM  319 

carefully  clearing  this  away  the  dorsal  ganglion  of  the  spinal  nerve  will  be  found 
imbedded  in  it.  It  is  a  rounded  brownish  body  from  which  spring  two  nerves, 
the  ventral  ramus  just  followed  and  the  smaller  dorsal  ramus  which  supplies  the 
epaxial  muscles. 

2.  The  limb  plexi. — The  ventral  rami  of  the  spinal  nerves  form  a  plexus 
for  each  limb,  the  motor  nerves  for  the  limb  muscles  arising  from  this  plexus. 
The  brachial  plexus  is  located  as  follows.     Make  a  cut  in  the  ventral  body  wall 
just  medial  to  the  base  of  the  fore  limb.     Separate  the  pectoral  and  shoulder 
muscles  from  the  sternohyoid  muscle.     The  brachial  plexus  is  then  easily  seen 
running  posterior  to  the  scapula.     It  consists  of  the  ventral  rami  of  three  spinal 
nerves  (3,  4,  and  5)  which  have  cross-connections  with  each  other.     Beyond  the 
plexus,  nerves  proceed  into  the  fore  limb. 

The  lumbosacral  plexus  is  located  by  cutting  through  the  skin  and  both  layers 
of  oblique  muscles  longitudinally  just  dorsal  to  the  base  of  the  hind  limb.  On 
separating  the  oblique  from  the  transverse  muscle  layers  the  plexus  is  exposed. 
It  consists  of  the  ventral  rami  of  three  (sometimes  four)  spinal  nerves.  The 
anterior  one  of  the  three  gives  off  a  slight  branch  to  the  nerve  next  posterior  to 
it,  sometimes  receives  a  contribution  from  the  ramus  anterior  to  it,  and  as  the 
crural  nerve,  passes  into  the  limb.  The  middle  of  the  three  nerves  is  much  the 
stoutest,  and  after  receiving  contributions  from  the  nerve  next  posterior  to  it 
enters  the  limb  as  the  ischiadic  (sciatic)  nerve. 

3.  The  sense  organs  of  the  head. — Owing  to  the  small  size  of  the  eye  and  its 
similarity  to  that  of  other  vertebrates  it  will  not  be  investigated. 

a)  Lateral  line  system:  Lateral  line  sense  organs  are  present  in  Necturus  but 
impractical  to  find  in  gross  dissection.     They  are  situated  along  lines  similar  to 
those  of  fishes. 

b)  Nose:  Probe  into  the  external  naris  and  follow  your  probe  with  the  cut, 
opening  the  entire  nasal  passage  to  the  internal  naris.     Note  the  folds  or  olfactory 
lamellae  in  the  interior  of  the  passage.     Unlike  the  condition   in   fishes,  the 
olfactory  sac  opens  into  the  oral  cavity  and  has  both  olfactory  and  respiratory 
functions. 

c)  Ear:  The  ear  as  in  fishes  consists  of  the  internal  ear  only,  imbedded  in 
the  otic  region  of  the  skull.     Expose  the  dorsal  surface  of  the  skull  by  cleaning 
away  the  muscles.     Locate  the  otic  capsules,  one  at  either  side  of  the  posterior 
end  of  the  skull.     Cautiously  shave  away  the  cartilage  here  and  locate  the  three 
semicircular  ducts  and  vestibule  as  done  in  the  dogfish.     There  are  three  of  the 
former — anterior  vertical,  posterior  vertical,  and  horizontal — each  with  an  ampulla, 
all  arranged  exactly  as  in  the  dogfish.     The  vestibule  is  divided  indistinctly  into 
a  dorsal  utriculus,  from  which  the  ducts  spring,  and  a  ventral  sacculus.    The 
latter  contains  a  crystalline  mass,  the  otolith. 

4.  The  dorsal  aspect  of  the  brain. — Remove  the  roof  of  the  skull.     This  is 
best  done  by  stripping  it  off  in  slivers  with  a  knife.    After  the  brain  is  revealed 


320       LABORATORY  MANUAL  FOR  VERTEBRATE  ANATOMY 

study  its  dorsal  surface.  The  brain  is  covered  by  a  membrane,  the  primitive 
meninx,  which  is  more  or  less  divisible  into  the  pia  mater,  a  delicate  pigmented 
membrane  adhering  to  the  brain,  and  an  outer  dura  mater,  which  is  separated 
from  the  skull  by  the  peridural  space. 

The  most  anterior  portion  of  the  brain  consists  of  the  two  elongated  lobes  of 
the  telencephalon.  These  are  very  indistinctly  subdivided  into  anterior  olfactory 
lobes  and  posterior  cerebral  hemispheres.  The  olfactory  lobes  include  the  bulbs 
and  tracts  of  the  dogfish.  Between  and  behind  the  posterior  ends  of  the  tel- 
encephalic  lobes  is  a  thin  roof  containing  a  chorioid  plexus.  The  anterior  part 
of  this  bears  a  process,  the  paraphysis,  projecting  forward  in  the  groove  between 
the  two  cerebral  hemispheres.  It  is  regarded  as  a  part  of  the  telencephalon. 
The  very  small  region  posterior  to  this  is  the  roof  of  the  diencephalon.  It  bears 
a  dorsally  projecting  process,  the  pinal  body  or  epiphysis,  which  lies  immediately 
posterior  to  the  paraphysis.  Posterior  to  the  diencephalon  are  the  two  elongated 
optic  lobes,  representing  the  dorsal  part  of  the  mesencephalon.  Behind  this  is 
another  chorioid  plexus,  a  dark  pigmented  membrane.  This  may  be  removed. 
There  is  then  revealed  the  triangular  cavity  of  the  fourth  ventricle.  The  anterior 
end  of  this  cavity  is  overhung  by  a  narrow  shelf  at  the  posterior  margins  of  the 
optic  lobes.  This  shelf  is  the  cerebellum.  The  remainder  of  the  brain  consists 
of  the  medulla  oblongata  which  incloses  the  cavity  of  the  fourth  ventricle.  The 
medulla  extends  forward  at  the  sides  of  the  optic  lobes  as  projections,  the  greater 
part  of  which  consists  of  the  acustico-lateral  area.  The  walls  of  the  medulla  are 
divided  into  dorsal  or  sensory  portions  and  ventral  or  motor  portions,  the  latter 
forming  a  broad  band  on  either  side  of  the  median  ventral  groove.  The  further 
subdivisions  of  these  are  not  evident  in  Necturus. 

Draw  the  dorsal  view  of  the  brain. 

5.  The  cranial  nerves. — 

a)  The  first  or  olfactory  nerve:  This  is  a  stout  band  passing  from  the  olfactory 
sac  to  the  anterior  end  of  the  telencephalon. 

b)  The  second  or  optic  nerve:  This  small  nerve  may  be  seen  in  the  floor  of 
the  cranial  cavity  by  pressing  the  telencephalon  to  one  side.     It  passes  obliquely 
caudad  to  the  ventral  surface  of  the  diencephalon. 

c)  The  eye  muscle  nerves:  The  third  or  oculomotor,  fourth  or  trochlear,  and 
sixth  or  abducens  nerves  are  so  small  in  Necturus  as  to  be  scarcely  discernible  in 
gross  dissection.     They  originate  from  the  same  regions  of  the  brain  and  supply 
the  same  eye  muscles  as  in  elasmobranchs. 

d)  The  fifth  or  trigeminus  nerve:  The  fifth  nerve  is  the  large  trunk  arising 
from  the  side  of  the  anterior  end  of  the  medulla.     Trace  it  out  through  the 
skull.     It  passes  in  front  of  the  otic  capsule  and  immediately  enters  a  large 
ganglion,  the  semilunar  or  Gasserian  ganglion  of  the  trigeminus.     From  the 
ganglion  three  large  nerves  are  given  off,  the  ophthalmic,  maxillary,  and  man- 
dibular  branches.     The  former,  corresponding  to  the  deep  ophthalmic  of  fishes, 
passes  forward  through  the  quadrate  cartilage  and  runs  forward  anteriorly  along- 


COMPARATIVE  ANATOMY  OF  THE  NERVOUS  SYSTEM  321 

side  the  frontal  bones.  The  maxillary  nerve  runs  along  the  margin  of  the  upper 
jaw.  The  mandibular  nerve  passes  laterally  to  the  angle  of  the  jaws  and  then 
turns  forward  along  the  lower  jaw. 

e)  The  seventh  or  facial  nerve:  This  arises  just  behind  the  trigeminus  and 
sends  a  branch  forward  to  join  the  latter  at  the  semilunar  ganglion.  This  branch 
of  the  facial  passes  out  with  the  ophthalmic  nerve  as  the  superficial  ophthalmic 
branch  of  the  facial  and  with  the  maxillary  nerve  as  the  buccal  branch.  Both  of 
these  go  to  lateral  line  organs.  The  greater  part  of  the  facial  arises  from  the 
medulla  in  common  with  the  auditory  nerve  ventral  to  the  above-named  branch. 
This  common  acustico-facial  trunk  of  facial  and  auditory  passes  into  the  anterior 
part  of  the  otic  capsule.  From  here  the  main  trunk  of  the  facial  or  hyomandib- 
ular  may  be  followed  laterally.  It  branches  to  muscles,  lateral  line  organs,  etc. 

/)  The  eighth  or  auditory  nerve:  This  arises  in  common  with  the  facial  and 
is  distributed  to  the  internal  ear.  Its  branches  are  readily  noted  in  the  otic 
capsule. 

g)  The  ninth  or  glossopharyngeal  and  tenth  or  vagus  nerves:  These  arise 
together  from  the  medulla  posterior  to  the  acustico-facial  trunk,  by  three  roots. 
The  common  trunk  passes  along  the  posterior  margin  of  the  ear  capsule  and 
enters  a  large  ganglion.  From  this  several  nerves  arise  which  may  be  traced 
into  the  external  gills  (these  nerves  are  not  homologous  with  the  pre-  and  post- 
trematic  branches  of  elasmobranchs),  and  to  the  visceral  arches.  The  most 
posterior  branch  of  the  vagus  gives  off  a  lateral  branch  which  passes  to  the  lateral 
line  which  it  accompanies.  The  vagus  also  supplies  the  viscera. 

Add  the  cranial  nerves  to  your  drawing  of  the  brain,  as  far  as  seen. 

6.  Ventral  aspect  of  the  brain. — Remove  the  brain  by  cutting  across  the 
spinal  cord,  the  olfactory,  and  other  nerves.  The  ventral  view  reveals  some 
additional  parts  of  the  diencephalon.  At  the  anterior  end  of  the  ventral  surface 
of  the  diencephalon  is  the  small  optic  chiasma  formed  by  the  optic  nerves.  Pos- 
terior to  this  is  the  large  infundibulum  from  the  posterior  end  of  which  projects 
the  hypophysis.  The  ventral  surface  of  the  midbrain  consists  of  the  two  cerebral 
peduncles. 

Draw  the  ventral  view. 

D.   THE  NERVOUS  SYSTEM  AND  SENSE  ORGANS  OF  THE  TURTLE 

i.  The  spinal  nerves,  the  sympathetic  system,  and  the  spinal  cord. — Remove 
all  of  the  viscera  from  the  pleuroperitoneal  cavity,  leaving  the  large  neck  muscles 
in  the  mid-dorsal  region  undisturbed.  Identify  the  spinal  nerves  as  the  white 
cords  passing  along  the  sutural  lines  of  the  costal  plates.  The  sympathetic 
chain  should  be  identified  as  a  white  cord  or  cords  located  on  the  sides  of  the 
mass  of  neck  muscles.1 

1  The  fact  that  the  sympathetic  system  was  not  described  in  elasmobranchs  and  Necturus  should 
not  be  taken  to  indicate  that  the  system  is  absent  in  those  forms.  It  is  present  but  impractical  to 
dissect  out. 


322       LABORATORY  MANUAL  FOR  VERTEBRATE  ANATOMY 

a)  Spinal  nerves  and  limb  plexi:  Carefully  expose  the  spinal  nerves  of  the 
trunk,  avoiding  injury  to  the  sympathetic  system.   These  nerves,  called  the  dor- 
sal spinal  nerves,  run  along  the  sutures  between  the  costal  plates  of  the  cara- 
pace.    In  most  cases  each  consists  of  two  branches,  a  smaller  dorsal  ramus 
and  a  larger  ventral  ramus.     On  tracing  these  toward  the  vertebral  column  they 
will  be  found  to  come  from  a  large  ganglion,  the  dorsal  or  spinal  ganglion,  situated 
in  contact  with  the  center  of  the  centrum. 

Expose  the  brachial  plexus  in  the  depression  between  the  neck  and  the  dorsal 
end  of  the  scapula.  It  is  generally  formed  by  the  cross-unions  between  the 
ventral  rami  of  the  last  four  cervical  spinal  nerves  and  the  first  dorsal  spinal 
nerve.  The  last-named  nerve  may  be  identified  as  the  one  in  front  of  the  first 
typical  rib  (really  the  second  rib).  The  four  cervical  nerves  form  a  complex 
network  on  the  surface  of  the  shoulder  muscles.  From  this  network  the  large 
median  nerve  proceeds  along  the  anterior  surface  of  these  muscles  and  the 
smaller  ulnar  and  radial  nerves  along  the  posterior  surface;  the  radial  is  the  most 
dorsally  situated  one.  The  first  dorsal  nerve  sends  a  branch  near  its  ganglion 
to  the  brachial  plexus  and  is  then  distributed  to  the  carapace  just  posterior  to 
the  fore  limb. 

The  next  six  dorsal  nerves  are  similar  to  the  first  description  given.  The 
ventral  rami  of  the  eighth,  ninth,  and  tenth  dorsal  nerves  together  with  the  two 
sacral  nerves  form  the  lumbosacral  plexus  for  the  hind  limb.  This  lies  on  the 
medial  surface  of  the  muscles  covering  the  ilium  and  is  found  by  carefully  separat- 
ing the  ilium  with  its  muscles  from  the  median  region.  The  branches  from  the 
three  dorsal  nerves  unite  to  a  sort  of  knot  from  which  several  nerves  proceed  to 
the  anterior  part  of  the  leg.  The  two  sacral  nerves,  receiving  also  a  contribu- 
tion from  the  tenth  dorsal  nerve,  unite  to  form  a  large  trunk,  the  sciatic  nerve, 
situated  among  the  muscles  on  the  posterior  side  of  the  leg. 

There  is  a  pair  of  caudal  spinal  nerves  corresponding  to  each  caudal  vertebra, 
but  these  need  not  be  looked  for.  There  are  nine  pairs  of  cervical  nerves  which 
will  be  found  by  looking  in  the  neck  at  the  same  level  as  the  level  of  emergence 
of  the  nerves  of  the  brachial  plexus. 

b)  The  sympathetic  system:  Locate  the  vagus  nerve  in  the  neck.     It  is  the 
conspicuous  white  cord  running  along  the  side  of  the  neck.     Trace  it  posteriorly. 
The  sympathetic  trunk  is  bound  with  it  but  at  about  the  level  of  the  first  nerve 
of  the  brachial  plexus  separates  from  the  vagus  and  enters  a  swelling  or  ganglion, 
the  middle  cervical  ganglion.     The  sympathetic  cord  proceeds  dorsally  from  this 
ganglion  and  lies  on  the  ventral  surface  of  the  brachial  plexus  where  it  presents 
two  successive  swellings,  which  together  constitute  the  inferior  cervical  ganglion. 
Observe  branches  from  these  ganglia.     The  sympathetic  cord  passes  to  the 
ganglion  of  the  first  dorsal  spinal  nerve  to  which  its  own  ganglion  is  fused.     It 
then  proceeds  as  a  delicate  white  cord  across  the  second  rib  and  again  forms  a 
ganglion  which  is  fused  to  the  ganglion  of  the  second  dorsal  nerve.     The  sympa- 
thetic cord  then  passes  more  ventrally,  lying  on  the  side  of  the  long  neck  muscles. 


COMPARATIVE  ANATOMY  OF  THE  NERVOUS  SYSTEM  323 

Follow  it  here  and  note  the  ganglia  which  it  bears  at  intervals  and  the  branches 
from  these  ganglia.  Note  particularly  the  branches  between  the  sympathetic 
ganglia  and  the  adjacent  spinal  ganglia.  These  branches  constitute  the  ramus 
communicans  and  consist  of  the  visceral  motor  and  visceral  sensory  fibers  passing 
between  the  sympathetic  and  central  nervous  systems.  The  ganglia  and 
branches  of  the  sympathetic  are  particularly  noticeable  in  the  urogenital  region. 

Draw  spinal  nerves,  plexi,  and  sympathetic  system  as  far  as  seen. 

2.  The  sense  organs  of  the  head. — 

a)  The  nasal  cavities:  The  external  nares  lead  into  wide  chambers,  the  nasal 
cavities.     Cut  off  the  external  nares  and  the  roof  of  the  skull  posterior  to  them, 
thus  revealing  the  nasal  cavities.     They  are  separated  by  a  median  septum, 
partly  bony.     From  the  ventral  region  of  the  septum  a  conspicuous  fold  projects 
into  the  nasal  cavity.     On  the  posterior  wall  of  the  nasal  cavity  is  a  slight  pro- 
jection, a  concha  or  turbinal.     Posterior  to  this  the  nasal  cavity  connects  by  a 
passage  with  the  roof  of  the  mouth  cavity,  the  nasal  cavities  thus  serving  as 
respiratory  passages. 

b)  The  eye:  Although  the  eye  is  small  it  can  be  dissected  with  a  little  care. 
Its  parts  are  very  similar  to  those  of  the  fish  eye.     Make  an  incision  through  the 
skin  around  one  eye  and  with  the  bone  scissors  remove  the  skull  dorsal  to  and 
between  the  eyes.     The  two  eyes  are  seen  to  be  close  together,  separated  by  a 
median  membranous  interorbital  septum.    Near  this  septum  on  each  side  runs 
an  artery.     On  the  anterior  dorsal  surface  of  the  eyeball  is  a  gland,  the  Harderian 
gland.     Over  the  posterior  and  ventral  surface  of  the  eyeball  extends  the  much 
larger  lacrimal  or  tear  gland.     Remove  these  glands,  thus  exposing  the  surface 
of  the  eyeball  and  the  eye  muscles.     Extending  from  the  interorbital  septum  to 
the  dorsal  surface  of  the  eyeball  is  the  superior  oblique  muscle.     Posterior  to 
this  and  inserted  in  the  eyeball  near  it  is  the  superior  rectus.    Between  and  ven- 
tral to  these  two  is  the  internal  rectus.     Passing  above  the  internal  rectus  are 
two  nerves,  the  trochlear  to  the  superior  oblique  muscle  and  the  ophthalmic 
branch  of  the  trigeminus.    Loosen  the  eyeball  ventrally  and  raising  it  as  far  as 
possible  examine  the  ventral  surface.     The  anterior  part  of  this  surface  is  covered 
by  a  flat  muscle,  the  pyramidalis,  which  originates  on  the  eyeball  and  passes  to 
the  eyelids  and  nictitating  membrane.     Remove  this  and  clean  the  ventral 
surface  of  the  eyeball.     The  inferior  oblique  and  inferior  rectus  muscles  are  then 
seen  converging  to  their  insertions  on  the  ventral  surface  of  the  eyeball.     The 
external  rectus  is  posterior  to  them. 

Remove  the  eyeball  and  open  it  by  cutting  off  its  dorsal  side.  Identify  the 
coats  of  the  eyeball,  the  lens,  the  cavities  of  the  eye,  and  the  two  humors  as  in 
the  elasmobranch  eye,  as  the  structure  is  practically  identical  with  the  latter. 
Note,  however,  the  difference  in  the  shape  of  the  lens  of  the  turtle  eye. 

c)  The  ear:  The  ear  consists  of  two  parts,  a  middle  ear  and  an  internal  ear. 
The  former  is  located  posterior  to  the  angle  of  the  jaws  internal  to  a  circular  area 


324       LABORATORY  MANUAL  FOR  VERTEBRATE  ANATOMY 

of  skin.  Remove  this  piece  of  skin  and  find  beneath  it  a  smaller  cartilaginous 
circular  plate,  the  tympanic  membrane  or  eardrum.  Make  a  cut  around  the 
margin  of  this  and  carefully  raise  it.  Attached  to  its  internal  surface,  posterior 
to  the  center,  is  a  rod-shaped  bone,  the  columella,  whose  inner  end  is  fastened  to 
the  wall  of  a  large  cavity.  This  cavity  is  the  tympanic  cavity  or  cavity  of  the 
middle  ear.  It  is  an  evagination  from  the  first  visceral  pouch.  Ventral  to  the 
inner  end  of  the  columella  is  a  slit  bounded  by  raised  lips.  This  slit  is  the  open- 
ing of  the  auditory  tube  connecting  the  pharyngeal  cavity  with  the  cavity  of 
the  middle  ear  and  representing  the  stalk  of  the  evagination  by  which  the  latter 
was  formed.  Considerably  internal  to  the  point  of  attachment  of  the  inner  end 
of  the  columella  lies  the  internal  ear.  It  will  be  more  definitely  located  later. 
It  is  similar  in  structure  to  the  internal  ear  of  elasmobranchs. 

3.  Dorsal  aspect  of  the  brain. — Remove  the  roof  of  the  skull  and  expose  the 
brain.  The  brain  is  covered  by  a  tough  membrane,  the  dura  mater.  On  cutting 
carefully  through  this  a  more  delicate  membrane,  the  pia  mater,  will  be  found 
adhering  to  the  brain.  It  is  more  or  less  pigmented  and  contains  numerous  blood 
vessels.  Strands  cross  between  the  two  membranes.  The  two  membranes  are 
derived  by  the  splitting  of  the  original  single  membrane  of  the  brain,  the  primitive 
meninx  of  fishes.  The  space  between  the  dura  mater  and  pia  mater  is  the  sub- 
dural  space;  between  the  former  and  the  skull,  the  peridural  space.  Remove  the 
dura  mater  from  the  dorsal  surface  of  the  brain. 

The  brain  possesses  the  same  divisions  as  in  the  preceding  forms,  but  the 
relative  proportions  of  the  parts  are  somewhat  altered.  The  anterior  end  con- 
sists of  the  two  olfactory  lobes,  which  include  the  olfactory  bulbs  and  tracts  seen 
in  the  elasmobranch ;  the  whole  olfactory  apparatus  is  evidently  much  reduced 
in  the  turtle  as  compared  with  the  fish.  Posterior  to  the  olfactory  lobes  and 
separated  from  them  by  a  slight  groove  are  the  two  cerebral  hemispheres,  relatively 
much  larger  than  in  the  elasmobranchs  and  Necturus.  Between  the  posterior 
ends  of  the  two  cerebral  hemispheres  is  the  diencephalon,  its  roof  consisting  of 
a  chorioid  plexus,  which  projects  dorsally  as  a  sac  that  adheres  to  the  dura  mater 
and  is  generally  torn  off  in  removing  the  latter.  This  sac  consists  of  the 
paraphysis  and  the  epiphysis.  When  it  is  removed  the  diencephalon  is  seen 
as  a  depressed  area  posterior  and  ventral  to  the  cerebral  hemispheres.  Posterior 
to  the  diencephalon  are  the  two  rounded  optic  lobes,  belonging  to  the  midbrain. 
Behind  them  is  the  cerebellum,  smaller  than  in  elasmobranchs  but  larger  than 
in  Amphibia.  Posterior  to  the  cerebellum  and  partly  overhung  by  it  is  the  me- 
dulla oblongata.  Its  thin  dorsal  roof  contains  as  usual  a  chorioid  plexus.  On 
removing  the  roof  of  the  medulla,  the  cavity  of  the  fourth  ventricle  is  revealed 
and  the  cerebellum  is  then  seen  to  project  like  a  roof  over  the  anterior  part 
of  the  medulla.  The  dorsal  rims  of  the  medulla  consist  as  usual  of  the  somatic 
sensory  columns,  and  on  raising  the  cerebellum  the  anterior  ends  of  these  col- 
umns which  are  auditorv  in  nature  are  seen  to  be  connected  with  the  cerebel- 


COMPARATIVE  ANATOMY  OF  THE  NERVOUS  SYSTEM       325 

lum.  In  the  floor  of  the  medulla  the  two  somatic  motor  columns  are  conspicuous, 
one  on  either  side  of  the  midventral  groove. 

Draw  the  dorsal  aspect  of  the  brain. 

4.  The  cranial  nerves. — The  dissection  of  the  cranial  nerves  is  a  matter  of 
some  difficulty  and  the  following  description  is  consequently  not  complete. 

a)  The  olfactory  nerves:  These  are  the  two  stout  nerves  extending  from  the 
dorsal  portions  of  the  olfactory  sacs  to  the  anterior  end  of  the  olfactory  lobes  of 
the  brain. 

b)  The  optic  nerves:  On  cutting  through  the  olfactory  nerves  and  raising  the 
anterior  end  of  the  brain,  the  optic  nerves  are  seen  as  two  stout  trunks  situated 
below  the  cerebral  hemispheres  and  passing  out  of  the  orbit. 

c)  The  trochlear  nerve:  This  small  nerve  arises  on  each  side  of  the  brain  in 
the  dorsolateral  angle  between  the  optic  lobe  and  the  cerebellum.     It  passes 
ventrally  and  forward,  and  will  be  seen  by  pressing  the  cerebral  hemisphere  away 
from  the  skull.     It  lies  behind  the  larger  oculomotor  nerve.     To  find  the  course 
of  the  trochlear  nerve  in  the  orbit  expose  the  undissected  eye  as  before,  clearing 
away  the  glands.     Cut  through  the  superior  oblique  muscle  at  its  point  of 
insertion  on  the  eyeball  and  find  below  it  the  trochlear  nerve  terminating  in  this 
muscle.     Medial  to  the  trochlear  is  the  ophthalmic  branch  of  the  trigeminus. 

d)  The  oculomotor  nerve:  This  nerve  originates  from  the  floor  of  the  midbrain 
immediately  in  front  of  the  trochlear  and  is  seen  by  pressing  the  cerebral  hemi- 
sphere away  from  the  skull.    Loosen  the  ventral  side  of  the  eyeball  and  raise 
it.    Among  the  loose  tissues  between  the  eyeball  and  the  floor  of  the  orbit, 
generally  adhering  to  the  eyeball,  is  a  nerve,  the  maxillary  branch  of  the  tri- 
geminus.    Free  this  from  the  eyeball.     Cut  away  the  pyramidal  muscle,  and 
raising  the  eyeball  and  pressing  it  as  far  medially  as  possible,  separate  the  inferior 
and  external  rectus  muscles  and  find  between  and  above  them  the  stout  white 
trunk  of  the  optic  nerve.     The  oculomotor  is  in  contact  with  the  ventral  surface 
of  the  optic  nerve  and  branches  to  the  same  four  eye  muscles  as  in  the  dogfish. 
These  are  not  readily  followed. 

e)  The  trigeminus  nerve:  This  is  a  stout  trunk  whose  origin  from  the  anterior 
end  of  the  medulla  will  be  seen  by  pressing  the  cerebellar  region  of  the  brain  away 
from  the  skull.     The  trunk  passes  laterally  and  enters  its  ganglion,  the  semilunar 
ganglion,  which  lies  in  a  depression  in  the  medial  wall  of  the  skull.     The  tri- 
geminus has  three  branches,  the  ophthalmic,  the  maxillary,  and  the  mandibular, 
distributed  to  the  orbit  and  nose,  the  upper,  and  the  lower  jaw,  respectively. 
Remove  the  eyeball  on  the  side  where  it  is  still  present  leaving  the  ophthalmic 
and  maxillary  nerves  intact.     Cut  through  the  roots  of  the  nerves  anterior  to 
the  trigeminus  so  as  to  raise  the  brain  and  bend  it  away  from  the  side  being 
dissected.     Follow  the  ophthalmic  nerve  forward  and  note  its  distribution  to  the 
nasal  sacs.     Follow  it  posteriorly  toward  the  root  of  the  trigeminus.     It  enters 
the  skull,  runs  in  company  with  the  trochlear  nerve  between  the  dura  mater  am? 


326       LABORATORY  MANUAL  FOR  VERTEBRATE  ANATOMY 

the  skull,  and  finally  joins  the  semilunar  ganglion.  Follow  the  maxillary  nerve 
posteriorly.  Besides  the  branch  below  the  eyeball  already  noted  there  is  a 
branch  in  the  floor  of  the  orbit  running  obliquely  forward.  These  two  branches 
unite  to  form  the  main  trunk  of  the  maxillary  nerve  at  the  posterior  end  of  the 
orbit.  Trace  this  nerve  posteriorly  among  the  muscles  to  where  it  pierces  the 
skull.  At  this  point  it  is  joined  by  the  mandibular  branch  of  the  trigeminus. 
Trace  this  laterally.  After  branching  into  adjacent  muscles,  the  mandibular 
nerve  proceeds  ventrally  and  enters  the  lower  jaw.  Mandibular  and  maxillary 
branches  pass  together  through  a  foramen  in  the  skull  and  connect  with  the 
semilunar  ganglion. 

/)  The  facial  and  auditory  nerves:  These  arise  together  from  the  side  of  the 
medulla  just  behind  the  root  of  the  trigeminus  and  immediately  separate  into 
an  anterior  facial  nerve  and  a  posterior  auditory.  The  latter  is  distributed  to 
the  internal  ear.  This  is  situated  in  the  skull  opposite  the  acustico-facial  root. 
This  part  of  the  skull  may  be  broken  open  with  the  bone  forceps.  The  semi- 
circular ducts,  ampullae,  and  vestibule  of  the  internal  ear  will  be  noted.  The 
auditory  nerve  will  be  seen  branching  among  these  structures.  The  facial  nerve 
passes  through  the  anterior  part  of  the  ear  capsule  and  will  be  seen  again 
later. 

g)  The  glossopharyngeal  nerve:  This  arises  by  a  small  root  from  the  medulla 
immediately  posterior  to  the  acustico-facial  root  and  passes  out  through  the 
posterior  part  of  the  ear  capsule. 

h)  The  vagus,  the  spinal  accessory,  and  the  hypoglossal:  The  vagus  and  spinal 
accessory  (eleventh)  nerves  arise  together  by  a  number  of  roots  from  the  side 
of  the  medulla  posterior  to  the  preceding  nerve,  the  more  anterior  roots  belong- 
ing to  the  vagus  and  the  posterior  ones  to  the  accessory.  On  cutting  through 
these  roots  the  more  ventrally  situated  roots  of  the  hypoglossal  (twelfth)  nerve 
will  be  seen.  The  three  nerves  pass  out  from  the  skull  close  together. 

i)  The  abducens  nerve:  Cut  through  all  of  the  nerve  roots  on  one  side  of  the 
brain  and  tilt  the  brain  toward  the  opposite  side.  The  abducens  nerves  will  be 
seen  springing  from  the  ventral  surface  of  the  medulla  at  about  the  same  level 
as  the  acustico-facial  root. 

The  seventh,  ninth,  tenth,  and  twelfth  nerves  may  be  traced  farther  as 
follows.  Turn  the  head  ventral  side  up  and  remove  the  skin  and  superficial 
muscles  from  the  hyoid  apparatus.  Locate  the  anterior  and  posterior  horns  of 
the  hyoid.  On  the  side  of  the  neck,  near  the  dorsal  end  of  the  anterior  horn  and 
posterior  to  it,  the  hypoglossal  nerve  will  be  seen  emerging.  It  branches  into 
the  muscles  over  the  anterior  horn  and  sends  a  branch  forward  into  the  tongue 
muscles.  Very  near  the  point  of  emergence  of  the  hypoglossal  but  situated  more 
deeply  and  nearer  to  the  cartilage  of  the  anterior  horn  will  be  found  the  glosso- 
pharyngeal nerve.  It  runs  between  the  two  horns  toward  the  median  ventral 
line  and  supplies  adjacent  muscles  and  lining  of  the  mouth  cavity.  Lateral 


COMPARATIVE  ANATOMY  OF  THE  NERVOUS  SYSTEM  327 

to  these  nerves,  a  branch  of  the  facial  will  be  found  crossing  the  anterior  horn 
near  its  dorsal  end  and  passing  into  the  muscles  lying  along  the  posterior  border 
of  the  mandible. 

Make  a  median  longitudinal  incision  through  the  whole  floor  of  the  mouth 
and  pharyngeal  cavities  and  open  the  two  flaps  so  that  the  roof  of  these  cavities 
is  revealed.  Locate  the  vagus  (really  vago-sympathetic)  trunk  in  the  neck  and 
trace  it  anteriorly  to  its  point  of  exit  from  the  skull,  removing  the  mucous  membrane 
from  the  roof  of  the  pharyngeal  cavity.  The  vago-sympathetic  trunk  passes  to 
the  dorsal  side  of  the  hypoglossal  nerve  seen  above  and  there  enters  a  ganglion, 
the  superior  cervical  ganglion  of  the  sympathetic.  F-om  this  ganglion  numerous 
branches  pass  out.  Internal  to  the  hypoglossal  locate  the  glossopharyngeal 
nerve,  the  carotid  artery  being  situated  between  the  two.  Slightly  anterior  to 
these  will  be  found  the  facial  nerve,  as  it  exits  from  the  skull.  Its  branches  pass 
to  the  muscles  between  the  anterior  horn  of  the  hyoid  and  the  lower  jaw,  one  of 
them  curving  over  the  ventral  surface  of  the  horn  as  noted  above.  The  vagus 
nerve  proceeds  posteriorly  and  supplies  the  heart  and  other  viscera.  It  will  be 
noted  that  the  facial,  glossopharyngeal,  and  vagus  nerves  are  much  reduced, 
owing  to  the  loss  of  the  lateral  line  system  and  the  gill  apparatus.  Note,  how- 
ever, that  these  nerves  continue  to  supply  the  remains  of  the  visceral  arches  and 
their  visceral  muscles. 

Add  the  cranial  nerves  to  your  drawing  of  the  brain. 

5.  Ventral  aspect  of  the  brain. — Remove  the  brain  from   the  skull  and 
examine  the  ventral  surface.     On  the  ventral  surface  of  the  diencephalon  note 
the  optic  chiasma,  the  infundibulum  just  behind  this,  with  the  hypophysis  pro- 
jecting ventrally  from  the  latter.     Note  the  roots  of  the  abducens  and  hypogossal 
nerves  arising  from  the  ventral  surface  of  the  medulla. 

6.  Median  sagittal  section. — Make  a  median  sagittal  section  and  study  the 
cut  surface.     Identify  the  fourth  ventricle  in  the  medulla,  the  aqueduct  or  passage 
below  the  cerebellum,  the  optic  ventricle  in  the  optic  lobe,  the  third  ventricle  in 
the  diencephalon.     Note  the  increased  size  of  the  diencephalon  as  compared 
with  the  elasmobranch,  and  the  backward   extension   of  the   cerebral   hemi- 
sphere over  the  diencephalon.     The  diencephalon  is  divided  into  epithalamus, 
thalamus,  and  hypothalamus  as  in  the  dogfish,  each  including  the  parts  previ- 
ously enumerated.     Note  that  the  cerebral  hemisphere  presents  a  solid  medial 
wall,  called  the  septum.     Cut  into  the  roof  or  pallium  of  the  hemisphere.     Note 
its  cavity,  the  lateral  ventricle,  and  the  large  mass  protruding  from  the  floor  into 
the  ventricle;  this  mass  is  the  corpus  striatum. 

The  functions  of  the  parts  of  the  turtle  brain  are  similar  to  those  stated  for 
the  elasmobranch  brain.  The  cerebral  hemisphere  is  still  largely  olfactory, 
although  its  lateral  surface  is  beginning  to  assume  the  functions  characteristic 
of  the  mammalian  hemisphere. 

Draw  the  section. 


328  LABORATORY  MANUAL  FOR  VERTEBRATE  ANATOMY 

E.      THE  NERVOUS   SYSTEM  AND   SENSE   ORGANS   OF  THE   PIGEON 

i.  The  spinal  nerves  and  the  sympathetic  system. — Carefully  remove  the 
remaining  viscera  from  one-half  of  the  trunk.  Note  the  ventral  rami  of  the 
spinal  nerves  passing  laterally  along  the  dorsal  body  wall  between  the  ribs  in 
the  trunk  region.  Trace  them  toward  the  vertebral  column  and  note,  at  the 
points  where  they  emerge  from  the  vertebrae,  the  ganglia  of  the  sympathetic 
system  lying  on  the  spinal  nerves  and  the  delicate  white  cords  of  the  sympathetic 
system  extending  between  the  ganglia  along  the  sides  of  the  vertebral  column. 

a)  Spinal  nerves  and  limb  plexi:  In  the  neck  the  cervical  spinal  nerves  will 
be  seen  by  separating  the  vertebral  column  from  the  skin.     They  pass  out  at 
segmental  intervals.     The  vagus  nerve  is  the  white  cord  which  passes  ventral 
to  the  proximal  portions  of  the  cervical  nerves. 

The  ventral  rami  of  the  last  cervical  nerves  together  with  that  of  the  first 
of  the  trunk  form  the  brachial  plexus  to  the  wing.  This  is  a  network,  formed 
by  the  union  of  branches  of  four  stout  nerves,  which  receives  a  small  branch 
from  the  succeeding  nerve. 

The  next  five  ventral  rami  pass  out  between  the  ribs.  Following  them  is 
the  lumbosacral  plexus,  divisible  into  three  parts,  the  lumbar,  the  sacral,  and  the 
pudendal  plexus.  The  lumbar  plexus  is  formed  by  three  nerves;  from  it  nerves 
pass  into  the  thigh.  The  sacral  plexus  arises  from  the  union  of  five  nerves,  the 
first  of  which  is  the  same  as  the  third  nerve  contributing  to  the  lumbar  plexus. 
These  five  unite  to  produce  a  large  trunk,  the  sciatic  nerve,  which  passes  along 
the  dorsal  side  of  the  thigh  between  the  muscles,  and  proceeds  down  the  leg.  It 
will  be  found  by  separating  the  muscles  along  the  middle  of  the  dorsal  surface 
of  the  thigh.  It  courses  alongside  the  femoral  artery  and  vein. 

The  remaining  spinal  nerves  posterior  to  the  sacral  plexus  form  the  pudendal 
plexus  and  pass  obliquely  posteriorly  to  the  tail  and  cloacal  region. 

b)  The  sympathetic  system:  This  has  already  been  identified  on  the  sides  of 
the  vertebral  column.     It  consists  on  each  side  of  a  chain  of  two  cords  and 
segmental  ganglia.     One  of  the  cords  passes  ventral  to  the  head  of  the  rib,  the 
other  dorsal  to  it.     A  sympathetic  ganglion  lies  fused  to  each  spinal  nerve  in 
the  trunk  region  as  the  latter  emerges  from  the  vertebral  column.     On  scraping 
off  one  of  these  sympathetic  ganglia,  the  spinal  ganglion  belonging  to  the  spinal 
nerve  will  be  found  dorsal  to  it.     At  about  the  middle  of  the  rib-bearing  region 
a  plexus  of  nerves  and  ganglia  will  be  seen  extending  ventrally  from  the  main 
sympathetic  cords  and  surrounding  the  dorsal  aorta  and  its  main  branches  to 
the  digestive  tract.     This  is  the  coeliac  plexus.     Posterior  to  this  region  the 
sympathetic  cords  are  reduced  and  consist  of  a  single  trunk  on  each  side.     A 
sympathetic  cord  accompanies  the  pudendal  plexus  and  has  a  ganglion  in  the 
middle  of  this  plexus.     Anteriorly  the  sympathetic  cords  pass  across  the  ventral 
side  of  the  brachial  plexus,  having  ganglionic  enlargements  on  the  latter,  and 
then  enter  the  vertebrarterial  canals. 


COMPARATIVE  ANATOMY  OF  THE  NERVOUS  SYSTEM  329 

2.  The  sense  organs  of  the  head. — 

a)  The  nasal  cavities:  Open  one  nasal  cavity  by  a  longitudinal  slit  just  above 
the  margin  of  the  upper  jaw  from  the  external  naris  to  the  head.  Note 
the  median  septum  between  the  two  nasal  cavities  and  the  swellings,  the 
turbinals  or  conchae,  projecting  from  the  septum  into  the  nasal  cavity.  There 
are  three  turbinals  in  a  row:  the  first  two  large  and  conspicuous,  the  third  and 
most  posterior  one  consisting  only  of  a  small  rounded  swelling  on  the  roof  of  the 
cavity  in  close  contact  with  the  posterior  end  of  the  second  concha.  Only  this 
third  concha  is  provided  with  olfactory  epithelium.  Beyond  the  conchae  the 
nasal  passages  connect  with  the  pharyngeal  cavity. 

I)  The  eye:  Cut  through  the  skin  around  the  eyeball  and  also  remove  the 
roof  of  the  skull  between  the  two  eyes.  Note  the  relatively  large  size  of  the 
eyeballs  and  the  interorbital  septum  between  them.  Along  the  dorsal  margin  of 
the  septum  course  the  two  olfactory  nerves.  Press  the  eyeball  outwardly  away 
from  the  skull.  Two  thin,  flat  muscles  will  be  seen  extending  to  the  eyeball 
from  the  orbit;  the  anterior  one  is  the  superior  oblique,  the  posterior  one  the 
superior  rectus.  Cut  through  the  superior  oblique  at  its  insertion  on  the  eyeball 
and  press  it  against  the  orbit.  The  internal  rectus  will  now  be  seen  extending 
to  the  eyeball  ventral  to  the  superior  oblique.  The  white  nerve  crossing  the 
orbit  against  the  internal  surface  of  the  superior  oblique  is  the  ophthalmic  branch 
of  the  trigeminus.  Dorsal  to  it  the  smaller  trochlear  nerve  is  seen  terminating 
on  the  superior  oblique.  The  thin  sheet  of  muscles  on  the  surface  of  the  eyeball 
is  the  quadrate,  a  muscle  of  the  eyelids.  On  the  anterior  surface  of  the  eyeball 
ventral  to  the  superior  oblique  is  a  white  fatlike  mass,  the  Harderian  gland. 
Press  the  eyeball  posteriorly  and  find  anterior  to  this  gland,  against  the  anterior 
wall  of  the  orbit,  the  inferior  oblique  muscle.  On  pulling  the  eyeball  forward 
the  external  rectus  is  seen  extending  to  the  posterior  surface  of  the  eyeball.  Free 
the  ventral  margin  of  the  eyeball.  In  the  posterior  ventral  region  on  raising  the 
eyeball  may  be  seen  a  small  gland,  the  lacrimal  gland.  Two  muscles  will 
be  seen  on  the  ventral  surface  of  the  eyeball.  The  anterior  one  is  the  inferior 
rectus,  the  posterior  one  the  external  rectus.  On  cutting  through  the  inferior 
rectus  the  pyramid,  a  muscle  of  the  eyelids,  will  be  revealed  internal  to  it.  Cut 
through  all  of  the  rectus  muscles  and  the  inferior  oblique  at  their  insertions  on 
the  eyeball  and  remove  the  eyeball,  severing  the  optic  nerve.  The  pyramid  and 
quadrate  muscles  are  now  more  readily  seen  extending  on  the  surface  of  the  eye- 
ball to  the  optic  nerve;  the  quadrate  muscle  is  broad  and  dorsally  situated, 
the  pyramid  narrow  and  ventral.  They  are  concerned  in  operating  the  nictitat- 
ing membrane.  In  the  orbit  note  the  extent  of  the  Harderian  gland. 

Cut  off  the  dorsal  part  of  the  eyeball  and  identify  the  structures  of  the  eye. 
Note  the  sclerotic  coat,  continuing  as  the  transparent  cornea  over  the  exposed 
part  of  the  eye;  the  conjunctiva,  passing  over  the  external  surface  of  the  cornea 
and  continuing  onto  the  eyelids;  the  black  chorioid  coat  internal  to  the  sclerotic 


330       LABORATORY  MANUAL  FOR  VERTEBRATE  ANATOMY 

and  forming  the  iris  in  front;  the  soft  retina.  Note  the  peculiar  ridged  structure, 
the  pecten,  projecting  from  the  chorioid  coat  through  the  retina  in  the  medial 
wall  of  the  eyeball  and  extending  to  the  lens.  Its  function  is  unknown.  Loosen 
the  lens  and  observe  that  it  is  encircled  by  a  structure  formed  chiefly  from  the 
chorioid  coat.  This  structure,  which  holds  the  lens  in  place,  is  called  the  ciliary 
body.  It  is  marked  by  radiating  ridges,  the  ciliary  processes,  and  contains 
muscles,  the  ciliary  muscles,  which  change  the  shape  and  position  of  the  lens. 
Note  the  shape  of  the  lens,  nearly  flat  externally,  more  convex  internally.  The 
chambers  of  the  eye  and  the  two  humors  are  as  in  the  dogfish.  Peel  off  the  iris 
from  the  cornea  and  note  the  stiff,  bony  ring  encircling  the  cornea;  it  is  composed 
of  a  number  of  sclerotic  bones. 

Draw,  showing  structure  of  the  eye. 

c)  The  ear:  The  ear  of  birds  consists  of  three  parts — the  external  ear,  the 
middle  ear,  and  the  internal  ear.  The  external  ear  comprises  the  passage,  the 
external  auditory  meatus,  situated  below  and  behind  the  eye.  Cut  into  this  on 
the  same  side  of  the  head  on  which  the  eye  was  dissected  and  find  at  its  internal 
end  a  circular  transparent  membrane,  the  tympanic  membrane.  Through  the 
membrane  the  columella  can  be  seen  extending  from  its  internal  surface  inwardly. 
Remove  the  tympanic  membrane,  noting  the  columella  adhering  to  its  internal 
surface.  The  cavity  of  the  middle  ear  is  now  exposed;  medially  and  ventrally 
it  is  connected  to  the  pharnygeal  cavity  by  the  auditory  tube;  posterior  and 
slightly  dorsal  to  it  is  situated  the  internal  ear.  The  inner  end  of  the  columella 
adjoins  a  tiny  bone,  the  stapes,  which  fits  into  an  opening,  the  fenestra  ovalis  or 
vestibuli,  which  leads  into  the  internal  ear.  Look  for  these  at  the  inner  end  of 
the  columella.  Next,  carefully  break  away  in  small  pieces  the  spongy  bone 
behind  the  middle  ear.  Three  bony  semicircular  canals  are  revealed.  Each  of 
them  contains  a  membranous  semicircular  duct,  as  will  be  seen  by  breaking  open 
one  of  them.  The  three  ducts  are  situated  in  the  same  planes  and  have  the 
same  names  as  in  elasmobranchs.  The  remaining  structures  of  the  internal  ear, 
consisting  of  two  small  chambers — the  utriculus  and  the  sacculus — are  too  difficult 
to  dissect. 

3.  Dorsal  aspect  of  the  brain. — Expose  the  brain,  removing  the  roof  of  the 
skull  and  the  side  of  the  skull  where  the  sense  organs  were  dissected,  including 
the  medial  wall  of  the  orbit.  Note:  the  dura  mater  inclosing  the  brain;  on 
removing  this,  the  very  delicate  pia  mater  next  to  the  brain  substance.  The 
origin  of  these  membranes  is  the  same  as  given  for  the  turtle,  and  the  spaces 
bounding  them  have  the  same  names.  Unlike  the  condition  in  the  preceding 
forms,  the  brain  completely  fills  the  cranial  cavity. 

The  brain  is  short  and  broad  and  strongly  curved,  in  correlation  with  the 
biped  gait.  The  curvature  results  from  flexures  of  the  brain  in  three  regions. 
The  chief  or  primary  flexure  occurs  in  the  region  of  the  midbrain,  with  the  result 
that  the  posterior  part  of  the  brain  is  bent  nearly  at  right  angles  to  the  anterior 


COMPARATIVE  ANATOMY  OF  THE  NERVOUS  SYSTEM  331 

part.  The  second  or  nuchal  flexure  takes  place  in  the  medulla,  bending  the 
medulla  at  an  angle  to  the  spinal  cord.  The  pontal  flexure  in  the  region  ventral 
to  the  cerebellum  bends  the  brain  in  the  opposite  direction  from  the  other  two 
flexures,  with  the  result  that  this  region  of  the  brain  is  depressed. 

The  anterior  end  of  the  brain  consists  of  the  two  very  small  olfactory  lobes. 
Posterior  to  them  are  the  large  cerebral  hemispheres  separated  by  a  deep  sagittal 
fissure.  These  are  so  enlarged  posteriorly  as  to  completely  conceal  the  dien- 
cephalon  from  dorsal  view,  with  the  exception  of  the  delicate  pineal  body  which 
is  seen  in  the  posterior  end  of  the  sagittal  fissure.  The  large  optic  lobes  of  the 
midbrain  are  ventral  to  the  posterior  ends  of  the  cerebral  hemispheres.  Posterior 
to  the  hemispheres  is  the  curved  cerebellum,  marked  by  transverse  grooves.  Pos- 
terior and  ventral  to  this  is  the  medulla  oblongata,  its  anterior  end  depressed 
beneath  the  cerebellum.  The  roof  of  the  medulla  is  composed  as  usual  of  a 
chorioid  plexus. 

Draw  the  brain  from  the  side. 

4.  The  cranial  nerves. — These  are  somewhat  difficult  to  follow  in  detail. 
Work  on  the  side  left  intact.  There  are  twelve  pairs  of  cranial  nerves. 

a)  The  olfactory  nerves:  These  are  two  stout  and  elongated  nerves  passing 
from  the  nasal  sacs  along  the  dorsal  margin  of  the  interorbital  septum  to  the 
olfactory  lobes. 

b)  The  optic  nerves:  On  the  side  where  the  wall  of  the  orbit  was  removed  note 
the  stout  white  optic  tract  in  front  of  the  optic  lobe.     Follow  this  toward  the 
orbit  and  find  the  optic  nerve  connected  with  its  anterior  end. 

c)  The  trochlear  nerve:  The  cranial  origin  of  this  nerve  is  difficult  to  see  at 
the  present  stage  of  the  dissection.     It  arises  in  the  deep  groove  between  the 
optic  lobe  and  the  cerebellum,  and  passes  ventrally  between  the  optic  lobe  and 
medulla.     It  runs  forward  in  the  floor  of  the  cranial  cavity  to  the  orbit.     To 
find  it  in  the  orbit,  expose  the  intact  eye  as  before.     Cut  through  the  superior 
oblique  at  its  insertion  on  the  eyeball  and  lifting  it  note  the  trochlear  nerve  pass- 
ing to  it  and  spreading  out  on  its  ventral  surface.     The  ophthalmic  branch  of 
the  trigeminus  runs  close  to  the  trochlear  nerve. 

d)  The  oculomotor  nerve:  The  cranial  origin  of  this  nerve  will  be  seen  later. 
It  branches  to  the  inferior  oblique,  superior,  inferior,  and  internal  rectus  muscles 
Remove  the  eye  which  is  still  in  place,  cutting  the  eye  muscles  as  near  the  eye- 
ball as  possible  and  preserving  the  ophthalmic  nerve  intact.    Look  for  the 
branches  of  the  oculomotor  among  the  eye  muscles  in  question.     The  branch 
to  the  inferior  oblique  in  the  floor  of  the  orbit  is  the  most  conspicuous  of  them. 

e)  The  abducens  nerve:  This  will  be  found  on  examining  the  posterior  surface 
of  the  external  rectus  muscle.     This  nerve  also  supplies  the  pyramid  and  quadrate 
muscles. 

f)  The  trigeminus  nerve:  This  has  three  branches,  the  ophthalmic,  the  max- 
illary, and  the  mandibular.     The  ophthalmic  has  already  been  noted  in  the  dorsal 


332       LABORATORY  MANUAL  FOR  VERTEBRATE  ANATOMY 

part  of  the  orbit.  Follow  it  forward,  noting  its  distribution  to  the  walls  of  the 
nasal  cavities.  In  the  floor  of  the  orbit,  near  its  outer  margin,  locate  the  maxillary 
nerve.  Trace  it  forward,  noting  its  branches  to  the  orbit  and  upper  jaw.  Trace 
the  maxillary  nerve  posteriorly,  carefully  cutting  away  tissues  in  its  path. 
Caudad  of  the  orbit  it  is  joined  by  the  mandibular  nerve.  Trace  this,  noting 
branches  to  muscles  and  main  trunk  passing  into  the  lower  jaw.  Trace  the 
common  trunk  of  the  maxillary  and  mandibular  nerve  toward  the  skull  and  note 
that  they  are  joined  in  the  skull  by  the  ophthalmic  nerve.  At  the  point  of  union 
is  the  semilunar  ganglion,  lying  in  the  skull.  From  the  ganglion  the  trigeminus 
nerve  may  be  traced  to  its  origin  from  the  side  of  the  medulla  below  the  optic 
lobe. 

g)  The  facial  nerve:  This  arises  from  the  medulla  just  back  of  the  root  of 
the  trigeminus  and  passes  through  the  anterior  part  of  the  ear  capsule,  where 
it  will  be  found  by  scraping  away  the  latter. 

H)  The  auditory  nerve:  This  arises  close  to  the  facial  and  passes  out  with  it 
into  the  ear  capsule  to  the  various  parts  of  which  it  is  distributed. 

i)  The  glossopharyngeal  and  the  vagus:  These  nerves  arise  close  together  just 
behind  the  ear  capsule  and  will  be  found  there  by  carefully  dissecting  in  the 
muscles.  The  glossopharyngeal  is  the  smaller  of  the  two  and  anterior  in  position. 
It  enters  a  ganglion,  the  petrosal  ganglion,  beyond  which  it  is  distributed  to  the 
palate,  pharynx,  and  larynx.  The  vagus  nerve  is  considerably  larger  than  the 
glossopharyngeal.  It  passes  laterally  parallel  and  posterior  to  the  glossopharyn- 
geal and  enters  its  ganglion,  the  jugular  ganglion,  which  is  united  with  the 
petrosal  ganglion,  the  two  forming  a  mass.  Beyond  this  the  vagus  turns  pos- 
teriorly and  passes  down  the  neck,  supplying  respiratory  system,  heart,  and 
other  viscera.  Portions  of  the  sympathetic  system  are  intermingled  with  the 
ninth  and  tenth  nerves. 

j)  The  spinal  accessory  and  the  hypoglossal:  The  former  passes  out  with  the 
vagus  and  is  distributed  to  certain  muscles.  The  hypoglossal  is  found  just 
posterior  to  the  vagus.  It  is  distributed  to  certain  neck  muscles  and  sends  a 
branch  forward  to  the  tongue. 

5.  Ventral  aspect  of  the  brain. — Remove  the  brain  from  the  cranial  cavity, 
preserving  the  roots  of  the  cranial  nerves  as  far  as  possible.  Those  not  kept 
attached  to  the  brain  will  be  found  in  the  cranial  cavity. 

Note  form  of  the  olfactory  lobes  and  cerebral  hemispheres  from  the  ventral 
aspect.  Between  the  optic  lobes  is  the  diencephalon.  In  the  center  of  this  is 
the  optic  chiasma,  marked  by  cross  lines.  From  the  optic  chiasma  the  strong 
white  optic  tracts  pass  laterad  and  dorsad  to  the  optic  lobes  and  dorsal  part  of 
the  diencephalon.  Behind  the  chiasma  is  a  depressed  area,  the  infundibulum 
from  which  the  hypophysis  extends  ventrally.  The  latter  is  usually  left  behind 
in  removing  the  brain,  and  will  be  found  in  a  deep  pit,  the  sella  turcica,  in  the 
floor  of  the  cranial  cavity.  The  infundibulum  bears  a  central  cleft  where  the 


COMPARATIVE  ANATOMY  OF  THE  NERVOUS  SYSTEM  333 

hypophysis  was  torn  from  it.  At  the  sides  of  the  infundibulum  are  the  roots  of 
the  oculomotor  nerves.  Posterior  to  the  diencephalon  is  the  depressed  medulla. 
Between  the  medulla  and  the  optic  lobe  is  the  slender  root  of  the  trochlear  nerve. 
On  the  ventral  surface  of  the  medulla  are  the  roots  of  the  abducens  nerves;  they 
should  also  be  sought  in  the  floor  of  the  cranial  cavity.  On  the  sides  of 
the  medulla  look  for  the  roots  of  the  fifth,  seventh,  eighth,  ninth,  and  tenth 
nerves,  situated  in  a  row.  The  twelfth  nerve  arises  from  the  ventral  surface  of 
the  medulla  about  on  the  same  level  as  the  ninth  and  tenth  roots.  The  eleventh 
nerve  arises  from  the  spinal  cord  by  several  roots  and  ascends  to  a  position 
immediately  behind  the  tenth  root. 

Draw  the  ventral  view  of  the  brain. 

6.  Sagittal  section. — Make  a  median  sagittal  section  of  the  brain  and  study 
the  cut  surface.  In  the  medulla  note  the  fourth  ventricle  overhung  by  the  cere- 
bellum. Note  the  thick  ventral  wall  of  the  medulla  and  the  pontal  flexure  caus- 
ing a  ventrally  directed  bend  in  the  medulla.  In  the  cerebellum  observe  the 
small  cerebellar  ventricle  and  the  arrangement  of  the  gray  and  white  matter 
resulting  in  section  in  a  treelike  appearance,  called  the  arbor  vitae.  Each  fold 
of  the  cerebellum  consists  of  a  central  plate  of  white  matter  surrounded  by  a 
thick  covering  of  gray  matter.  Anterior  to  the  cerebellum  is  a  region  consisting 
dorsally  of  the  mesencephalon  and  ventrally  of  the  diencephalon.  The  optic 
lobes  do  not  appear  in  the  section,  but  the  median  part  of  the  midbrain  forms 
the  dorsal  part  of  the  section.  A  narrow  cavity,  the  third  ventricle,  is  present  in 
the  diencephalon  and  extends  into  the  infundibulum.  In  front  of  the  latter 
appears  the  optic  chiasma.  Note  how  the  cerebral  hemisphere  arches  back  over 
the  diencephalon  and  midbrain,  and  note  the  strong  connection  of  the  diencepha- 
lon with  the  hemisphere.  The  cavity  of  the  cerebral  hemisphere  is  not  visible 
in  the  median  section.  The  medial  wall  of  the  hemisphere  is  called  the  septum, 
its  dorsal  wall  the  pallium.  Cut  into  the  latter,  noting  its  thinness,  and  find 
inside  the  cavity  or  lateral  ventricle  of  the  hemisphere  and  the  great  mass,  the 
corpus  striatum,  bulging  from  the  floor.  The  function  of  the  corpus  striatum  is 
not  definitely  known,  but  it  seems  to  have  a  steadying  effect  on  voluntary  move- 
ments, and  the  delicacy  and  precision  of  movement  necessary  in  flight  may 
account  for  the  relatively  enormous  size  of  the  corpus  striatum  in  birds. 

F.   THE  NERVOUS  SYSTEM  AND  SENSE  ORGANS  OF  THE  MAMMAL 

For  the  complete  dissection  of  the  nervous  system  a  new  specimen  is  neces- 
sary, but  the  greater  part  of  this  system  can  be  worked  out  on  the  same  specimen 
as  used  for  preceding  systems.  If  a  new  animal  is  provided,  open  it  by  a  longi- 
tudinal cut  from  the  perineum  through  the  anterior  end  of  the  sternum.  If  the 
old  specimen  is  used  it  will  not  be  possible  to  see  the  branches  of  the  sympathetic 
system  and  the  vagus  to  the  viscera  or  the  peripheral  distribution  of  some  of 
the  cranial  nerves.  In  working  on  the  nerves  all  structures  other  than  nerves 


334       LABORATORY  MANUAL  FOR  VERTEBRATE  ANATOMY 

may  be  removed  in  order  to  expose  the  latter.     In  following  one  nerve,  adjacent 
nerves  must  not  be  destroyed. 

i.  The  spinal  nerves,  the  sympathetic  system,  and  the  vagus. — 

a)  Cervical  portion  of  the  sympathetic  and  the  vagus:  Locate  the  vagus  nerve 
at  a  point  near  the  larynx.     It  lies  alongside  the  carotid  artery.     The  nerve 
crossing  the  vagus  near  the  larynx  and  giving  off  branches  into  the  sternohyoid, 
sternothyroid,  and  related  muscles  is  the  descending  branch  of  the  twelfth  or 
hypoglossal  nerve. 

Rabbit:  The  vagus  nerve  and  the  cervical  part  of  the  sympathetic  trunk  lie 
together  on  the  dorsal  surface  of  the  carotid  artery.  The  vagus  is  larger  and 
more  lateral.  To  the  medial  side  of  the  sympathetic  trunk  posterior  to  the 
larynx  may  be  separated  a  delicate  nerve,  the  cardiac  branch  of  the  vagus  (depressor 
nerve  of  the  heart) .  Trace  the  sympathetic  posteriorly.  Just  in  front  of  the  sub- 
clavian  artery  it  enters  a  ganglion,  the  inferior  cervical  ganglion.  From  this 
ganglion  cords  pass  to  either  side  of  the  subclavian  artery,  forming  the  ansa  sub- 
clavia,  and  unite  again  to  another  ganglion,  the  first  thoracic  ganglion,  situated 
posterior  and  dorsal  to  the  artery. 

Cat:  The  sympathetic  trunk  is  inseparably  bound  with  the  vagus,  the  two 
forming  a  large  vago-sympathetic  trunk  coursing  lateral  to  the  carotid  artery  and 
bound  with  it  by  a  common  sheath.  Trace  it  caudad.  Just  in  front  of  the  first 
rib  branches  of  sympathetic  origin  arise  from  the  trunk  and  proceed  toward  the 
esophagus.  Shortly  posterior  to  this  point  the  sympathetic  separates  from  the 
vagus  and  generally  enters  a  ganglion,  the  middle  cervical  ganglion,  which  lies 
in  contact  with  the  vagus.  From  this  ganglion  cords  pass  on  either  side  of  the 
subclavian  artery,  forming  the  ansa  subclavia,  and  proceeding  directly  dorsally 
unite  to  form  a  large  ganglion,  the  inferior  cervical  ganglion,  which  lies  against 
the  neck  muscles  between  the  heads  of  the  first  and  second  ribs. 

From  the  inferior  cervical  ganglion  in  both  animals  cardiac  branches  pass  to 
the  heart.  The  conspicuous  nerve  lying  lateral  to  the  vagus  is  the  phrenic  nerve 
or  nerve  of  the  diaphragm.  The  right  vagus  just  after  passing  ventral  to  the 
subclavian  artery  gives  off  the  recurrent  or  posterior  laryngeal  nerve  which  runs 
anteriorly  along  the  side  of  the  trachea  to  the  larynx.  The  left  recurrent  nerve 
arises  much  farther  posteriorly  from  the  left  vagus. 

b)  The  anterior  cervical  spinal  nerves:  The  spinal  nerves  emerge  from  the 
spinal  cord  in  pairs  between  successive  vertebrae,  passing  out  through  the  inter- 
vertebral  foramina.     Those  of  the  cervical  region  are  called  the  cervical  nerves; 
there  are  eight  pairs  of  them.     The  ventral  rami  of  the  first  four  cervical  nerves 
are  loosely  united  with  each  other  to  form  the  cervical  plexus;    the  last  four 
together  with  the  first  thoracic  form  the  brachial  plexus.     As  the  first  two  are 
small  and  more  or  less  mingled  with  the  posterior  cranial  nerves  they  will  not 
be  studied  at  this  stage  of  the  dissection. 

To  expose  the  cervical  nerves,  pull  the  muscles  which  are  inserted  on  the 
anterior  end  of  the  sternum  (sternomastoid,  sternohyoid,  sternothyroid)  laterally 


COMPARATIVE  ANATOMY  OF  THE  NERVOUS  SYSTEM  335 

or  cut  across  them  where  necessary,  thus  exposing  the  musculature  of  the  verte- 
bral column.  Look  along  the  side  of  this,  dorsal  to  the  carotid  artery,  and  note 
the  ventral  rami  of  the  spinal  nerves  emerging  at  intervals.  At  about  the  level 
of  the  posterior  end  of  the  larynx  lies  the  third  cervical  nerve  in  the  rabbit,  fourth 
in  the  cat.  As  already  stated,  the  nerves  thus  exposed  are  the  ventral  rami  only; 
the  dorsal  rami  are  exposed  only  by  more  radical  dissection,  which  will  not  be 
attempted  here.  The  dorsal  rami  supply  the  epaxial  musculature.  Note  the 
branches  of  the  exposed  ventral  rami  to  the  muscles  of  the  side  of  the 
neck. 

From  the  ventral  ramus  of  the  fourth  cervical  nerve  (rabbit)  and  fifth 
cervical  nerve  (cat)  arises  the  phrenic  nerve.  It  passes  posteriorly  parallel  to 
the  vagus,  in  the  rabbit  close  to  the  vertebral  musculature.  It  receives  a  branch 
from  the  fifth  (rabbit)  or  sixth  (cat)  cervical  nerve  and  then  continues  posteriorly 
into  the  thorax.  As  it  passes  the  sympathetic  ganglia  it  receives  contributions 
from  them.  In  the  thorax  it  lies  at  the  side  of  the  pericardial  sac,  just  ventral 
to  the  root  of  the  lung.  Trace  it  posteriorly  and  note  how  it  spreads  on  the 
surface  of  the  diaphragm.  The  phrenic  nerves  are  the  motor  nerves  of  the  dia- 
phragm; their  origin  from  the  cervical  nerves  shows  that  the  muscles  of  the 
diaphragm  are  derived  from  cervical  myotomes. 

c)  The  brachial  plexus:  The  ventral  rami  of  the  fourth  to  eighth  (rabbit)  or 
fifth  to  eighth  (cat)  cervical  nerves  together  with  the  ventral  ramus  of  the  first 
thoracic  nerve  are  united  by  intercommunicating  branches,  called  ansae,  to  form 
the  brachial  plexus,  which  innervates  the  muscles  of  the  shoulder,  breast,  fore 
limb,  and  diaphragm.  The  fourth  cervical  (rabbit),  or  fifth  (cat),  take  part  in 
plexus  only  through  their  contribution  to  the  phrenic  nerve. 

To  expose  the  brachial  plexus  cut  through  the  pectoral  muscles  near  the 
midventral  line  and  separate  the  pectoral  muscles  from  the  underlying  serratus 
muscle.  The  plexus  lies  in  the  axilla  along  with  the  axillary  artery  and  vein. 
Then  cut  through  the  pectoral  muscles  as  near  as  possible  to  their  insertion  on 
the  humerus  and  separate  them  from  the  muscles  of  the  upper  arm.  In  this 
way  the  course  of  the  nerves  into  the  fore  limb  is  exposed. 

The  connections  of  the  nerves  of  the  plexus  are  so  intricate  that  it  is 
impossible  to  describe  them.  The  following  points  may  be  noted,  however. 
Rabbit:  The  fifth  cervical  immediately  sends  a  branch  to  the  sixth  cervical  and 
then  proceeds  laterally  into  the  neck  muscles.  The  sixth  cervical  is  a  broad 
nerve  which,  after  communicating  with  the  seventh  nerve,  passes  to  the  shoulder 
muscles.  The  seventh  is  smaller  and  after  contributing  to  the  eighth  likewise 
innervates  the  shoulder  muscles.  The  eighth  cervical  and  first  thoracic  unite  to 
one  trunk  as  they  emerge  from  the  vertebral  column.  From  this  trunk 
arise  the  nerves  of  the  limb.  Cat:  The  sixth  cervical  has  a  broad  connection 
with  the  seventh  and  then  proceeds  to  the  shoulder.  The  seventh  and  eighth 
cervicals  and  the  first  thoracic  are  very  stout  trunks  which  are  intricately  con- 
nected with  each  other  and  from  which  ororeed  the  nerves  of  the  fore  liml 


336       LABORATORY  MANUAL  FOR  VERTEBRATE  ANATOMY 

The  chief  nerves  from  the  brachial  plexus  are  the  following: 

1.  The  phrenic  nerve.     This  was  described  above. 

2.  The  suprascapular  nerve.     This  is  the  most  anterior  nerve  arising  from 
the  sixth  cervical.     The  main  part  of  this  nerve  passes  between  the  supraspinatus 
and  subscapular  muscles  to  supply  the  supraspinatus  and  infraspinatus.     In 
the  cat  a  branch  of  this  nerve  passes  over  the  shoulder  to  more  superficial  parts. 

3.  The  ventral  thoracic  nerves.     These  nerves  supply  the  pectoral  muscles 
and  will  be  found  entering  the  inner  surface  of  these  muscles  between  the  two 
incisions  made  above.     They  are  the  most  ventral  of  the  nerves  of  the  plexus. 
There  are  two  of  these  nerves — one  arising  from  the  seventh  cervical,  the  other 
from  the  eighth  cervical  and  first  thoracic.     The  former  is  small  in  the  rabbit. 

4.  The  subscapular  nerves.     There  are  three  of  these,  dorsally  situated  and 
passing  into  the  inner  surface  of  the  shoulder.     The  first  arises  from  the  sixth 
cervical  and  passes  to  the  subscapular  muscle;  the  second  arises  from  the  seventh 
cervical  and  supplies  chiefly  teres  major;  the  third  comes  from  the  seventh  and 
eighth  cervicals  and  runs  posteriorly  along  the  internal  surface  of  the  latissimus 
dorsi  muscle. 

5.  The  axillary  nerve.    This  nerve  originates  chiefly  from  the  seventh  cervical. 
It  passes  through  the  upper  part  of  the  upper  arm,  ventral  to  the  triceps  and 
emerging  on  the  lateral  surface  of  the  upper  arm  supplies  chiefly  the  deltoid 
muscles. 

6.  The  dorsal  or  long  thoracic  nerve.     This  nerve  is  best  located  by  examin- 
ing the  outer  surface  of  the  serratus  ventralis  muscle.     On  tracing  it  anteriorly 
the  nerve  will  be  found  to  pass  internal  to  the  scalenes  and  to  spring  from  the 
seventh  cervical  nerve  close  to  the  vertebral  column. 

7.  The  musculocutaneous  nerve  (cat  only).     This  arises  from  the  ventral 
surface  of  the  sixth  and  seventh  cervicals.     It  passes  to  the  biceps  muscle,  fork- 
ing as  it  approaches  the  muscle.     The  posterior  branch  continues  along  the 
surface  of  the  muscle  and  at  the  elbow  passes  to  the  lateral  surface  of  the  arm 
and  supplies  the  skin  of  the  forearm. 

8.  The  radial  nerve.     This  is  the  largest  nerve  springing  from  the  plexus. 
Seventh  and  eighth  cervicals  and  first  thoracic  nerves  contribute  to  its  formation. 
It  passes  to  the  upper  arm  and  coursing  between  the  humerus  and  the  triceps 
turns  dis tally.     It  supplies  many  muscles  of  the  fore  limb. 

9.  The  median  nerve.     This  nerve  lies  posterior  to  the  radial.     It  arises  in 
the  cat  by  branches  from  the  last  three  nerves  of  the  plexus  and  in  the  rabbit 
chiefly  from  the  first  thoracic.     It  passes  to  the  upper  arm,  and  then  turns 
distally  running  along  with  the  brachial  artery. 

10.  The  ulnar  nerve.     This  lies  just  posterior  and  parallel  to  the  median 
nerve,  originating  chiefly  from  the  first  thoracic  nerve.     The  ulnar  and  me- 
dian nerves  supply  the  limb  distal  to  the  elbow,  although  in  the  rabbit,  the 
median  nerve  innervates  the  biceps. 


COMPARATIVE  ANATOMY  OF  THE  NERVOUS  SYSTEM  337 

ii.  The  medial  cutaneous.  This  is  the  small  nerve  which  runs  in  contact 
with  the  ulnar  nerve.  It  turns  superficially  just  above  the  elbow  and  is  dis- 
tributed to  the  skin  of  the  forearm. 

Draw,  showing  the  main  parts  of  the  brachial  plexus. 

d)  The  thoracic  portions  of  the  vagus  and  the  sympathetic:  Trace  the  two  vagi 
toward  the  heart.     They  pass  dorsal  to  the  roots  of  the  lungs.     The  left  vagus 
just  caudad  of  the  aortic  arch  gives  off  the  left  recurrent  laryngeal  nerve  which 
turns  cephalad,  passing  on  the  dorsal  side  of  the  aorta,  and  ascends  along  the 
side  of  the  trachea.     At  the  roots  of  the  lungs  the  vagi  give  rise  to  a  plexus — 
the  pulmonary  plexus — to  the  lungs.     This  plexus  also  extends  to  the  heart  as 
the  cardiac  plexus.     The  cardiac  branches  of  the  sympathetic  system,  noted 
above,  join  the  cardiac  plexus.     The  cardiac  plexus  is  situated  at  the  bases  of 
the  aorta  and  pulmonary  arteries.     In  the  rabbit  the  cardiac  branches  of  the 
vagus  may  be  traced  into  this  plexus. 

Caudad  of  the  pulmonary  plexus  the  two  vagi  in  the  rabbit  continue  posteri- 
orly along  the  sides  of  the  esophagus,  to  which  they  furnish  small  branches  and 
penetrate  the  diaphragm.  In  the  cat  each  vagus  divides  just  posterior  to  the 
root  of  the  lungs  into  dorsal  and  ventral  branches.  The  ventral  branches  of  the 
two  sides  immediately  unite  into  one  trunk  which  proceeds  posteriorly,  lying  on 
the  left  ventrolateral  surface  of  the  esophagus.  The  two  dorsal  branches  con- 
tinue posteriorly,  lying  along  the  right  and  left  sides  of  the  esophagus;  near  the 
diaphragm  on  the  dorsal  side  of  the  esophagus  they  unite  to  one  trunk.  In  this 
manner  are  formed  the  dorsal  and  ventral  divisions  of  the  vagi;  they  pass  through 
the  diaphragm.  In  their  course  along  the  esophagus  they  furnish  branches  to  it. 

Locate  again  the  inferior  cervical  ganglion.  Note  the  communicating 
branches  from  this  ganglion  to  the  brachial  plexus.  In  the  cat  a  particularly 
stout  branch  extends  anteriorly  ventral  to  the  bases  of  the  sixth  to  eighth  cervical 
nerves,  giving  branches  to  them.  Trace  the  sympathetic  trunk  posteriorly  from 
the  inferior  cervical  ganglion.  The  contents  of  the  pleural  cavities  may  now  be 
cleaned  out.  The  sympathetic  trunk  is  a  white  cord  lying  to  each  side  of  the 
vertebral  column,  passing  ventral  to  the  heads  of  the  ribs.  At  segmental 
intervals  generally  in  the  places  between  the  ribs,  it  presents  a  ganglionic 
enlargement. 

e)  The  thoracic  spinal  nerves:  The  first  thoracic  nerve  contributes  to  the 
brachial  plexus  as  already  learned.     The  ventral  rami  of  the  remaining  thoracic 
nerves  pass  laterally  as  the  intercostal  nerves,  lying  along  the  posterior  side  of 
each  rib.     These  nerves  are  readily  exposed  by  running  the  point  of  an  instru- 
ment along  the  posterior  side  of  each  rib,  slitting  open  the  fascia  of  the  intercostal 
muscles.     As  each  nerve  emerges  from  the  intervertebral  foramen  it  receives  one 
or  two  communicating  branches  (rami  communicantes)  from  the  adjacent  sym- 
pathetic ganglion.     These  branches  are  rather  delicate  and  the  student  may  not 
be  able  to  see  them.     The  dorsal  rami  of  the  thoracic  spinal  nerves  supply  the 


338       LABORATORY  MANUAL  FOR  VERTEBRATE  ANATOMY 

epaxial  muscles.  To  see  them,  turn  the  animal  dorsal  side  up  and  carefully 
cut  down  through  the  mass  of  epaxial  muscles  close  to  the  vertebrae.  The 
dorsal  rami  will  then  be  seen  emerging  from  the  vertebral  column  and  penetrat- 
ing the  epaxial  mass,  accompanied  by  blood  vessels.  There  are  twelve  or  thirteen 
pairs  of  thoracic  nerves. 

/)  The  abdominal  portions  of  the  vagus  and  the  sympathetic:  Trace  the  vagi 
into  the  peritoneal  cavity,  removing  the  liver  if  not  already  done.  In  the  rabbit 
the  left  vagus  crosses  the  ventral  surface  of  the  esophagus  obliquely  to  the  right 
and  is  distributed  to  the  lesser  curvature  and  ventral  surface  of  the  stomach. 
The  right  vagus  crosses  the  dorsal  surface  of  the  esophagus  obliquely  to  the  left 
and  is  distributed  to  the  dorsal  surface  of  the  stomach.  In  the  cat  the  ventral 
division  of  the  vagus  passes  to  the  lesser  curvature,  the  dorsal  division  to  the 
greater  curvature.  In  both  cases  the  vagi  form  plexi  on  the  stomach  called  the 
ventral  and  dorsal  gastric  plexi,  which  also  connect  with  the  nearby  sympathetic 
plexi,  described  in  the  next  paragraph. 

Locate  again  the  posterior  part  of  the  thoracic  portion  of  the  sympathetic 
trunk.  Expose  it  and  note  the  nerve,  the  greater  splanchnic  nerve,  which  arises 
from  the  sympathetic  trunk  on  each  side  and  passes  obliquely  ventrally  toward 
the  diaphragm.  In  the  cat  this  nerve  is  accompanied  by  additional  smaller 
nerves,  the  lesser  splanchnic  nerves,  arising  from  the  sympathetic  shortly  posterior 
to  the  origin  of  the  greater  splanchnic  nerve.  The  splanchnic  nerves  pass  to 
either  side  of  the  crura  of  the  diaphragm  into  the  peritoneal  cavity.  (The  crura 
of  the  diaphragm  are  the  muscular  cords  which  fasten  the  diaphragm  to  the 
lumbar  vertebrae.)  Turn  the  abdominal  viscera  to  the  right  and  look  on  the 
left  surface  of  the  superior  mesenteric  artery  near  its  origin  from  the  aorta. 
Two  prominent  sympathetic  ganglia  will  be  found  lying  on  the  superior  mesenteric 
artery.  These  are  the  coeliac  and  superior  mesenteric  ganglia;  the  former  lies  in 
front  of  or  on  the  left  surface  of  the  artery;  the  latter  behind  or  on  the  ventral 
surface  of  the  vessel.  The  two  ganglia  are  bound  together  by  a  strong  connec- 
tion. The  splanchnic  nerves  of  both  sides  may  be  traced  into  the  coeliac  gang- 
lion. From  this  ganglion  a  prominent  coeliac  plexus  will  be  seen  extending 
toward  the  stomach,  where  it  connects  with  the  gastric  plexi  of  the  vagi.  This 
great  sympathetic  plexus  formed  around  and  dorsal  to  the  stomach  is  often 
called  the  solar  plexus.  From  the  coeliac  and  superior  mesenteric  ganglia  and 
adjacent  plexi  also  arise  plexi  for  the  liver,  spleen,  adrenal  glands,  gonads,  and 
the  great  blood  vessels.  Some  of  these  will  probably  be  seen.  The  inferior 
mesenteric  ganglion  of  the  sympathetic  system  lies  in  the  mesocolon  alongside 
the  inferior  mesenteric  artery.  It  is  situated  in  the  inferior  mesenteric  plexus 
from  which  networks  extend  to  adjacent  structures. 

The  main  sympathetic  trunk  of  the  abdominal  region  should  now  be  traced 
caudad  from  the  place  of  origin  of  the  splanchnic  nerves.  The  two  trunks 
descend  deep  dorsally  lying  in  the  groove  between  two  muscle  masses.  At 
segmental  intervals  they  have  ganglionic  enlargements  from  which  nerves  pass 


COMPARATIVE  ANATOMY  OF  THE  NERVOUS  SYSTEM  339 

to  the  ganglia  and  plexi  already  noted.  At  the  posterior  end  of  the  peritoneal 
cavity  the  sympathetic  trunks  gradually  diminish  and  disappear. 

g)  The  lumbar  and  sacral  spinal  nerves  and  the  lumbosacral  plexus:  There 
are  seven  pairs  of  lumbar  nerves  and  four  (rabbit)  or  three  (cat)  pairs  of  sacral 
nerves.  The  ventral  rami  of  the  last  four  lumbar  nerves  form  a  lumbar  plexus, 
those  of  the  sacral  nerves  a  sacral  plexus;  but  since  the  two  plexi  are  united  with 
each  other,  they  may  be  considered  together  as  the  lumbosacral  plexus. 

Remove  all  viscera  from  the  peritoneal  cavity,  including  the  postcaval  vein 
and  aorta.  In  the  dorsal  wall  is  a  muscular  mass  extending  from  the  vertebrae 
to  the  pelvic  girdle.  This  consists  of  a  lateral  larger  muscle,  the  iliopsoas,  and  a 
smaller  medial  one,  the  psoas  minor.  In  the  rabbit  the  psoas  minor  is  a  slender 
muscle  which  occupies  only  the  posterior  part  of  the  mid-dorsal  region ;  its  stout 
shining  tendon  passes  to  the  dorsal  side  of  the  inguinal  ligament.  In  the  cat  the 
psoas  minor  extends  nearly  as  far  anteriorly  as  the  iliopsoas;  it  narrows  pos- 
teriorly to  a  tendon,  which  passes  obliquely  laterally  on  the  ventral  surface  of 
the  iliopsoas  which  is  thus  exposed  both  medially  and  laterally  to  the  tendon  of 
the  psoas  minor.  The  psoas  minor  covers  a  part  of  the  iliopsoas  in  both  animals, 
and  the  greater  part  of  the  lumbar  plexus  is  situated  between  the  two  muscles. 
Note  the  abdominal  parts  of  the  sympathetic  cords  between  the  posterior  portions 
of  these  muscles. 

Locate  the  last  thoracic  spinal  nerve.  It  lies  about  one-half  inch  posterior 
to  the  last  rib.  The  first  nerve  posterior  to  this  on  the  dorsal  wall  is  the  ventral 
ramus  of  the  first  lumbar  nerve.  Shortly  posterior  to  this  is  the  second  lumbar 
nerve.  These  two  nerves  pass  to  the  muscles  and  skin  of  the  abdominal  wall; 
in  the  cat  each  divides  into  two  branches.  The  third  lumbar  nerve  emerges  dorsal 
to  the  iliopsoas  muscle  and  divides  into  a  larger  lateral  branch  to  the  abdominal 
wall  and  a  more  slender  medial  branch,  which  passes  obliquely  caudad,  reaching 
and  following  the  course  of  the  iliolumbar  artery  and  vein.  The  fourth  lumbar 
nerve  is  the  first  of  the  lumbar  plexus.  It  has  two  main  branches,  the  lateral 
cutaneous  nerve  and  the  genitofemoral  nerve.  The  former  is  the  stout  trunk 
which  emerges  between  the  iliopsoas  and  psoas  minor  muscles  and  accompanies 
the  course  of  the  iliolumbar  artery  and  vein,  passing  to  the  thigh.  The  genito- 
femoral nerve  is  a  long  slender  nerve  which  runs  along  the  medial  border  of  the 
psoas  minor  muscle,  lateral  to  the  sympathetic  cords.  In  the  posterior  part  of 
its  course  it  accompanies  the  external  iliac  artery.  It  supplies  the  thigh  and 
abdominal  wall  of  and  adjacent  to  the  inguinal  region.  After  locating  these 
two  branches  of  the  fourth  lumbar  trace  them  toward  the  vertebral  column, 
removing  the  psoas  minor  as  far  as  necessary.  Find  the  point  of  emergence  of 
the  fourth. lumbar  from  the  vertebral  column  and  note  the  connection,  very 
stout  in  the  cat,  between  the  fourth  lumbar  and  the  fifth. 

The  fifth  lumbar  contributes  by  means  of  its  connection  with  the  fourth 
lumbar  to  the  lateral  cutaneous  branch  named  above  and  also  forms  a  strong 
union  with  the  sixth  lumbar.  To  expose  these  remove  the  rest  of  the  psoas 


340       LABORATORY  MANUAL  FOR  VERTEBRATE  ANATOMY 

minor.  The  common  trunk  formed  by  the  union  of  branches  from  the  fifth 
and  sixth  lumbar  nerves  passes  laterally  as  the  large  femoral  nerve.  Trace  this 
to  the  thigh.  It  courses  along  the  center  of  the  medial  surface  of  the  thigh  in 
company  with  the  femoral  artery  and  vein.  It  innervates  adjacent  muscles  of 
the  thigh  and  then  continues  down  the  shank  and  foot  as  the  saphenous  nerve. 

The  obturator  nerve  arises  from  the  connecting  band  between  the  sixth  and 
seventh  lumbar  nerves  and  passes  obliquely  caudad,  dorsal  to  the  pubis,  through 
the  obturator  foramen  and  into  the  gracilis  and  other  muscles. 

The  seventh  lumbar  together  with  the  first  sacral  unite  to  form  a  very  large 
trunk,  the  sciatic  nerve.  The  sixth  lumbar  and  second  sacral  also  contribute 
small  branches  to  this  nerve.  Follow  the  sciatic  nerve.  It  turns  dorsally,  pass- 
ing between  the  ilium  and  the  vertebral  column.  Thrust  an  instrument  through 
the  place  where  it  turns  and  dissect  where  the  instrument  emerges  on  the  dorsal 
side  of  the  animal.  On  separating  the  muscles  at  this  place  the  sciatic  nerve  is 
exposed.  Expose  it  as  near  to  the  vertebral  column  as  possible.  The  gluteal 
nerves  will  be  seen  separating  from  the  anterior  side  of  the  main  trunk  and  pass- 
ing into  the  gluteus  muscles.  (The  nerve  on  the  posterior  side  of  the  sciatic 
trunk  is  the  posterior  cutaneous,  described  below.)  Follow  the  sciatic  nerve 
down  the  leg.  After  giving  off  branches  into  the  thigh  muscles  it  divides  shortly 
above  the  knee  into  a  lateral  branch,  the  peroneal  nerve,  which  passes  between 
the  insertion  of  the  biceps  femoris  and  the  gastrocnemius,  and  a  more  medial 
branch,  the  tibial  nerve,  passing  between  the  two  heads  of  the  gastrocnemius. 

The  sacral  nerves  are  united  by  ansae  to  form  the  sacral  plexus.  The  first 
sacral  also,  as  seen  above,  takes  part  in  the  formation  of  the  sciatic  nerve.  The 
chief  nerves  arising  from  the  sacral  plexus  are  the  pudendal  nerve  and  the  inferior 
haemorrhoidal.  The  latter  arises  in  the  cat  from  the  point  of  union  of  the  three 
sacral  nerves  and  passes  to  the  bladder  and  rectum.  The  pudendal  nerve  arises 
from  the  large  trunk  formed  by  the  union  of  the  second  and  third  sacral  nerves 
and  may  also  in  the  cat  receive  a  branch  from  the  sciatic.  This  trunk  passes 
laterally  parallel  and  posterior  to  the  sciatic.  From  it  arises  the  pudendal  nerve 
which  turns  toward  the  rectum  and  urogenital  organs,  and  the  posterior  cutaneous 
nerve  which  continues  laterally  into  the  biceps  femoris  muscle.  It  will  be  found 
by  turning  the  animal  dorsal  side  up  and  looking  where  the  sciatic  nerve  was 
exposed.  The  nerve  in  question  lies  immediately  posterior  to  the  sciatic  nerve 
and  enters  the  biceps  femoris.  The  fourth  sacral  nerve  in  the  rabbit  is  of  moder- 
ate size;  it  passes  laterally  and  then  turns  to  the  sides  of  the  rectum  which  it 
innervates  in  common  with  the  pudendal  nerve. 

Draw,  showing  the  lumbosacral  plexus. 

The  foregoing  nerves  are  all  the  ventral  rami  of  the  lumbar  and  sacral  nerves. 
To  see  the  small  dorsal  rami  of  the  lumbar  nerves  proceed  as  directed  for  the 
dorsal  rami  of  the  thoracic  nerves.  The  caudal  spinal  nerves  will  not  be  con- 
sidered. 


COMPARATIVE  ANATOMY  OF  THE  NERVOUS  SYSTEM       341 

2.  The  spinal  cord  and  the  roots  of  the  spinal  nerves. — With  the  bone  scissors 
cut  out  a  piece  of  the  vertebral  column  two  or  three  inches  long  from  the  posterior 
thoracic  and  anterior  lumbar  region.  Remove  the  epaxial  muscles  from  this 
piece  so  as  to  expose  the  vertebrae,  and  with  the  bone  scissors  cut  off  the  neural 
arches  of  the  vertebrae,  thus  exposing  the  neural  canal.  In  this  canal,  but  not 
completely  filling  it,  lies  the  spinal  cord.  Note  that  the  spinal  cord  is  loosely 
inclosed  in  a  tough  membrane,  the  dura  mater,  from  which  strands  pass  to  the 
walls  of  the  neural  canal.  The  space  between  the  dura  mater  and  the  spinal 
cord  is  the  subdural  space.  Slit  open  the  dura  mater.  The  spinal  cord  is  closely 
invested  by  a  membrane,  the  pia  mater,  which  cannot  be  separated  frora  its 
surface.  Between  these  two  is  a  delicate  membrane,  the  arachnoid,  which  is 
almost  impossible  to  identify  in  gross  dissection.  The  arachnoid  and  pia  mater 
of  mammals  together  correspond  to  the  pia  mater  of  lower  vertebrates.  The 
spaces  around  and  between  these  membranes  are  filled  in  life  with  the  cerebro- 
spinal  fluid,  which  is  a  modified  lymph. 

From  the  sides  of  the  spinal  cord  observe  the  roots  of  the  spinal  nerves  aris- 
ing in  pairs  at  segmental  intervals.  They  are  insheathed  in  the  dura  mater  which 
follows  them  to  their  exit  from  the  intervertebral  foramina  and  is  continuous 
with  their  sheaths  outside  of  the  vertebral  column.  Examine  one  of  the  roots 
in  detail.  Although  it  appears  at  first  glance  to  be  single,  a  little  gentle  picking 
in  the  center  of  the  root  with  the  point  of  a  probe  will  reveal  that  it  is  composed 
of  two  parts.  One  of  these,  the  dorsal  root,  is  attached  to  the  dorsolateral  region 
of  the  cord  and  near  the  intervertebral  foramen  bears  a  large  oval  swelling,  the 
dorsal  or  spinal  ganglion.  The  dorsal  root  carries  sensory  fibers  only  (except 
in  fishes)  and  the  nerve  cells  from  which  the  sensory  fibers  originate  are 
located  in  the  spinal  ganglion.  The  other  root,  the  ventral  root,  arises  from  the 
ventrolateral  region  of  the  cord  by  several  branches  which  unite  to  one  trunk. 
The  ventral  root  carries  motor  fibers  only,  arising  from  motor  cells  in  the  cord. 
The  dorsal  and  ventral  roots  unite  beyond  the  ganglion  to  form  the  spinal  nerve, 
which  then  exits  through  the  intervertebral  foramen  and  divides  into  the  dorsal 
ramus  to  the  epaxial  muscles  and  adjacent  skin,  the  ventral  ramus  to  the  hypaxial 
muscles  and  adjacent  skin,  and  the  communicating  rami  to  the  sympathetic 
system.  These  rami  were  already  seen. 

Cut  through  the  roots  of  the  spinal  nerves  and  remove  a  small  section  of 
the  spinal  cord  for  examination.  Identify  in  the  median  dorsal  line  a  groove, 
the  dorsal  median  sulcus;  in  the  median  ventral  line,  another  groove,  the 
ventral  median  fissure.  Lateral  to  the  dorsal  median  sulcus  is  the  dorsolateral 
sulcus,  along  which  the  dorsal  roots  enter  the  cord.  The  region  between  the 
dorsal  median  and  dorsolateral  sulci  is  called  the  dorsal  funiculus.  The  lateral 
region  of  the  cord  between  the  dorsolateral  sulcus  and  the  line  along  which 
the  ventral  roots  emerge  is  the  lateral  funiculus.  Between  this  and  the  ventral 
median  fissure  is  the  ventral  funiculus. 


342       LABORATORY  MANUAL  FOR  VERTEBRATE  ANATOMY 

Make  a  diagram  through  the  cord  showing  its  funiculi  and  the  roots  of  the 
spinal  nerves. 

Make  a  clean  cut  across  the  cord  and  examine  the  cut  surface.  The  section 
is  divisible  into  a  central  darker  material,  the  gray  matter,  shaped  like  a  butter- 
fly, in  which  the  nerve  cells  of  the  cord  are  located;  and  a  much  thicker  white 
material,  the  white  matter,  surrounding  the  gray  matter  and  composed  of  nerve 
fibers.  The  white  matter  is  subdivisible  into  the  funiculi  named  above.  Each 
funiculus  consists  of  a  number  of  tracts  or  bundles  of  fibers  whose  functions  are 
known,  but  these  tracts  are  not  visibly  differentiated  from  each  other. 

3.  The  peripheral  distribution  of  the  posterior  cranial  nerves. — In  this 
section  will  be  described  the  peripheral  course  of  the  fifth,  seventh,  and  ninth  to 
twelfth  cranial  nerves.  For  the  complete  dissection  of  these,  it  is  necessary  to 
have  a  specimen  of  which  the  head  is  intact,  but  most  of  them  can  be  found,  in 
part  at  least,  on  the  same  specimen  on  which  the  previous  dissections  were  made. 

a)  The  eleventh  or  spinal  accessory  nerve:  This  nerve  supplies  the  sterno- 
mastoid,  cleidomastoid,  levator  scapulae  ventralis,  and  trapezius  muscles.     It  is 
a  pure  motor  nerve  and  is  apparently  derived  from  the  vagus. 

Rabbit:  Separate  the  sternomastoid  and  cleidomastoid  on  the  one  hand 
from  the  basioclavicularis  and  levator  scapulae  ventralis  on  the  other.  Running 
near  the  dorsal  border  of  the  levator  scapulae  ventralis  and  parallel  to  it  is  the 
spinal  accessory  nerve.  Branches  of  the  second  to  fourth  spinal  nerves  pass 
ventral  to  it  and  unite  with  it  by  branches.  Trace  it  posteriorly  and  note  its 
branches  on  the  inner  surface  of  the  trapezius.  Trace  it  anteriorly  and  note 
branches  to  the  levator  scapulae  ventralis,  sternomastoid,  and  cleidomastoid. 

Cat:  Cut  through  the  clavo trapezius  near  its  origin  and  deflect  it  ventrally, 
thus  exposing  the  levator  scapulae  ventralis.  On  the  inner  surface  of  the  clavo- 
trapezius  along  the  dorsal  border  of  the  levator  scapulae  ventralis  runs  the  main 
part  of  the  spinal  accessory  nerve.  Trace  it  posteriorly,  noting  branches  into 
the  trapezius  muscles  and  the  levator  scapulae  ventralis.  Trace  it  anteriorly. 
It  passes  dorsal  to  the  second  cervical  nerve  to  which  it  is  connected  by  a  net- 
work, and  near  this  region  gives  branches  to  the  sternomastoid  and  cleidomastoid 
muscles.  It  then  passes  through  the  cleidomastoid  muscle. 

b)  The  vagus,  the  sympathetic,  and  the  hypoglossal  nerves:  Follow  the  vagus 
and  sympathetic  anteriorly.     Stretch  the  head  forward  by  cutting  across  the 
lateral  muscles  of  the  neck.     At  about  the  level  of  the  posterior  end  of  the  larynx 
the  vagus  and  carotid  artery  are  crossed  ventrally  by  the  descending  branch  of  the 
hypoglossal  or  twelfth  cranial  nerve.     This  passes  obliquely  caudad  toward 
the  median  line  and  supplies  the  sternohyoid,  sterno thyroid,  and  thyrohyoid 
muscles.     Continue  forward.     At  about  the  place  where  the  common  carotid 
artery  divides  into  external  and  internal  carotids  a  conspicuous  nerve  is  seen 
crossing  the  ventral  surface  of  the  vagus  and  carotid  artery  and  curving  ante- 
riorly.    This  is  the  main  part  of  the  hypoglossal  nerve.     Follow  it  forward.     It 


COMPARATIVE  ANATOMY  OF  THE  NERVOUS  SYSTEM  343 

passes  to  the  dorsal  side  of  the  mylohyoid  muscle,  which  may  be  cut,  and  inner- 
vates the  muscles  of  the  tongue. 

About  halfway  between  the  descending  branch  and  main  part  of  the  hypo- 
glossal  nerve,  but  deeper  dorsally  and  passing  to  the  dorsal  side  of  the  carotid 
artery,  is  situated  the  superior  laryngeal  branch  of  the  vagus  nerve.  It  runs 
obliquely  caudad  to  the  larynx  which  it  penetrates,  passing  through  the  fibers 
of  the  thyrohyoid  muscle. 

Follow  the  vagus  and  sympathetic  once  more.  At  the  place  where  the 
descending  branch  of  the  hypoglossal  crosses  them  the  two  separate  in  the  cat. 
Shortly  anterior  to  this  the  vagus  in  both  animals  presents  an  elongated  swelling, 
the  nodosal  ganglion.  At  about  the  same  level,  but  more  medial  in  position,  the 
sympathetic  trunk  enters  the  superior  cervical  ganglion  of  the  sympathetic,  an 
elongated  pinkish  body.  The  two  ganglia  lie  just  posterior  to  the  hypoglossal 
as  it  curves  forward  into  the  tongue. 

The  hypoglossal,  the  accessory,  the  vagus,  and  the  sympathetic  are  all 
involved  in  a  plexus  in  which  the  first  cervical  nerves  also  take  part. 

c)  The  ninth  or  glossopharyngeal  nerve:  This  lies  very  close  to  the  main 
part  of  the  hypoglossal  nerve  but  more  deeply  dorsal.     Dissect  directly  internal 
to  the  hypoglossal  where  it  curves  anteriorly  to  the  tongue.     The  glossopharyngeal 
is  a  smaller  nerve  lying  dorsal  to  the  hypoglossal  along  the  sides  of  the  pharynx 
anterior  to  the  larynx.     It  is  situated  between  the  two  horns  of  the  hyoid.     It 
divides  into  two  branches:   a  smaller  pharyngeal  branch  passing  medially  into 
the  pharynx  and  a  main  lingual  branch  which  enters  the  tongue.     The  former 
is  a  motor  nerve  to  muscles  of  the  pharynx,  while  the  lingual  branch  is  a  nerve 
of  taste. 

Follow  the  nerves  thus  far  described  toward  the  point  where  they  emerge 
from  the  skull.  They  are  found  to  converge  to  a  point  to  the  medial  side  of 
the  tympanic  bulla.  Here  the  ninth,  tenth,  and  eleventh  nerves  emerge  from  the 
brain  through  the  jugular  foramen,  located  on  the  medial  side  of  the  bulla. 
The  twelfth  nerve  emerges  near  the  others  through  the  hypoglossal  foramen 
(consisting  of  several  openings  in  the  rabbit). 

d)  The  seventh  or  facial  nerve:  The  main  part  of  this  nerve  is  very  super- 
ficial in  position.     It  emerges  at  the  posterior  end  of  the  masseter  muscle  at  the 
base  of  the  ear,  in  a  sort  of  depression.     On  carefully  searching  in  this  region 
it  will  be  found  as  a  stout  white  band,  in  contact  with  the  main  part  of  the  external 
carotid  artery.     At  this  place  the  facial  gives  of!  a  branch  to  the  posterior  part 
of  the  digastric  muscle,  and  the  posterior  auricular  nerve  to  the  pinna.     (The 
large  nerve  to  the  pinna  which  may  be  noticed  dorsal  to  this  branch  of  the  facial 
is  the  great  auricular  nerve  originating  in  the  cervical  plexus.)     The  facial  then 
proceeds  forward,  branching  over  the  external  surface  of  the  masseter  muscle, 
and  passes  to  the  lips  and  region  of  the  eye.     It  supplies  the  various  parts  of 
the  platysma  muscle,  which  it  may  be  recalled  is  a  dermal  muscle  of  the  head 


344       LABORATORY  MANUAL  FOR  VERTEBRATE  ANATOMY 

and  neck,  serving  to  move  the  ears,  lips,  eyelids,  whiskers,  etc.  The  platysma 
muscle  is  a  visceral  muscle  originally  belonging  to  the  hyoid  arch,  hence  its 
inner vation  by  the  facial  nerve. 

e)  The  fifth  or  trigeminus  nerve:  This  nerve  has  three  main  branches:  the 
ophthalmic,  the  maxillary,  and  the  mandibular.  The  former  is  best  studied  with 
the  eye  since  it  passes  into  the  orbit. 

To  locate  the  mandibular  branch  of  the  trigeminus  proceed  as  follows,  free- 
ing one  half  of  the  mandible.  Cut  through  the  attachment  of  the  digastric  to 
the  mandible  and  deflect  the  digastric  backward.  Cut  through  the  attachments 
of  all  of  the  muscles  along  the  medial  surface  of  the  mandible,  keeping  the  knife 
against  the  bone.  Next,  free  the  lateral  or  outer  surface  of  the  body  of  the 
mandible  from  muscle  attachments,  chiefly  the  masseter.  Cut  through  the 
symphysis  of  the  mandible  (place  of  junction  of  the  two  halves  of  the  mandible 
at  their  anterior  tips).  Carefully  bend  the  half  of  the  mandible  thus  freed  out- 
ward, so  as  to  expose  the  side  of  the  muscular  mass  which  forms  the  floor  of  the 
mouth  and  pharyngeal  cavities.  The  main  part  of  the  mandibular  branch  of 
the  trigeminus,  the  inferior  alveolar  nerve,  will  now  be  seen  passing  into  a  fora- 
men, the  mandibular  foramen,  situated  on  the  medial  surface  of  the  mandible. 
In  the  rabbit  the  mylohyoid  nerve,  another  branch  of  the  mandibular,  will  be 
noted  to  the  medial  side  of  the  inferior  alveolar  and  proceeding  ventrally 
to  muscles  of  the  floor  of  the  mouth  cavity.  The  inferior  alveolar  nerve  runs 
in  the  interior  of  the  mandible  supplying  the  teeth  and  then  emerges  through 
the  mental  foramen  on  the  lateral  surface  of  the  mandible  at  the  level  of  the 
diastema.  There  the  nerve,  now  named  the  mental  nerve,  may  be  found  and 
followed  into  the  lower  lip. 

Trace  the  inferior  alveolar  nerve  posteriorly.  It  converges  toward  another 
branch  of  the  mandibular  nerve,  the  lingual  nerve,  which  should  then  be  followed 
forward.  It  passes  into  the  tongue,  lying  close  to  the  hypoglossal.  The  lingual 
branch  of  the  trigeminus  innervates  the  mucous  membrane  of  the  tongue,  but 
is  not  a  nerve  of  taste. 

Follow  both  lingual  and  inferior  alveolar  nerves  centrally  again.  In  front 
of  the  tympanic  bulla  behind  the  point  where  the  body  of  the  mandible  bends 
dorsally  into  the  ramus  of  the  mandible  will  be  seen  the  auriculotemporal  branch 
of  the  mandibular  nerve  joining  the  other  two.  On  tracing  it  peripherally  it  is 
found  to  pass  to  the  skin  of  the  cranial  side  of  the  pinna  and  in  the  cat  also  sends 
branches  along  the  side  of  the  face  in  company  with  the  branches  of  the  facial. 

The  tympanic  bulla  may  now  be  exposed.  Emerging  from  the  bulla  will  be 
found  a  slender  nerve  which  very  soon  joins  the  lingual  branch  of  the  man- 
dibular. This  is  the  chorda  tympani  (so  called  because  it  runs  in  the  tympanic 
membrane),  a  branch  of  the  facial  nerve.  Its  fibers  pass  out  with  the  lingual 
nerve  and  supply  the  taste  buds  on  the  anterior  part  of  the  tongue;  it  also  inner- 
vates the  sublingual  and  submaxillary  salivary  glands. 


COMPARATIVE  ANATOMY  OF  THE  NERVOUS  SYSTEM  345 

Besides  the  branches  of  the  mandibular  nerve  here  named  there  are  branches 
to  the  muscles  of  mastication,  namely,  the  temporal,  the  masseter,  the  anterior 
part  of  the  digastric,  and  the  pterygoids. 

Remove  the  half  of  the  mandible.  This  will  reveal  additional  branches  of 
the  mandibular  nerve.  One  of  these,  the  buccinator,  will  probably  be  noticed 
extending  to  the  angle  of  the  mouth,  where  it  supplies  the  masseter  muscle  and 
the  lips.  The  main  trunk  of  the  maxillary  nerve,  the  second  branch  of  the 
trigeminus,  may  now  be  sought.  It  is  a  very  stout  trunk  lying  at  the  sides  of 
the  palate  in  front  of  and  more  deeply  situated  than  the  main  trunk  of  the  man- 
dibular nerve.  It  is  somewhat  concealed  by  an  artery  (internal  maxillary)  which 
runs  along  its  ventral  surface  and  should  be  removed.  The  maxillary  nerve  is  then 
revealed  as  a  large  trunk  which  passes  forward  along  the  side  of  the  hard  palate 
and  disappears  dorsal  to  the  teeth.  Cut  away  the  zygomatic  arch  on  the  same 
side  on  which  the  half  of  the  mandible  was  removed;  in  the  rabbit  cut  away 
also  the  ridge  which  holds  the  molar  and  premolar  teeth.  By  this  operation 
the  contents  of  the  orbit  are  revealed.  Note  in  the  cat  the  small  reddish  infra- 
orbital  salivary  gland  lying  close  to  the  maxillary  nerve.  In  the  rabbit  the  very 
large  reddish  mass  of  the  Harderian  gland  with  the  smaller  yellowish  mass  of 
the  infraorbital  salivary  gland  anterior  to  it  are  readily  noticed.  The  maxillary 
nerve  should  now  be  investigated.  It  is  seen  to  divide  into  a  large  main  trunk , 
the  infraorbital  nerves,  and  a  small  medial  branch,  the  spheno palatine  nerve, 
which  passes  into  the  hard  palate.  The  infraorbital  nerves  pass  forward  above 
the  teeth,  which  they  supply,  and  emerge  through  the  infraorbital  foramen, 
situated  internal  to  the  root  of  the  zygomatic  arch.  On  separating  the  upper 
lip  from  the  teeth  the  foramen  is  readily  found  and  the  nerve  is  seen  emerging 
from  it  to  supply  the  upper  lip  and  side  of  the  nose.  Follow  the  sphenopalatine 
nerve  toward  the  palate,  cutting  away  the  bone.  It  connects  with  a  ganglion, 
the  sphenopalatine  ganglion  of  the  sympathetic  system.  This  ganglion  lies  near 
the  sphenopalatine  foramen.  The  chief  branch  of  the  sphenopalatine  nerve 
is  the  palatine  branch  which  passes  into  the  hard  palate  by  a  foramen.  In  the 
cat  this  nerve  arises  before  the  ganglion  is  reached,  in  the  rabbit  beyond  the 
ganglion.  Other  branches  of  the  sphenopalatine  nerve  pass  from  the  ganglion 
into  the  nasal  cavity. 

4.  The  sense  organs  of  the  head. — 

a)  The  eye,  the  eye  muscles,  and  the  nerves  of  the  orbit:  Dissect  on  the  other 
side  from  that  on  which  the  cranial  nerves  were  worked  out.  Identify  upper 
and  lower  eyelids  and  the  nictitating  membrane,  a  fold  projecting  from  the 
anterior  corner  of  the  eye.  Make  a  slit  through  the  junction  of  upper  and  lower 
lids  at  the  posterior  corner  of  the  eye  so  that  the  eyelids  can  be  pulled  away 
from  the  eyeball.  Note  that  the  skin  passes  onto  the  inner  surface  of  the  eyelids 
and  continues  over  the  exposed  surface  of  the  eyeball,  thus  forming  the  outer- 
most covering  membrane,  the  conjunctiva,  for  this  part  of  the  eyeball.  Make  an 


346       LABORATORY  MANUAL  FOR  VERTEBRATE  ANATOMY 

incision  through  the  skin  above  the  eye  and  deflect  the  skin  downward  toward  the 
eye  on  which  you  are  working,  stretching  the  skin  away  from  the  head.  On 
the  skin  of  the  inner  surface  of  the  upper  eyelid  note  a  thin  sheet  of  muscle  fibers, 
proceeding  in  a  somewhat  circular  direction.  This  is  orbicularis  oculi,  a  part  of 
the  platysma,  and  has  the  function  of  closing  the  eyelids. 

Rabbit:  Stretch  the  upper  eyelid  away  from  the  head  and  clean  away  the 
connective  tissue  between  it  and  the  eyeball.  A  thin  sheet  of  muscle  will  be 
found  extending  from  beneath  the  supraorbital  arch  to  the  upper  eyelid.  This 
is  the  levator  palpebrae  superioris,  which  raises  the  eyelid.  Repeat  the  foregoing 
directions  on  the  lower  eyelid,  stretching  the  skin  away  from  the  eyeball.  On 
the  inner  surface  of  the  lower  eyelid  note  the  rest  of  the  orbicularis  oculi.  The 
depressor  palpebrae  inferioris  may  be  noted  extending  from  the  zygomatic  arch 
to  the  lower  eyelid;  it  lowers  the  eyelid.  Remove  the  surrounding  skin  and 
eyelids,  cutting  them  away  from  the  eyeball.  With  the  bone  clippers  cut  away 
the  supraorbital  arch  and  clean  away  tissue  between  the  dorsal  surface  of  the 
eyeball  and  the  orbit.  A  slender  but  strong  muscle  will  now  be  seen  extending 
from  about  the  middle  of  the  wall  of  the  orbit  to  the  dorsal  surface  of  the  eye- 
ball; this  is  the  superior  oblique  muscle.  It  separates  the  thin  sheet  of  the 
levator  palpebrae  superioris  into  two  parts  which  pass  on  either  side  of  it.  Trace 
the  superior  oblique  to  the  wall  of  the  orbit.  Here  there  will  be  found  a  tendi- 
nous cord,  the  trochlea,  over  which  the  muscle  passes.  Next,  remove  the  levator 
palpebrae  superioris  and  find  underneath  its  posterior  portion  the  thin  flat 
superior  rectus  muscle.  The  insertion  of  the  superior  oblique  on  the  eyeball  is 
concealed  under  the  margin  of  the  superior  rectus. 

Remove  the  half  of  the  mandible  and  the  zygomatic  arch  on  the  side  on 
which  you  are  working.  This  fully  exposes  the  ventral  side  of  the  eyeball. 
Along  the  ventral  surface  of  the  outer  part  of  the  eyeball  extends  the  yellowish 
infraorbital  salivary  gland.  Medial  to  this  extending  beneath  the  eyeball  is  the 
larger  Harderian  gland,  which  pours  its  secretion  onto  the  nictitating  membrane. 
Remove  these  glands;  note  the  white  part  of  the  Harderian  gland  extending  far 
medially.  The  inferior  oblique  muscle  is  now  seen  extending  to  the  eyeball 
from  the  anteroventral  region  of  the  orbit.  Posterior  to  it  is  the  inferior  rectus 
muscle  originating  from  the  posteroventral  region  of  the  orbit.  Note  the  branch 
of  the  oculomotor  nerve  running  along  the  anterior  border  of  the  inferior  rectus 
and  supplying  both  muscles.  The  nerve  which  runs  along  the  posterior  border 
of  the  inferior  rectus,  innervating  the  lower  eyelid,  is  the  zygomatic  branch  of 
the  maxillary  nerve.  Immediately  behind  the  inferior  rectus  is  the  external  or 
lateral  rectus.  The  nerve  passing  along  the  posterior  margin  of  the  external 
rectus  is  the  lacrimal  branch  of  the  maxillary.  It  passes  to  the  lacrimal  gland 
and  to  the  skin  between  the  eye  and  base  of  the  pinna.  The  lacrimal  gland  is 
a  small  reddish  body  which  will  be  found  by  pressing  the  eyeball  forward  and 
searching  against  the  posterodorsal  wall  of  the  orbit.  Two  nerves  pass  the  point 


COMPARATIVE  ANATOMY  OF  THE  NERVOUS  SYSTEM  347 

of  origin  of  the  external  rectus  from  the  orbit.  The  larger  is  the  oculomotor,  the 
smaller  the  abducens.  Cut  through  the  insertions  of  the  inferior  oblique  and 
inferior  and  external  recti  at  the  eyeball  and  deflect  them  ventrally.  Above  the 
inferior  rectus  the  internal  or  medial  rectus  will  be  seen  inserted  on  the  eyeball. 
Look  on  the  inner  surface  of  the  external  rectus  and  find  the  abducens  nerve, 
curving  around  the  posterior  border  of  the  origin  of  this  muscle  and  passing  on- 
to its  surface.  Return  to  the  dorsal  surface  of  the  eyeball,  cut  through  the  inser- 
tion of  the  superior  oblique  at  the  eyeball,  and  press  the  eyeball  ventrally.  Two 
nerves  will  be  seen  on  the  medial  wall  of  the  orbit.  The  lower  one  is  the  trochlear 
nerve.  Trace  it  to  the  medial  surface  of  the  superior  oblique.  The  upper  nerve 
is  the  frontal  nerve,  one  of  the  main  branches  of  the  ophthalmic  branch  of  the 
trigeminus.  It  passes  to  the  dorsal  part  of  the  orbit  and  exits  through  the 
anterior  supraorbital  foramen  to  be  distributed  to  the  upper  eyelid  and  skin  in 
front  of  the  orbit.  It  may  have  been  cut  in  removing  the  supraorbital  arch. 
The  white  part  of  the  Harderian  gland  will  be  noted  in  the  anterior  part  of  the 
orbit.  Cut  through  all  of  the  insertions  of  the  eye  muscles  at  the  eyeball  and 
through  the  optic  nerve,  removing  the  eyeball.  The  optic  nerve  is  the  stout 
white  trunk  near  the  superior  rectus.  The  muscles  around  the  optic  nerve, 
exclusive  of  those  already  identified,  belong  to  the  retractor  bulbi.  Find  the 
main  trunk  of  the  oculomotor  nerve  and  trace  its  branches  to  the  retractor 
bulbi  and  superior  and  internal  recti.  The  main  nerve  curves  below  the  optic 
nerve.  The  nasociliary  branch  of  the  ophthalmic  nerve  may  be  noted  passing 
between  the  superior  oblique  and  the  retractor  bulbi.  Its  main  portion,  the 
ethmoidal  nerve,  leaves  the  orbit  by  a  small  foramen  in  front  of  the  superior 
oblique  muscle.  On  tracing  this  nerve  posteriorly  fine  branches  to  the  orbit 
may  be  seen. 

Trace  the  nerves  of  the  orbit  to  their  exits  from  the  skull.  The  third,  fourth, 
and  sixth  nerves  and  the  ophthalmic  and  maxillary  branches  of  the  trigeminus 
pass  through  the  orbital  fissure.  The  mandibular  branch  of  the  trigeminus 
passes  through  the  foramen  lacerum. 

Cat:  Remove  the  eyelids  and  the  surrounding  skin,  cutting  them  away  from 
the  eyeball.  Remove  the  half  of  the  mandible  and  the  zygomatic  arch  from 
the  side  on  which  you  are  working.  Press  the  eyeball  ventrally  away  from  the 
supraorbital  arch.  In  the  anterodorsal  angle  of  the  orbit  a  strong  fibrous  con- 
nection will  be  found  between  the  wall  of  the  orbit  and  the  eyeball.  On  investi- 
gating this  it  is  found  to  consist  of  two  fibrous  bands  which  form  a  pulley;  this 
is  known  as  the  trochlea.  The  tendon  of  the  superior  oblique  muscle  passes  over 
the  trochlea  and  is  inserted  on  the  eyeball.  Its  insertion  is  much  expanded  and 
extends  caudad  from  the  trochlea.  Posterior  to  the  insertion  of  the  superior 
oblique  is  a  thin  flat  muscle,  the  levator  palpebrae  superioris,  or  elevator  of  the 
upper  eyelid.  This  passes  to  the  dorsoposterior  surface  of  the  eyeball.  Cut 
through  this  at  its  insertion.  Posterior  to  this  muscle  in  the  dorsoposterior 


348       LABORATORY  MANUAL  FOR  VERTEBRATE  ANATOMY 

angle  of  the  orbit  is  the  flattened  lacrimal  gland.  Cut  out  the  nictitating 
membrane  and  examine  its  internal  surface.  It  is  found  to  be  roughened,  owing 
to  the  presence  of  the  Harderian  gland  in  its  wall. 

Turn  to  the  ventral  surface  of  the  eyeball  exposed  by  the  removal  of  the 
mandible,  the  zygomatic  arch,  and  part  of  the  hard  palate.  Identify  again  the 
small  reddish  infraorbital  salivary  gland,  situated  back  of  the  last  tooth.  On 
clearing  away  connective  tissue  and  fat,  the  inferior  oblique  eye  muscle  will  be 
seen  extending  from  the  anterior  part  of  the  orbit  to  the  ventral  surface  of  the 
eyeball.  Ventral  and  at  right  angles  to  the  inferior  oblique  is  the  inferior  rectus. 
The  branch  of  the  oculomotor  which  innervates  the  inferior  oblique  runs  along 
the  posterior  border  of  the  inferior  rectus.  The  zygomatic  branch  of  the  maxil- 
lary nerve  passes  along  the  posterior  border  of  the  inferior  rectus  to  the  lower 
eyelid,  but  may  have  been  destroyed.  Posterior  to  the  inferior  rectus  is  the 
external  rectus;  between  and  internal  to  them  appears  one  of  the  four  parts  of  the 
retractor  bulbi  muscle.  Along  the  posterior  border  of  the  external  rectus  runs 
the  lacrimal  branch  of  the  maxillary  nerve,  supplying  the  lacrimal  gland  and 
adjacent  skin.  On  detaching  the  eyeball  from  the  posterior  wall  of  the  orbit 
another  part  of  the  retractor  bulbi  will  be  seen  next  posterior  to  the  external 
rectus;  dorsal  to  this  is  the  superior  rectus.  Cut  through  both  obliques  at  their 
insertions  and  press  the  eyeball  posteriorly.  Note  the  internal  rectus  on  the 
anterior  surface  of  the  eye  and  above  it  the  remainder  of  the  retractor  bulbi. 

Cut  through  all  of  the  eye  muscles  and  the  optic  nerve  at  their  insertion  on 
the  eyeball  and  remove  the  latter.  Note  the  four  parts  of  the  retractor  bulbi 
around  the  optic  nerve.  Deflect  the  external  rectus  ventrally  and  note  the 
abducens  nerve  ascending  on  its  inner  surface.  Running  along  the  ventral  sur- 
face of  the  optic  nerve  note  a  slender  nerve,  the  long  ciliary  branch  of  the  oph- 
thalmic branch  of  the  trigeminus;  it  accompanies  the  optic  nerve  into  the 
eyeball.  Look  on  the  inner  surface  of  the  inferior  rectus  for  the  branch  of  the 
oculomotor  to  this  muscle.  Note  the  ciliary  ganglion  of  the  sympathetic  near 
this  branch  and  observe  branches  between  this  ganglion  and  the  oculomotor 
and  long  ciliary  nerves  and  the  short  ciliary  nerves  passing  from  the  ganglion 
along  the  optic  nerve.  Find  the  main  trunk  of  the  oculomotor  ventral  to  the 
optic  nerve  at  the  place  of  passage  of  both  through  the  wall  of  the  orbit,  and 
note  branches  of  the  oculomotor  to  the  retractor  bulbi  and  superior  rectus. 
Bend  all  eye  muscles  except  the  superior  oblique  ventrally,  leaving  the  superior 
oblique  against  the  medial  wall  of  the  orbit.  Crossing  the  inner  surface  of  the 
superior  oblique  obliquely  forward  are  two  nerves.  They  are  parts  of  the  oph- 
thalmic branch  of  the  trigeminus.  The  lower  one  is  the  ethmoidal  nerve;  it 
passes  through  a  foramen  into  the  nasal  cavity.  The  upper  one  is  the  infra- 
trochlear  nerve.  It  goes  to  the  anterior  part  of  the  upper  eyelid.  Posterior 
to  and  parallel  to  the  posterior  margin  of  the  superior  oblique  is  the  frontal 
branch  of  the  ophthalmic.  It  innervates  the  upper  eyelid  and  integument 


COMPARATIVE  ANATOMY  OF  THE  NERVOUS  SYSTEM  349 

anterior  to  the  eyelid.  The  trochlear  nerve  lies  slightly  dorsal  and  medial  to 
the  proximal  portions  of  the  ethmoidal  and  infratrochlear  nerves.  It  runs 
obliquely  dorsad  and  anteriorly,  and  enters  the  superior  oblique  muscle  at  about 
the  middle  of  its  posterior  margin. 

The  structure  of  the  eyeball  may  now  be  investigated.  It  is  very  similar 
to  that  of  all  vertebrates.  The  outer  tough  sclerotic  coat  or  sclera  is  continuous 
with  the  transparent  cornea  covering  the  exposed  surface  of  the  eye.  As  found 
above,  the  cornea  is  covered  externally  by  the  conjunctiva.  Cut  off  the  top  or 
dorsal  side  of  the  eyeball  and  look  within.  The  large  lens  will  be  observed. 
Internal  to  the  sclera  is  the  black  chorioid  coat  of  the  eye  and  internal  to  that 
the  greenish-gray  retina.  Between  the  lens  and  the  retina  is  a  large  chamber, 
the  cavity  of  the  vitreous  humor,  containing  a  gelatinous  mass,  the  vitreous  humor 
or  vitreous  body.  Remove  the  lens.  The  chorioid  coat  terminates  behind  the 
cornea  as  a  black  curtain,  the  iris,  bearing  in  its  center  a  round  hole,  the  pupil. 
The  space  between  the  cornea  and  the  iris  is  called  the  anterior  chamber  of  the 
eye  and  is  filled  in  life  with  a  fluid,  the  aqueous  humor.  The  boundary  between 
the  iris  and  the  rest  of  the  chorioid  coat  constitutes  a  ring  known  as  the  ciliary 
body.  It  consists  of  two  parts :  a  ring  of  thickened  processes,  the  ciliary  processes, 
next  to  the  iris,  and  a  ring  of  radially  arranged  ridges,  the  orbiculus  ciliaris, 
extending  to  the  main  part  of  the  chorioid  coat.  Both  parts  Of  the  ciliary  body 
contain  the  ciliary  muscle;  this  is  a  smooth  muscle  having  both  meridional  and 
circular  fibers.  It  originates  on  the  sclera,  is  inserted  on  the  walls  of  the  ciliary 
body,  and  has  the  function  of  changing  the  shape  of  the  lens.  By  making  a 
new  cut  parallel  to  the  first  around  the  equator  of  the  eyeball  the  relations  of 
cornea,  iris,  and  ciliary  body  will  be  more  clearly  observable.  Note  the  marked 
thickening  due  to  the  ciliary  body.  Examine  the  lens.  Note  its  biconvex  form 
as  compared  with  the  spherical  form  of  the  lens  of  the  fish  eye.  Around  the 
equator  of  the  lens  will  be  found  the  torn  attachment  of  a  membrane.  This 
membrane  holds  the  vitreous  body.  Where  it  is  attached  to  the  lens  it  exhibits 
parallel  ridges,  the  zonular  fibers,  which  in  life  fit  into  the  hollows  between  the 
ciliary  processes.  The  zonular  fibers  constitute  the  suspensory  ligament  of  the 
lens  which  passes  from  the  lens  to  the  ciliary  processes.  By  means  of  this  liga- 
ment traction  can  be  exerted  on  the  lens  and  its  shape  altered  to  some  extent. 
The  small  space  between  the  suspensory  ligament  and  the  iris  is  the  posterior 
chamber  of  the  eye.  Peel  the  lens  and  note  that  it  is  composed  of  concentric 
coats  or  lamellae  like  the  coats  of  an  onion,  each  lamella  being  composed  of  lens 
fibers. 

Draw  the  section  of  the  eye. 

b)  The  nasal  cavities:  Detach  the  head  of  the  animal  at  the  joint  between 
the  occipital  condyles  and  the  altas,  and  discard  the  body.  Cut  off  the  pinnae. 
Clear  the  dorsal  surface  of  the  skull  down  to  the  bone.  With  a  saw,  saw  com- 
pletely through  the  head  slightly  to  one  side  of  the  median  sagittal  plane.  Use 


350       LABORATORY  MANUAL  FOR  VERTEBRATE  ANATOMY 

the  saw  only  for  the  bony  parts.  After  having  sawed  through  the  roof  of  the 
skull,  cut  down  through  the  brain  with  a  single  sliding  stroke  of  a  blunt  knife 
like  a  table  knife.  The  brain  and  skull  should  thus  be  cut  in  two,  one  part 
being  slightly  larger  than  the  other.  Wash  the  cut  surfaces  gently  under  the  tap, 
and  study  the  nasal  cavities. 

The  nasal  cavities  are  very  long  in  the  rabbit,  shorter  in  the  cat.  They  are 
divided  into  right  and  left  cavities  or  fossae  by  a  perpendicular  plate,  the  septum 
of  the  nose,  which  is  present  on  the  larger  section  of  the  head.  The  septum 
consists  of  cartilage  anteriorly  and  of  thin  bone  posteriorly,  the  latter  being  the 
perpendicular  plate  of  the  ethmoid  bone.  On  the  smaller  section  the  lateral  and 
posterior  walls  of  the  nasal  fossa  are  seen  to  be  occupied  by  delicate  scrolled  and 
folded  bones,  the  turbinated  bones  or  conchae.  In  the  rabbit  these  are  readily 
divisable  into  an  anterior  concha,  the  inferior  concha  or  maxilloturbinal,  much 
folded  and  located  on  a  separate  small  bone  of  the  skull;  a  middle  concha  or 
nasoturbinal,  a  long  single  fold  dorsal  to  the  preceding  and  dependent  from  the 
nasal  bone;  and  the  superior  concha  or  ethmoturbinal,  part  of  the  ethmoid  bone. 
In  the  cat  the  turbinals  are  closely  crowded  together,  but  by  prying  them  apart 
gently  there  can  be  distinguished  a  small  anterior  maxilloturbinal  on  the  maxilla ; 
above  this  a  single  fold,  the  nasoturbinal,  dependent  from  the  nasal  bone;  and  a 
great  mass  of  folds,  the  ethmoturbinal,  filling  the  greater  part  of  the  nasal  fossa. 
The  ethmoturbinals  are  also  called  the  ethmoid  labyrinths,  and  the  spaces  inclosed 
by  the  bony  folds  are  called  the  ethmoid  cells.  Definite  passages  known  as  the 
meatuses  of  the  nose  run  between  the  conchae  and  conduct  the  air  to  the  naso- 
pharynx. They  connect  with  the  nasopharynx  below  the  ethmoturbinals. 

The  posterior  dorsal  part  of  the  nasal  fossa  is  closed  by  the  cribriform  plate 
of  the  ethmoid  which  unites  with  the  perpendicular  plate  of  the  ethmoid  bone 
medially  and  with  the  parts  of  the  ethmoid  which  bear  the  layrinths  laterally. 
The  anterior  end  of  the  brain  (olfactory  bulbs)  is  readily  seen  to  abut  against 
the  cribriform  plate,  and  through  this  plate  the  fibers  of  the  olfactory  nerve  pass 
from  the  olfactory  membrane  covering  the  ethmoid  labyrinths  to  the  olfactory 
bulbs. 

c)  The  structure  of  the  ear:  Carefully  remove  the  brain  from  the  two  halves 
of  the  skull,  preserving  the  latter.  In  doing  this  the  roof  of  the  skull  may  be 
cut  away.  Loosen  the  brain  on  all  sides  by  passing  a  blunt  instrument  between 
the  brain  and  the  skull.  The  tough  membrane,  the  dura  mater,  which  covers 
the  brain  should  be  retained  with  the  brain.  Carefully  cut  the  cranial  nerves 
where  they  pass  through  the  foramina  of  the  skull,  leaving  their  roots  attached 
to  the  brain.  Note  the  small  round  reddish  body,  the  pituitary  body,  attached  to 
the  ventral  surface  of  the  brain,  set  into  a  depression  in  the  floor  of  the  skull; 
keep  this  body  attached  to  the  brain  if  possible.  Preserve  the  two  halves  of  the 
brain  in  a  vessel  of  water  or,  if  they  are  to  be  kept  for  some  time,  in  weak  formal- 
dehyde. 


COMPARATIVE  ANATOMY  OF  THE  NERVOUS  SYSTEM  351 

After  removal  of  the  brain  examine  the  cavities  of  the  skull  on  the  larger 
piece.  Anteriorly  behind  the  cribriform  plate  is  the  small  anterior  or  olfactory 
fossa  in  which  the  olfactory  bulbs  are  situated.  Posterior  to  this  is  the  large 
middle  or  cerebral  fossa  lodging  the  cerebrum.  Behind  this  is  the  smaller  posterior 
or  cerebellar  fossa  for  the  cerebellum.  The  cerebral  and  cerebellar  fossae  are 
partly  separated  by  a  bony  ledge,  the  tentorium,  which  is  continued  in  life  by 
the  dura  mater.  In  the  floor  of  the  cerebral  fossa  in  the  basisphenoid  bone  is 
the  sella  turcica  lodging  the  pituitary  body.  Note  also  the  optic  foramen  in  front 
of  this  and  behind  this,  near  the  ventral  end  of  the  tentorium,  the  foramina  for 
the  passage  of  the  third  to  sixth  cranial  nerves.  In  the  wall  of  the  cerebellar 
fossa  observe  an  area  of  hard,  white  bone;  this  is  the  petrous  portion  of  the 
temporal.  In  the  center  of  this  is  a  foramen  for  the  passage  of  the  auditory 
nerve  into  the  internal  ear.  Above  this  in  the  rabbit  is  a  depression,  thefloccular 
fossa,  which  lodges  a  part  of  the  cerebellum  called  the  flocculus.  In  removing 
the  rabbit  brain  the  flocculus  is  left  behind  in  the  fossa.  In  front  of  the  ventral 
part  of  the  petrous  bone,  just  behind  the  tentorium,  is  the  internal  opening  of 
the  facial  canal  for  the  passage  of  the  facial  nerve.  Behind  the  middle  of  the 
petrous  bone  is  the  jugular  foramen  for  the  passage  of  the  ninth,  tenth,  and 
eleventh  nerves.  Behind  this  the  twelfth  nerve  passes  through  one  or  more 
foramina. 

The  ear  of  mammals  consists  of  three  parts,  the  external,  the  middle,  and  the 
internal  ear.  The  external  ear  includes  the  pinna  or  auricle  and  the  external  audi- 
tory meatus  leading  into  the  interior  of  the  bulla;  these  have  already  been  noted. 
The  middle  ear  is  situated  in  the  tympanic  bulla,  the  internal  ear  in  the  petrous 
portion  of  the  temporal  bone.  Both  are  consequently  in  the  wall  of  the  cere- 
bellar fossa.  With  the  bone  clippers  remove  this  wall  in  one  piece  and  discard 
the  remainder  of  the  skull.  Clean  away  the  muscles  from  its  external  surface, 
exposing  the  tympanic  bulla. 

Rabbit:  With  the  bone  clippers  cut  away  the  ventral  wall  of  the  tympanic 
bulla.  A  large  cavity,  the  cavity  of  the  middle  ear  or  tympanic  cavity,  is  revealed. 
In  the  lateral  wall  of  this  cavity  is  a  ringlike  elevation  of  bone  across  which  is 
stretched  a  delicate  membrane,  the  tympanic  membrane  or  eardrum.  By  probing 
into  the  external  auditory  meatus  determine  that  the  meatus  terminates  at  the 
eardrum,  which  closes  its  internal  opening.  The  tympanic  membrane  has  a 
nearly  vertical  position.  Extending  toward  the  tympanic  membrane  from  the 
medial  wall  is  a  short  calcareous  process  which  supports  the  chorda  tympani 
branch  of  the  facial  nerve  as  it  crosses  from  the  facial  to  the  tympanic  membrane. 
Anterior  to  the  tympanic  membrane  is  a  depression  in  which  are  lodged  the  three 
little  ear  bones.  These  bones  are  so  small  and  so  deeply  lodged  in  the  depression 
that  they  cannot  be  distinctly  seen,  but  on  picking  in  the  depression  with  a 
forceps  it  is  usually  possible  to  extract  one  or  more  of  them.  Compare  them  with 
K,  page  202,  Figure  210. 


352       LABORATORY  MANUAL  FOR  VERTEBRATE  ANATOMY 

Cat:  Remove  the  fleshy  part  of  the  external  auditory  meatus  down  to  the 
tympanic  bulla.  The  meatus  will  be  found  to  terminate  at  an  oval  opening  with 
a  slightly  elevated  rim.  Across  the  rim  is  stretched  the  delicate  tympanic  mem- 
brane or  eardrum.  The  handle  of  the  malleus  or  hammer  is  visible  through 
the  eardrum  attached  to  its  internal  surface.  Next,  remove  with  the 
bone  clippers  the  ventral  wall  of  the  bulla.  The  interior  is  the  tympanic 
cavity  of  the  middle  ear.  Note  that  it  is  divided  by  a  bony  plate  into  a 
larger  posteroventral  chamber  and  a  smaller  anterodorsal  chamber.  The  latter 
is  the  one  covered  above  by  the  tympanic  membrane.  Break  open  the  plate  of 
bone,  exposing  this  cavity,  which  is  called  the  tympanum  proper  and  which  con- 
tains the  ear  bones.  Note  the  membrane  which  lines  it  and  the  eardrum  forming 
its  anterodorsal  wall.  From  the  posterodorsal  region  of  the  cavity  a  calcareous 
process  projects  toward  the  eardrum  and  carries  the  chorda  tympani  nerve,  a 
branch  of  the  facial,  to  the  eardrum.  From  the  internal  surface  of  the  eardrum 
the  three  little  ear  bones  are  plainly  seen  extending  into  a  depression  in  the  inter- 
nal wall  of  the  tympanum.  These  may  be  extracted  and  examined.  Compare 
with  K,  page  202,  Figure  210. 

There  now  remains  between  the  bulla  and  the  cerebellar  fossa  the  hard  white 
mass  of  the  petrous  bone  already  noted.  This  contains  the  internal  ear.  Owing 
to  the  complexity  of  the  internal  ear  and  its  small  size,  a  dissection  of  it  is  imprac- 
tical, but  its  main  parts  can  be  seen  by  breaking  away  the  petrous  bone  in  small 
fragments.  The  tiny  spirally  coiled  chamber  in  the  bone  is  the  cochlea;  it 
contains  a  spiral  tube,  the  cochlear  duct,  in  which  the  organ  of  sound  perception 
(organ  of  Corti)  is  located.  In  the  thicker  harder  part  of  the  petrous  bone  are 
semicircular  channels,  the  semicircular  canals,  inclosing  the  semicircular  ducts. 

The  internal  ear  is  thus  seen  to  be  inclosed  in  channels  in  the  petrous  bone,  consisting 
of  the  cochlea,  the  semicircular  canals,  and  the  vestibule  or  connecting  chamber;  together, 
these  constitute  the  bony  labyrinth.  The  internal  ear  proper,  or  membranous  labyrinth,  is 
contained  in  the  bony  labyrinth.  Its  parts  are:  the  sacculus  and  utriculus  inclosed  in  the 
vestibule;  the  semicircular  ducts  arising  from  the  utriculus  and  situated  inside  of  the  semi- 
circular canals,  and  the  cochlear  duct  arising  from  the  sacculus  and  inclosed  in  the  cochlea. 
The  cochlear  duct  is  a  new  structure  characteristic  of  mammals,  although  it  begins  to  appear 
in  birds.  It  is  an  outgrowth  of  the  sacculus  and  is  the  real  organ  of  hearing,  the  semicircular 
ducts  serving  equilibratory  functions  only. 

5.  The  structure  of  the  brain. — 

a)  The  membranes  or  meninges  of  the  brain:  With  the  two  halves  of  the  brain 
previously  removed  before  you,  study  the  membranes  of  the  brain.  The  brain  is 
covered  by  a  tough  membrane,  the  dura  mater.  This  consists  of  the  dura  mater 
of  lower  forms  fused  to  the  internal  lining  (periosteum)  of  the  skull.  A  consider- 
able space,  the  subdural  space,  is  present  between  the  dura  mater  and  the  other 
membranes  of  the  brain.  The  dura  mater  dips  down  between  the  larger  divisions 
of  the  brain.  The  surface  of  the  brain  is  covered  by  the  delicate  pia  mater,  in 


COMPARATIVE  ANATOMY  OF  THE  NERVOUS  SYSTEM  353 

which  the  blood  vessels  run.  The  pia  mater  follows  closely  all  of  the  folds  of 
the  brain  surface.  Between  the  pia  mater  and  the  dura  mater  is  another  mem- 
brane, the  arachnoid,  very  delicate  and  difficult  to  see.  It  is  best  found  covering 
the  depressions  on  the  surface  of  the  brain,  for  the  pia  mater  dips  down  into  these 
depressions  while  the  arachnoid  passes  over  them.  Between  the  arachnoid  and 
the  pia  mater  is  the  subarachnoid  space  crossed  by  a  delicate  web  of  tissue.  All 
of  the  spaces  between  the  meninges  of  the  brain  are  filled  in  life  with  the  cerebro- 
spinal  fluid. 

b)  The  dorsal  aspect  of  the  brain:  Remove  the  dura  mater.  Fit  the  two  halves 
of  the  brain  and  study  the  dorsal  surface.  At  the  anterior  end  of  the  brain  are 
the  two  olfactory  bulbs,  relatively  small,  rounded  masses  into  whose  anterior 
surfaces  the  fibers  of  the  olfactory  nerve  enter.  Posterior  to  them  are  the 
enlarged  pear-shaped  cerebral  hemispheres.  Their  surfaces  are  much  convoluted 
in  the  cat,  consisting  of  folds,  the  gyri,  with  grooves,  the  sulci,  between  the  gyri. 
The  two  hemispheres  are  separated  from  each  other  by  a  deep  median  sagittal 
fissure,  the  longitudinal  cerebral  fissure  (which  is  on  the  larger  piece  of  the  brain). 
Gently  spread  open  the  fissure  and  note  at  its  bottom  a  thick,  white  mass  con- 
necting the  two  hemispheres.  This  is  the  corpus  callosum,  a  structure  character- 
istic of  the  mammalian  brain.  It  is  composed  of  nerve  fibers  passing  between 
the  hemispheres.  At  the  posterior  end  of  the  longitudinal  fissure  is  a  small  red- 
dish mass  of  folded  tissue,  which  is  part  of  the  chorioid  plexus  of  the  roof  of  the 
diencephalon.  The  diencephalon  or  region  of  the  brain  posterior  to  the  cerebral 
hemispheres  is  in  mammals  completely  concealed  from  dorsal  view  by  the  poste- 
rior extension  of  the  hemispheres  above  it.  The  posterior  ends  of  the  cerebral 
hemispheres  are  in  contact  with  the  cerebellum,  a  large  mass  with  a  much  con- 
voluted surface.  Between  the  cerebellum  and  the  cerebral  hemispheres  is  the 
midbrain,  also  concealed  from  dorsal  view  by  the  hemispheres.  It  is  readily 
revealed  by  bending  the  hemispheres  and  the  cerebellum  apart.  It  consists  of 
four  rounded  lobes  or  hillocks  known  as  the  corpora  quadrugemina  or  colliculi. 
The  two  anterior  ones  are  named  the  superior  colliculi,  the  two  posterior  ones 
the  inferior  colliculi.  The  cerebellum  consists  of  a  median  lobe,  the  vermis  or 
worm,  and  a  pair  of  lateral  lobes,  the  hemispheres.  The  vermis  corresponds  to 
the  entire  cerebellum  of  lower  vertebrates.  From  each  hemisphere  in  the  rabbit 
arises  by  a  narrow  stalk  another  lobe,  the  flocculus,  which  as  already  seen  is  left 
behind  in  the  floccular  fossa  of  the  petrous  bone  when  the  brain  is  removed  from 
the  skull.  Identify  on  the  hemispheres  the  cut  surfaces  where  the  flocculi  were 
attached.  Posterior  to  the  cerebellum  and  partly  overlapped  by  the  vermis  is 
the  medulla  oblongata.  Lift  the  vermis  and  note  the  cavity  of  the  fourth  ventricle 
in  the  medulla.  It  is  normally  roofed  over  by  a  membrane,  the  medullary  velum, 
which  contains  a  chorioid  plexus;  this  has  probably  been  removed  in  sectioning 
the  brain.  In  the  cat  the  chorioid  plexus  projects  on  each  side  between  cere- 
bellum and  medulla  as  a  little  tuft  of  vascular  tissue.  At  each  side  of  the 


354       LABORATORY  MANUAL  FOR  VERTEBRATE  ANATOMY 

posterior  pointed  end  of  the  fourth  ventricle  is  a  tract  terminating  in  a  club-shaped 
area,  the  dava.  Lateral  to  each  clava  is  another  elongated  area,  the  tuberculum 
cuneatum.  These  two  belong  to  the  somatic  sensory  column.  Anterior  to  them 
a  white  bundle  passes  toward  the  cerebellum,  disappearing  ventral  to  an  elevation 
which  lies  just  ventral  to  the  hemisphere  of  the  cerebellum.  The  bundle  is  the 
restiform  body  or  posterior  peduncle  of  the  cerebellum  which  conveys  impulses 
from  the  medulla  and  spinal  cord  to  the  cerebellum.  The  elevation  over  the 
restiform  body  is  the  area  acustica  or  primary  auditory  center.  The  general 
resemblance  of  these  structures  to  those  found  in  the  dogfish  should  be  evident. 

Draw  the  dorsal  view  of  the  brain. 

c)  The  ventral  aspect  of  the  brain:  Note  the  basilar  artery  (continuation  of 
the  two  vertebral  arteries)  running  in  the  midventral  line  and  forming  a  circle 
around  some  structures  in  the  center  of  the  ventral  surface.  This  circle,  the 
circle  of  Willis,  is  joined  on  each  side  by  the  internal  carotid  artery.  Note  the 
arteries  arising  from  the  basilar  and  circle  of  Willis  and  distributed  over  the  brain, 
coursing  in  the  pia  mater.  The  arteries  should  be  removed. 

At  the  anterior  end  of  the  ventral  surface  are  the  two  olfactory  bulbs.  From 
each  one  a  definite  white  tract,  the  olfactory  tract,  extends  obliquely  caudad  and 
terminates  posteriorly  in  a  lobe,  the  pyriform  lobe,  which  forms  the  posteroventral 
part  of  the  cerebral  hemispheres.  The  fissure  or  sulcus  which  separates  the  pyri- 
form lobe  from  the  rest  of  the  cerebral  hemisphere  is  called  the  rhinal  fissure. 
Inclosed  between  the  two  pyriform  lobes  is  the  ventral  side  of  the  diencephalon, 
or  thalamencephalon.  At  the  anterior  end  of  this  is  the  optic  chiasma  from  which 
the  optic  nerves  project.  The  region  between  the  optic  chiasma  and  the  olfactory 
tracts  is  called  the  anterior  perforated  substance.  Behind  the  optic  chiasma  is  a 
slight  rounded  elevation,  the  tuber  cinereum,  from  which  the  pituitary  body  or 
hypophysis  depends  by  a  stalk.  In  case  the  pituitary  body  was  torn  off  in 
removing  the  brain,  a  slitlike  aperture  will  be  noticed  in  the  center  of  the  tuber 
cinereum  marking  the  place  of  attachment  of  the  pituitary  body.  Immediately 
posterior  to  the  attachment  of  the  pituitary  body  is  an  area,  the  mammillary 
body,  not  distinctly  marked  off  from  the  tuber  cinereum.  Posterior  to  this  is  a 
depressed  area,  the  posterior  perforated  substance,  from  which  arise  the  two  third, 
or  oculomotor,  nerves.  From  beneath  (dorsal  to)  the  pyriform  lobes  a  thick  white 
bundle  will  be  seen  passing  obliquely  backward  on  each  side  of  the  posterior 
perforated  substance.  These  bundles  are  the  cerebral  peduncles,  belonging  to 
the  midbrain.  The  fourth  or  trochlear  nerves  arise  on  the  side  of  the  brain 
between  the  cerebellum  and  the  inferior  colliculi,  and  pass  ventrally  over  the 
outer  surface  of  the  peduncles. 

The  remainder  of  the  ventral  surface  of  the  brain  belongs  to  the  hindbrain, 
and  consists  of  the  pons  and  the  medulla  oblongata.  The  pons  is  the  heavy  band 
of  fibers  which  crosses  the  ventral  surface  of  the  hindbrain  immediately  behind 
the  posterior  perforated  substance.  By  following  it  around  to  the  sides  of  the 


COMPARATIVE  ANATOMY  OF  THE  NERVOUS  SYSTEM  355 

brain  it  will  be  seen  to  narrow  to  a  white  cord,  the  brachium  pontis  or  middle 
peduncle  of  the  cerebellum,  which  passes  into  the  substance  of  the  cerebellum. 
The  pons  is,  in  fact,  a  bridge  between  the  two  hemispheres  of  the  cerebellum. 
It  is  the  ventral  part  of  the  metencephalon  of  which  the  cerebellum  is  the  dorsal 
portion.  Immediately  posterior  to  the  brachium  pontis  and  partly  concealing 
it  is  the  thick  root  of  the  trigeminus  nerve.  On  close  examination  this  will  be 
seen  to  consist  of  a  large  dorsal  portion,  the  sensory  root  (porlio  major)  which 
consists  of  the  somatic  sensory  fibers  of  the  trigeminus,  and  a  very  small  ventral 
portion,  the  motor  root  (portio  minor)  which  contains  the  visceral  motor  fibers 
for  the  muscles  of  mastication  (masse ter,  temporal,  digastric,  etc.).  Posterior 
to  the  pons  and  of  about  half  its  width  is  another  bundle  of  transverse  fibers 
the  trapezoid  body.  Close  inspection  will  show  that  the  trapezoid  body  originates 
from  the  area  acustica  or  auditory  center;  it  passes  toward  the  median  line  but 
before  reaching  it,  turns  forward  and  disappears  dorsal  to  the  pons.  The  trape- 
zoid body  is  the  main  tract  which  carries  the  auditory  impulses  to  the  more 
anterior  portions  of  the  brain.  Attached  to  the  side  of  the  area  acustica  is  the 
root  of  the  eighth  or  auditory  nerve.  Just  ventral  to  this  and  behind  the  root 
of  the  trigeminus  is  the  root  of  the  facial  nerve  emerging  through  the  trapezoid 
body.  In  the  median  ventral  line  of  the  medulla  is  a  groove,  the  median  ventral 
fissure.  Along  each  side  of  this  runs  a  narrow  bundle  of  fibers;  each  emerges 
dorsal  to  the  posterior  margin  of  the  pons  and  proceeds  straight  posteriorly. 
These  two  tracts  are  the  pyramids  or  somatic  motor  tracts;  they  convey  impulses 
from  the  cerebral  hemispheres  to  the  voluntary  muscles.  At  the  place  where 
the  pyramids  emerge  from  above  the  pons  are  the  roots  of  the  sixth  or  abducens 
nerves.  The  small  root  of  the  ninth  or  glossopharyngeal  nerve  will  be  found  at 
the  posterior  boundary  of  the  acustic  area,  at  the  point  where  the  restiform  body 
passes  dorsal  to  it  and  about  on  a  line  with  the  root  of  the  eighth  nerve.  The 
equally  small  root  of  the  tenth  or  vagus  nerve  lies  immediately  posterior  to  and 
on  a  line  with  the  root  of  the  ninth  nerve.  Posterior  to  the  vagus  are  the  numer- 
ous roots  of  the  eleventh  or  spinal  accessory  nerve,  arising  in  a  line.  The  main 
root  of  the  accessory  ascends  from  the  spinal  cord,  but  is  probably  missing  in 
the  specimen.  The  roots  of  the  twelfth  or  hypoglossal  nerve  emerge  along  the 
lateral  border  of  the  pyramid,  posterior  to  the  preceding  roots. 

Draw  the  ventral  view  of  the  brain,  including  the  roots  of  the  cranial  nerves 
as  far  as  you  have  seen  them. 

d)  The  median  sagittal  section:  Now  cut  the  larger  half  of  the  brain  along 
the  longitudinal  cerebral  fissure  so  as  to  obtain  an  exact  median  sagittal  section. 
In  making  such  a  cut  use  a  dull  knife  and  pass  it  through  the  brain  with  one 
sliding  stroke.  Examine  the  cut  surface.  The  cerebral  hemisphere  forms  a 
thick  roof  which  arches  posteriorly  above  the  diencephalon  and  midbrain.  In 
the  cerebral  hemisphere  identify  the  section  of  the  corpus  callosum.  This  is  an 
obliquely  placed  longitudinal  band  of  white  material.  Both  anterior  and  posterior 


356       LABORATORY  MANUAL  FOR  VERTEBRATE  ANATOMY 

ends  are  enlarged,  the  former  being  named  the  genu,  the  latter  the  splenium. 
From  about  the  middle  of  the  corpus  callosum  a  band  of  fibers,  the  fornix,  curves 
ventrally.  Between  the  fornix  and  the  anterior  half  of  the  corpus  callosum 
stretches  a  thin  membrane,  the  septum  pellucidum,  consisting  of  two  leaves.  If 
the  brain  is  sectioned  exactly  in  the  median  sagittal  plane,  the  section  will  pass 
between  the  two  leaves  of  the  septum  pellucidum;  but  often  the  whole  septum  is 
left  on  one  half;  in  this  case  a  slitlike  opening  into  a  cavity,  the  lateral  ventricle, 
will  appear  on  the  other  half  between  the  fornix  and  the  corpus  callosum.  The 
fornix  passes  downward  and  soon  turns  (as  the  column  of  the  fornix)  into 
the  interior  of  the  brain  where  it  is  lost  to  view.  Immediately  in  front  of  the  point 
where  it  disappears  is  the  section  of  a  small  round  bundle,  the  anterior  commissure. 
From  the  anterior  commissure  a  delicate  membrane,  the  lamina  terminalis, 
extends  ventrally  to  the  optic  chiasma.  The  fornix,  the  anterior  commissure, 
and  the  lamina  terminalis  form  the  anterior  boundary  of  a  deep  but  narrow 
chamber,  the  third  ventricle)  which  lies  in  the  middle  of  the  diencephalon.  The 
cavity  of  the  third  ventricle  extends  ventrally  into  the  tuber  cinereum  and  the 
pituitary  body. 

The  diencephalon  is  the  massive  region  extending  between  the  fornix  and 
lamina  terminalis  and  midbrain.  It  consists  of  three  parts :  a  dorsal  region,  the 
epithalamus;  a  central  and  lateral  region,  the  thalamus  \  and  a  ventral  region, 
the  hypothalamus.  The  hypothalamus  includes  the  optic  chiasma,  the  tuber 
cinereum,  the  mammillary  body,  and  the  hypophysis  or  pituitary  body,  all  of 
which  should  be  identified  in  the  section.  The  epithalamus  includes  the  struc- 
tures in  the  roof  of  the  diencephalon.  These  are:  the  chorioid  plexus,  a  thin 
folded  vascular  membrane  between  the  cerebral  hemisphere  and  the  diencephalon; 
the  pineal  body,  a  stalked  body  lying  in  the  chorioid  plexus;  the  habenula,  a 
small  mass  just  in  front  of  the  attachment  of  the  pineal  body  to  the  diencephalon ; 
and  the  posterior  commissure,  a  small  circular  area  just  posterior  to  the  habenula. 
The  thalamus  constitutes  the  greater  part  of  the  diencephalon.  On  the  cut 
surface  it  presents  a  large,  round  mass,  the  intermediate  mass  or  middle  commis- 
sure; this  is  not  really  a  commissure  but  merely  the  cut  median  mass  of  the 
thalamus.  The  greater  part  of  the  thalamus  is  concealed  by  the  overhanging 
cerebral  hemisphere.  On  the  smaller  piece  of  the  brain  remove  the  cerebral 
hemisphere  and  then  examine  the  dorsal  and  lateral  regions  of  the  thalamus. 
Three  low  elevations  are  present.  The  most  dorsal  and  medial  one  is  the  puhinar. 
Lateral  to  this  and  whiter  in  color  is  the  lateral  geniculate  body.  A  white  band, 
the  optic  tract,  is  plainly  seen  ascending  from  the  optic  chiasma  and  terminating 
on  the  lateral  geniculate  body.  Posterior  and  ventral  to  the  lateral  geniculate 
body  is  a  smaller  swelling,  the  medial  geniculate  body.  Behind  the  geniculate 
bodies  will  be  recognized  the  corpora  quadrugemina  as  two  low  hillocks.  Ventral 
to  them  runs  the  stout  cerebral  peduncle,  the  anterior  part  of  which  is  crossed 
externally  by  the  optic  tract. 


COMPARATIVE  ANATOMY  OF  THE  NERVOUS  SYSTEM  357 

Returning  to  the  medial  sagittal  section,  identify  in  the  roof  of  the  midbrain 
the  two  hillocks  formed  by  the  superior  and  inferior  colliculi  or  corpora  quad- 
rugemina.  Below  them  is  a  narrow  passage,  the  aqueduct  of  the  brain,  which 
connects  the  third  ventricle  in  the  diencephalon  with  the  fourth  ventricle  in  the 
medulla.  Below  the  aqueduct  is  the  thick  floor  of  the  midbrain,  the  tegmentum. 
At  the  sides  of  this  are  the  cerebral  peduncles,  not  exposed  in  the  section.  In 
the  section  of  the  cerebellum  note  the  curious  branching  treelike  arrangement 
of  the  white  matter,  forming  the  arbor  vitae  or  tree  of  life.  This  appearance  is 
brought  about  by  the  fact  that  each  fold  of  the  cerebellar  surface  consists  of  gray 
matter  or  nerve  cells,  with  a  central  plate  of  white  matter  or  nerve  fibers.  The 
cerebellum  fits  into  the  fourth  ventricle  from  which,  however,  it  is  separated  in 
the  normal  condition  by  a  membrane,  the  medullary  velum.  Part  of  this  velum 
will  probably  be  found  below  the  anterior  part  of  the  cerebellum.  Identify 
in  the  section  the  mass  formed  by  the  pons.  The  section  of  the  medulla  has 
nothing  of  additional  interest. 

Draw  the  sagittal  section. 

e)  Further  structure  of  the  cerebral  hemispheres:  On  the  intact  half  of  the  brain 
begin  to  cut  away  the  roof  of  the  cerebral  hemisphere  in  thin  slices.  Note  that 
the  superficial  substance  of  the  roof  is  gray,  the  interior  white,  a  reversal  of  the 
condition  found  in  lower  vertebrates.  This  gray  outer  coat  of  the  mammalian 
brain  is  called  the  cortex;  it  is  gray  because  it  consists  of  nerve  cells  which  have 
migrated  to  the  surface  from  their  original,  more  central  position.  The  white 
matter  consists  of  fibers  which  carry  impulses  to  and  from  the  cortex.  In  the 
cat  the  cortex  is  much  convoluted.  Note  that  each  convolution  has  a  central 
core  of  white  matter  and  a  peripheral  thick  coat  of  gray  matter.  Continue  to 
shave  the  brain  ventrally  until  the  corpus  callosum  is  exposed.  This  is  a  narrow 
band  of  fibers  conveying  impulses  between  the  two  hemispheres.  Remove  the 
corpus  callosum  and  the  cortex  lateral  to  it.  This  exposes  the  cavity  of  the  cere- 
bral hemisphere,  called  the  lateral  ventricle.  It  is  filled  by  two  conspicuous 
elevations.  The  anterior  and  smaller  one,  of  a  darker  color,  is  the  corpus  striatum, 
a  mass  of  gray  matter.  The  posterior,  larger  one  is  the  hippocampus.  Remove 
the  side  of  the  hemisphere  so  as  to  expose  the  hippocampus.  It  is  a  curved  body 
with  an  anterior  free  margin,  the  fimbria.  Cut  through  the  medial  attachment 
of  the  hippocampus,  raise  the  anterior  border,  and  roll  the  hippocampus  back. 
Observe  that  the  part  of  the  hippocampus  still  attached  is  continuous  with  the 
pyriform  lobe.  The  hippocampus  is  a  part  of  the  original  external  surface  of 
the  brain,  which  has  been  invaginated  into  the  interior.  The  turning  back  of 
the  hippocampus  reveals  the  thalamus  Note  the  thick  stalk  extending  from  the 
thalamus  into  the  cerebrum,  immediately  in  front  of  the  pulvinar  and  lateral 
geniculate  body.  Scrape  the  surface  of  this  and  note  that  it  consists  of  a  great 
mass  of  fibers  radiating  from  the  thalamus  into  the  cerebral  hemisphere.  This  is 


358,      LABORATORY  MANUAL  FOR  VERTEBRATE  ANATOMY 

more  evident  in  the  rabbit  than  in  the  cat,  since  in  the  cat  the  fibers  turn  dorsally. 
This  radiating  mass  is  called  the  corona  radiata. 

f)  Functions  of  the  parts  of  the  brain:  As  an  aid  in  the  understanding  of  the  anatomy  of 
the  brain,  a  few  statements  may  be  made  concerning  the  functions  of  the  parts  identified  in 
the  preceding  sections.  The  olfactory  bulbs,  the  olfactory  tracts,  the  pyriform  lobe,  the  tuber 
cinereum,  the  fornix,  the  habenulae,  the  mammillary  body,  and  the  hippocampus  belong  to 
the  olfactory  apparatus  of  the  mammal.  The  olfactory  impulses  come  along  the  olfactory 
nerves  into  the  olfactory  bulbs,  which  are  the  primary  olfactory  centers;  they  are  then  relayed 
along  the  olfactory  tracts  to  the  pyriform  lobe,  which  is  the  secondary  olfactory  center; 
from  the  pyriform  lobe  they  pass  to  the  hippocampus,  the  tertiary  olfactory  center,  or  con- 
scious center  of  smell.  The  hippocampus  has  extensive  connections  with  other  parts  of  the 
brain  for  reflex  purposes.  These  connections  occur  by  way  of  the  fimbria,  a  mass  of  nerve 
fibers.  The  fimbria  connects  with  the  fornix  and  this  in  turn  with  the  mammillary  body. 
The  habenula  is  also  connected  with  the  hippocampus.  In  the  dogfish  practically  the  whole 
of  the  telencephalon  is  concerned  with  smell,  while  here  the  olfactory  functions  occupy  but 
a  part  of  the  telencephalon,  the  remainder  having  developed  new  connections  and  functions. 
The  diencephalon  is  the  great  center  of  correlations  in  the  mammalian  brain.  Its  relation 
to  the  olfactory  sense  has  already  been  noted.  The  lateral  geniculate  body  is  the  primary 
optic  center,  in  which,  as  we  saw,  the  optic  tracts  terminate  in  part.  From  the  lateral 
geniculate  body  the  optic  impulses  pass  to  the  cerebral  hemisphere  by  way  of  the  corona 
radiata.  The  pulvinar  and  the  superior  colliculus  of  the  midbrain  are  also  concerned  in 
optic  impulses,  the  latter  being  a  reflex  center  for  these  impulses.  The  primary  auditory 
center  is  located  in  the  area  acustica;  from  here  the  auditory  impulses  are  carried,  in  part  by 
the  trapezoid  body,  to  the  inferior  colliculus  and  the  medial  geniculate  body,  which  consti- 
tute secondary  and  tertiary  auditory  centers.  From  the  medial  geniculate  body  the  auditory 
impulses  are  carried  in  the  corona  radiata  to  the  cerebral  cortex.  In  a  similar  way  other 
sensations  such  as  pain,  touch,  temperature,  pressure,  consciousness  of  muscle  and  joint 
movements,  and  position  of  the  body  in  space  are  carried  by  definite  paths  (which  are  for 
the  most  part  invisible  externally  on  the  brain)  to  the  thalamus  from  which  they  are  re- 
layed to  the  cerebral  cortex.  (The  clava  and  tuberculum  cuneatum  are  concerned  with  joint 
and  muscle  sense  and  steadiness  of  body  movement  and  position.)  It  will  thus  be  seen  that 
practically  all  sensations  make  a  relay  in  the  diencephalon  from  which  they  ascend  by  a  new 
path,  the  corona  radiata,  to  the  cerebral  cortex.  The  corona  radiata  is  thus  the  great 
pathway  between  the  diencephalon  and  the  cerebral  cortex  by  means  of  which  the  sensa- 
tions are  projected,  as  it  were,  upon  the  cortex.  It  is  further  well  known  on  which  area  of 
the  cerebral  cortex  each  sensation  is  projected.  After  all  of  the  sensations  have  thus  been 
localized  upon  the  cortex,  there  still  remains  a  considerable  portion  of  the  cortex  to  which  no 
definite  tracts  from  below  can  be  traced.  It  is  presumed  that  these  areas  are  concerned  with 
co-ordination,  reason,  emotion,  etc. 

The  impulses  toward  voluntary  movements  originate  in  a  definite  part  of  the  cortex, 
pass  downward  in  the  corona  radiata  into  the  cerebral  peduncles,  and  appear  on  the  ventral 
surface  of  the  medulla  as  the  pyramidal  tracts  or  pyramids,  which  descend  the  whole  length 
of  the  spinal  cord  and  make  connections  with  the  motor  cells  of  the  ventral  columns  of  the 
cord.  The  cerebral  peduncles,  besides  carrying  the  pyramidal  tracts,  also  carry  large  tracts 
from  the  cortex  to  the  pons,  where  they  pass  into  the  cerebellum. 

The  cerebellum  is  the  great  center  for  equilibration  and  motor  co-ordination.  Its 
functions  are  involuntary  and  unperceived  by  the  conscious  mind.  It  is  connected  with 
the  rest  of  the  brain  by  means  of  its  peduncles:  the  posterior  peduncles  or  restiform  bodies, 


COMPARATIVE  ANATOMY  OF  THE  NERVOUS  SYSTEM  359 

which  connect  it  with  the  medulla  and  spinal  cord;  the  brachium  pontis  or  middle  peduncle, 
which  joins  the  two  sides  of  the  cerebellum  and  also  conveys  tracts  between  the  cerebellum 
and  cerebral  cortex;  and  the  anterior  peduncles  (which  were  not  seen  in  the  dissection) 
which  extend  between  the  cerebellum  and  midbrain  and  thalamus.  The  cerebellum  has 
extensive  connections  with  the  area  acustica,  since  the  impulses  from  the  ampullae  of  the 
semicircular  ducts,  which  are  concerned  with  equilibration,  terminate  in  the  area  acustica. 

G.      SUMMARY   OF  THE  NERVOUS   SYSTEM  AND  THE   SENSE  ORGANS 

1.  The  nervous  system  and  the  nervous  parts  of  the  sense  organs  are  derived  from  the 
ectoderm. 

2.  The  nervous  system  is  subdivided  into  central,  peripheral,  and  sympathetic  nervous 
systems.    The  first  includes  the  brain  and  spinal  cord,  the  second  the  cranial  and  spinal 
nerves,  and  the  third  the  sympathetic  cords,  ganglia,  and  nerves. 

3.  The  spinal  cord  consists  of  a  central  gray  region  and  a  peripheral   white   region 
subdivided  into  tracts.    The  spinal  nerves  arise  in  pairs  from  the  cord  at  segmental  intervals. 

4.  Each  spinal  nerve  arises  from  the  cord  by  two  roots,  a  dorsal  and  a  ventral.    The 
dorsal  root  is  sensory  and  bears  a  ganglion  composed  of  sensory  cells;   the  ventral  root  is 
motor.     The  two  roots  unite  to  form  a  spinal  nerve. 

5.  The  spinal  nerve  very  soon  divides  into  a  dorsal  ramus  which  passes  to  the  epaxial 
muscles  and  adjacent  skin,  a  ventral  ramus  which  passes  to  the  hypaxial  muscles  and 
adjacent  skin,  and  a  communicating  ramus  which  connects  with  -the  sympathetic  system. 

6.  The  ventral  rami  of  the  spinal  nerves  are  intricately  united  by  cross-connections  in 
the  region  of  the  appendages  to  form  plexi  from  which  the  motor  nerves  to  the  muscles  of  the 
appendages  arise.     The  chief  plexi  are  the  brachial  plexus  to  the  anterior  appendages  and 
the  lumbosacral  plexus  to  the  posterior  appendages 

7.  The  sympathetic  system  consists  chiefly  of  a  paired  ganglionated  cord  in  the  mid- 
dorsal  region  of  the  body  cavity.     These  are  connected  with  the  spinal  and  cranial  nerves 
by  communicating  branches  and  with  the  viscera,  glands,  blood  vessels,  etc.,  by  means  of 
branches,  networks  or  plexi,  and  ganglia. 

8.  The  vertebrate  brain  consists  at  first  of  three  vesicles,  the  forebrain  or  prosencephalon, 
the  midbrain  or  mesencephalon,  and  the  hindbrain  or  rhombencephalon.     The  first  and  third 
soon  divide  into  two  vesicles,  making  five  in  all. 

9.  The  five  vesicles  of  the  adult  vertebrate  brain  are  arranged  in  a  longitudinal  series 
and  are  named,  beginning  anteriorly:    the  telencephalon,  the  diencephalon,  the  mesencepha- 
lon, the  metencephalon,  and  the  myelencephalon. 

10.  The  telencephalon  differentiates  into  the  olfactory  bulbs,  the  olfactory  tracts, 
olfactory  lobes,  and  cerebral  hemispheres.     The  hemispheres  are  lateral  expansions  of  the 
telencephalon.     The  olfactory  part  of  the  telencephalon  is  of  paramount  importance  in  the 
lower  vertebrates,  but  later  becomes  subordinated  to  the  cerebral  hemispheres.     These 
latter  increase  in  size  in  the  vertebrate  scale  until  in  mammals  they  cover  most  of  the  remain- 
ing parts  of  the  brain.     In  mammals  the  two  hemispheres  are  connected  by  the  corpus 
callosum. 

n.  The  diencephalon  differentiates  into  the  hypothalamus,  the  thalamus,  and  the 
epithalamus.  _The  hypothalamus  includes  the  optic  chiasma,  optic  nerves,  and  nervous 
parts  of  the  eye  (these  are  often  regarded,  however,  as  belonging  to  the  telencephalon),  the 
infundibulum,  and  the  mammillary  body.  The  infundibulum  is  an  evagination  from  the 
floor  of  the  diencephalon;  its  most  ventral  portion  unites  with  an  outgrowth  from  the  roof 
of  the  mouth,  the  two  together  forming  the  pituitary  body.  The  thalamus  includes  the 


360       LABORATORY  MANUAL  FOR  VERTEBRATE  ANATOMY 

greater  central  mass  of  the  diencephalon  and  in  mammals  is  subdivided  into  several  parts. 
The  epithalamus  consists  of  the  pineal  body  and  other  evaginations  from  the  roof  of  the 
diencephalon,  and  adjacent  parts. 

12.  The  mesencephalon  or  midbrain  is  composed  dorsally  of  the  optic  lobes,  of  which 
there  are  two  in  most  vertebrates,  four  in  mammals,  named  the  corpora  quadrugemina. 
Ventrally  the  midbrain  consists  of  the  tegmentum  and  the  cerebral  peduncles. 

13.  The  metencephalon  includes  the  cerebellum  dorsally  and  the  pons  ventrally.    The 
pons  is  distinctly  developed  only  in  mammals. 

14.  The  myelencephalon  becomes  the  medulla  oblongata. 

15.  The  brain  is  hollow,  its  cavities  being  known  as  ventricles.    The  first  two  ventricles 
are  in  the  cerebral  hemispheres,  the  third  ventricle  in  the  diencephalon,  the  fourth  in  the 
medulla.     In  lower  vertebrates  ventricles  are  also  present  in  the  cerebellum  and  midbrain. 
The  ventricles  are  connected  with  each  other. 

1 6.  The  brain  of  fishes  and  Amphibia  is  provided  with  ten  cranial  nerves,  that  of 
amniotes  with  twelve.     The  last  two  in  amniotes  are  probably  not  new  formations. 

17.  The  first  or  olfactory  nerve  extends  from  the  olfactory  epithelium  in  the  nose  to 
the  olfactory  bulbs. 

18.  The  second  or  optic  nerve  extends  from  the  retina  to  the  diencephalon.     It  is  not 
a  true  nerve  but  a  tract  of  the  brain. 

19.  The  third  or  oculomotor  nerve  is  a  motor  nerve  to  the  inferior  oblique,  superior, 
inferior,  and  internal  recti  and  some,  accessory  muscles  of  the  eyeball.     It  originates  in  the 
midbrain. 

20.  The  fourth  or  trochlear  nerve  is  a  motor  nerve  to  the  superior  oblique  muscle  of 
the  eyeball.     It  arises  from  the  midbrain. 

21.  The  fifth  or  trigeminus  nerve  is  the  chief  somatic  sensory  nerve  of  the  head  and  the 
nerve  of  the  first  visceral  or  mandibular  arch.     It  has  in  all  vertebrates  three  branches: 
the  ophthalmic  (deep  ophthalmic  of  fishes)  branch  to  the  orbit  and  nasal  region;  the  maxillary 
to  the  upper  jaws  and  roof  of  the  mouth  and  pharyngeal  cavities;  the  mandibular  to  the  lower 
jaw  and  floor  of  these  cavities.    The  first  two  are  pure  somatic  sensory  nerves;   the  man- 
dibular also  contains  visceral  motor  fibers  to  the  visceral  muscles  belonging  to  the  lower  jaw. 
In  fishes  there  is  an  additional  branch  of  the  trigeminus,  the  superficial  ophthalmic,  which 
disappears  later.    The  trigeminus  arises  from  the  anterior  end  of  the  medulla. 

22.  The  sixth  or  abducens  nerve  is  the  motor  nerve  of  the  external  rectus  muscle  of  the 
eyeball.    It  takes  origin  in  the  floor  of  the  medulla. 

23.  The  seventh  or  facial  nerve  is  the  nerve  of  the  second  or  hyoid  visceral  arch.     It  is 
large  in  fishes  because  it  includes  branches  for  the  lateral  line  system.     These  disappear  in 
land  vertebrates,  leaving  only  that  part  of  the  facial  which  is  named  the  hyomandibular 
nerve  in  fishes.    This  nerve  is  a  sensory  nerve  (taste)  to  the  pharyngeal  cavity  and  a  visceral 
motor  nerve  to  the  muscles  of  the  hyoid  arch.    The  facial  springs  from  the  medulla. 

24.  The  eighth  or  auditory  nerve  is  a  sensory  nerve  extending  from  the  internal  ear  to 
the  acustic  area  of  the  medulla. 

25.  The  ninth  or  glossopharyngeal  nerve  is  the  nerve  of  the  third  visceral  arch.     In 
the  land  vertebrates  it  is  reduced  but  continues  to  supply  sensory  fibers  to  the  floor  of  the 
pharyngeal  cavity  and  visceral  motor  fibers  to  pharyngeal  muscles.     It  is  attached  to  the 
medulla. 

26.  The  tenth  or  vagus  nerve  is  the  nerve  of  the  remaining  visceral  arches.     In  fishes 
it  also  includes  a  large  branch  for  the  lateral  line.    With  the  loss  of  the  lateral  line  system  and 
the  branchial  mode  of  breathing  the  vagus  is  much  reduced  but  continues  to  supply  the 
corresponding  region  of  the  pharynx,  being  a  sensory  nerve  to  the  lining  of  this  region  and  a 


COMPARATIVE  ANATOMY  OF  THE  NERVOUS  SYSTEM  361 

visceral  motor  nerve  to  the  corresponding  muscles.  In  addition,  the  vagus  nerve  is  dis- 
tributed extensively  to  the  heart,  lungs,  stomach,  and  other  viscera.  The  vagus  arises  from 
the  medulla. 

27.  The  eleventh  or  spinal  accessory  nerve  of  amniotes  is  probably  a  part  of  the  vagus 
isolated  as  a  separate  nerve.     It  springs  from  the  medulla  and  upper  spinal  cord  and  is  a 
motor  nerve  to  certain  visceral  muscles. 

28.  The  twelfth  or  hypoglossal  nerve  of  amniotes  is  probably  a  spinal  nerve  which  has 
become  included  in  the  cranial  cavity.     It  arises  in  the  medulla  and  innervates  the  muscles 
of  the  tongue. 

29.  The  chief  sense  organs  of  the  head  are  the  eyes,  ears,  and  nose. 

30.  The  nose  at  first  consists  of  a  pair  of  olfactory  sacs  not  communicating  with  the 
mo.uth  cavity.    In  land  vertebrates  they  establish  connections  with  the  mouth  cavity  for 
respiratory  purposes.     The  nasal  cavities  thereafter  have  both  respiratory  and  olfactory 
functions,  the  latter  being  limited  to  the  posterior  regions  of  the  cavities. 

31.  In  higher  vertebrates,  particularly  mammals,  the  walls  of  the  nasal  cavities  develop 
complex  outgrowths,  the  turbinals  or  conchae,  for  the  purpose  of  increasing  both  respiratory 
and  olfactory  surfaces. 

32.  The  eyes  are  compound  structures.     The  nervous  part  of  the  eye  is  formed  by  an 
evagination  from  the  brain.    The  lens  of  the  eye  is  an  invagination  from  the  adjacent 
ectoderm.    The  coats  of  the  eye,  sclera,  and  chorioid,  are  formed  in  the  surrounding  mesen- 
chyme.    The  eye  is  moved  by  muscles  which  are  very  constant   in  arrangement  in  the 
different  vertebrate  classes,  except  that  in  mammals  the  superior  oblique  operates  by  means 
of  a  pulley. 

33.  The  ear  consists  of  internal,  middle,  and  external  portions.    The  internal  ear  is  an 
invagination  from  the  ectoderm.     It  differentiates  into  the  three  semicircular  ducts,  the 
sacculus,  the  utriculus,  and  the  endolymphatic  duct.     Fishes  and  many  urodeles  possess 
only  the  internal  ear. 

34.  The  middle  ear  is  added  to  the  internal  ear  beginning  with  Amphibia.    It  consists 
of  a  chamber  developed  by  outgrowth  from  the  first  visceral  pouch.    The  outer  wall  of  this 
chamber  comes  in  contact  with  the  skin,  producing  a  double-walled  membrane,  the  tympanic 
membrane.     Within  the  middle  ear  is  a  chain  of  ear  bones,  two  in  number  in  most  vertebrates, 
three  in  mammals. 

35.  Beginning  with  reptiles  the  tympanic  membrane  sinks  into  the  skull,  leaving  a 
passage,  the  external  auditory  meatus,  extending  from  the  tympanic  membrane  to  the 
exterior.     This  passage  is  deepened  in  birds  and  mammals,  and  in  the  latter  a  fold  of  skin, 
the  pinna,  develops  around  the  external  rim  of  the  meatus.     Pinna  and  meatus  constitute 
the  external  ear. 

36.  The  internal  ear  of  mammals  is  more  complicated  than  that  of  other  vertebrates, 
owing  to  the  development  of  a  spiral  outgrowth,  the  cochlear  duct,  from  the  sacculus. 


APPENDIX  A 


PRONUNCIATION  AND  DERIVATION  OF  TECHNICAL  WORDS 

Some  compound  words  and  names  combined  of  two  or  more  words  are  given  under  their 
components.     Some  common  prefixes  are  also  given.     (L,  Latin;   G,  Greek;  F,  French.) 


A  (G  prefix,  without) 

Ab  (L  prefix,  away  from) 

Abdomen,  abdominal — ab  dough'  men,  ab  dom'  i 

nal  (L,  of  uncertain  origin) 

Abducens — ab  due'  senz  (L,  ab,  from;  duco,  lead) 
Acanthias — a  kan'  the  as  (G,  acantha,  thorn) 
Acentrous — a  sen'  trous  (G,  a,  without;  kentron, 

center) 
Acetabulum — ass  i  tab'  yu  lum  (L,  name  of  a  kind 

of  cup) 

Acipenser — ass  i  pen'  ser  (L,  a  sturgeon) 
Acromion — a  krow'  me  on  (G,  akros,  top;  omos, 

shoulder) 
Actinopterygii — ak'  ti  nop'  ter  yg'ee  eye  (G,  ak- 

tinos,  ray;  pterygion,  whig,  fin) 
Acustico — a  kous'  ti  koh'  (G,  akoustikos,  related 

to  hearing) 

Ad — (L  prefix,  toward,  upon) 
Adductor — a  duck'  tore  (L,  ad,  to;  duco,  lead) 
Adrenal — add  ree'  nal  (L,  ad,  upon;   renes,  kid- 
neys) 

Afferent — aff  err  ent  (L,  ad,  to;  fero,  bear) 
All — al'  ee  (L  prefix,  ala,  wing) 
Allantois — a  Ian'  toe  iss  (G,  alias,  sausage;  eidos, 

form) 
Alveolar,  alveolus,  alveoli — al  vee'  oh  lar,  -lus,  -lie 

(L,  a  little  cavity) 

Amia — ami'  ee  ah  (G,  a  kind  of  fish) 
Amnion,  amniota,  amniote — am'  nee  on,  am'   nee 

oh'  tah,  am'  nee  oat  (G,  a  membrane  of  the 

embryo) 
Amphibia — am  fib'  ee  ah  (G,  amphi,  double;  bios, 

life) 
Amphicoelous — am'  fee  see'  lous  (G,  amphi,  double; 

koilos,  hollow) 
Amphioxus — am'  fee  ox'  us  (G,  amphi,  both;  oxys, 

sharp) 
Amphiplatyan — am'  fee  pla'  tee  an  (G,  amphi, 

both;  platys,  flat) 
Ampulla,  ampullae — am  pull'  ah,  am  pull'  ee  (L, 

flask) 
Anamnia — an  am'  nee  ah  (G,  an,  without;  amnion, 

embryonic  membrane) 

Anastomosis — a  nass'  toh  mow'  sis  (G,  an  opening) 
Anconeus — an'  ko  nee'  us  (L,  ancon,  the  bend  of 

the  arm) 
Ankylosis — an'  kee  lo'  sis  (G,  ankylos,  bent) 


Ansa — ann'  sah  (L,  a  handle) 

Anura — a  new'  rah  (G,  an,  without;  our  a,  tail) 

Anus — ay'  nus  (L,  ring) 

Aorta,  aortic — ay  or'  tah,  -tik  (G,  aorte,  to  lift) 

Aponeurosis — ap  oh'  new  row'  sis  (G,  apo,  from; 

neuron,  tendon) 

Apophysis — a  poff'  y  sis  (G,  an  outgrowth) 
Aqueduct — ak'  we   duct  (L,  aqua,   water;    duco, 

lead) 

Aqueous — ay'  kwee  us  (L,  aqua,  water) 
Arachnoid — a  rack'  noid  (G,  arachne,  spider) 
Archenteron — ar  ken'  ter    on   (G,  archos,  chief, 

first;  enter  on,  intestine) 
Arcualia — ar'  kiu  ale'  ee  ah  (L,  arcus,  bow) 
Arteriosus — ar  teer'  ee  oh'  sus  (L,  arteria,  artery) 
Arytenoid — ar'  ee  tee'  noid  (G,  arytaina,  pitcher, 

funnel) 

Astragalus — ass  trag'  a  lus  (G,  an  ankle  bone) 
Atlas— at'  lass  (G,  tlao,  to  bear) 
Atrium — ay'  tree  um  (L,  a  court) 
Auditory-yaw'  di  toe  ry  (L,  auditorius,  pertaining 

to  hearing) 
Auricle,  auricular — aw'  ree  kal,  aw  rik'  yu  lar  (L, 

auricula,  a  little  ear) 
Aves — ay'  veez  (L,  birds) 
Axilla,  axillary — axe  ill'  ah,  axe'  i  lay  ree  (L,  axilla, 

a  little  axis) 
Azygos — az'  ee  gos  (G,  a,  without;  zygon,  yoke) 

Balanoglossus — bahl'  an  oh  gloss'  us  (G,  balanos, 

acorn;  glossa,  tongue) 
Basalia — bah  sail'  ee  ah  (L,  basis,  the  base) 
Basi — base'  ee  (L,  prefix,  at  the  base  of) 
Basilar — bass'  ee  lar  (L,  basis,  the  base) 
Biceps — buy'  seps  (L,  bi,  two;  caput,  head) 
Blastocoel — bias'  toe  seal  (G,  Uastos,  germ;  koil- 

los,  hollow) 
Blastoderm — bias'  toe  derm  (G,  Uastos,   germ; 

derma,  skin) 

Blastula — bias'  tiu  lah  (L,  a  little  germ) 
Brachial,  brachialis,  brachium — bray' kee  al,-kee 

ay'  lis,  -kee  um  (L,  brachium,  arm) 
Brachiocephalic — bray'  kee  oh  se  phal'  ik  (L,  bra- 
chium, arm;  cephalicus,  pertaining  to  the  head) 
Branchia,  branchiae,  branchial — bran'  kee  ah,  -kee 
ee,  -kee  al  (G,  branchia,  gills) 


362 


APPENDIX  A 


363 


Branchiostegal — bran'  kee  oss'  te  gal  (G,  branchia, 

gills;  stego,  to  cover) 
Bronchus,   bronchi,   bronchial — bron'   kus,   bron' 

kai,  bron'  kee  al  (G,  bronchos,  windpipe) 
Buccal — buck'  al  (L,  bucca,  cheek  or  mouth) 
Buccinator — buck'  si  nay'  tor  (L,  pertaining  to 

blowing  a  trumpet) 

Bulla — bull'  ah  (L,  a  round  seal  or  locket) 
Bursa — burr'  sah  (L,  a  purse) 

Caecum,  caeca — see'  kum,  see'    kah  (L,  caecus, 

blind) 

Calcaneus,  kal  kay'  nee  us  (L,  calx,  heel) 
Canine — kay  nine'  (L,  cants,  dog) 
Capillary — kap'  ee  lay'  ry  (L,  capUlus,  hair) 
Capitular,  capitulum — ka  pit'  yu  lar,  -yu  lum,  (L, 

a  small  head) 
Carapace — kair'  a  pace  (F,  probably  from  L,  capa, 

hood) 
Cardia,  cardiac — kar'  dee  ah,  -dee  ak  (G,  kardia, 

heart) 

Carina — ka  rye'  nah  (L,  keel) 
Carnivora,  carnivore — kar  niv'  oh  rah,  kar'  ni  vore 

(L,  caro,  flesh;  voro,  devour) 
Carotid — ka  rot'  id  (G,  karos,  stupor) 
Carpales,  carpus — kar  pay'  les,  kar'  pus  (G,  kar- 

pos,  wrist) 

Caudal — kaw'  dal  (L,  cauda,  tail) 
Cava,  caval — kave'  ah,  -al  (L,  cavus,  hollow) 
Cavernosa,  cavernous — kav'  er  no'  sah,  -nous  (L, 

caverna,  hollow) 
Centrale,  centralia — sen  tray'  lee,  -tray'  lee  ah  (L, 

centralis,  central) 
Centrum — sen'  trum  (L,  center) 
Cephalic— se  phal'  ik  (G,  kephale,  head) 
Cephalization— seph'    al  i  za'    tion  (G,  kephale, 

head) 

Cephalochordata — seph'  a  low  chor  day'  tah  (G, 

kephale,  head;  chorde,  string) 
Cerato — ser'  a  toe  (G,  prefix,  keras,  horn) 
Cere — sear  (L,  cera,  wax) 
Cerebellum — ser'  e  bell'  um  (L,  a  little  Drain) 
Cerebral,  cerebrum — ser'  e  bral,  ser'  e  brum  (L, 

brain) 

Cervical — ser'  vi  kal  (L,  cervix,  neck) 
Chelonia — kee  low'  nee  ah  (G,  chelone,  turtle) 
Chiasma — kai  as'  ma  (G,  cross-mark) 
Choana,  choanae — koh'  a  nah,  -a  nee  (G,  funnel) 
Chondrocranium — kon'    dro    cray'    nee   um    (G, 

chondros,  cartilage;  kranion,  skull) 
Chondrostei — kon    dross'  tee    eye  (G,  chondros, 

cartilage;  osteon,  bone) 
Chordata,  chordate — kor  day'  tah,  kor'  date  (L, 

chorda,  cord  or  string) 
Chordocentrous — kor'  dough  sen'  trous  (L,  chorda, 

cord;  centrum,  center) 
Chorioid  (or  choroid) — koh'  ree  oid,  koh'  roid  (G, 

chorion,  membrane) 


Chorion — koh'  ree  on  (G,  membrane) 

Ciliary — sill'  ee  a  ry  (L,  cilium,  eyelid) 

Cinereum — si  nee'  ree  um  (L,  ashy) 

Ciona — sigh'  oh  na  (G,  kion,  pillar) 

Circulatory — sir'  kiu  la  tow'  ree  (L,  circulo,  to  form 

a  circle) 
Cirrus,  cirri — sir'  rus,  -ree  (L,  cirrus,  tuft,  lock  of 

hair) 

Clava — clay'  va  (L,  branch,  club) 
Clavicle — klav'  i  kel  (L,  claricula,  little  key) 
Clavo — clay'  voh  (L  combining  form,  clams,  key, 

referring  to  the  clavicle) 
Cleido — kly'  dough  (G  combining  form,  kleis,  key, 

referring  to  the  clavicle) 

Cleithrum — klyth'  rum  (G,  kleithron,  bar,  gate) 
Clitoris— kly'  to  riss  (G,  kleio,  to  close) 
Cloaca — klo  ay'  kah  (L,  sewer) 
Coccyx,  coccygeal — kock'  six,  -sij'  ee  al  (G  word) 
Cochlea — kock'  lee  ah  (L,  snail) 
Coeliac — see'  lee  ak  (G,  koilia,  stomach) 
Coelom — see'  loam  (G,  koilos,  hollow) 
Colon — koh'  Ion  (G,  kolon,  member) 
Columella — kol'  yu  mell'  ah  (L,  little  column) 
Concha — kon'  ka  (L,  shell) 
Condyle — kon'  dill  (L,  condylus,  knuckle) 
Conjunctiva — kon'    junk  tie'  va  (L,  conjunctus, 

join  together) 

Copula— kop'  yu  lah  (L,  cum,  together;  apo,  bind) 
Coracoid — kor'    a  koid    (G,  korakoeides,  like  a 

crow's  beak) 

Corium — koh'  ree  um  (L,  leather) 
Cornea,  corneum — kor'  nee  ah,  -um  (L,  corneus, 

horny) 

Cornu,  cornua — kor'  niu,  kor'  niu  ah  (L,  horn) 
Corona,  coronary — koh  row'  nah,  kor'  oh  nay  ry 

(L,  crown  or  wreath) 
Coronoid — kor'  oh  noid  (G,  korone,  crow) 
Corpora,  corpus — kor'  po  rah,  kor'  pus  (L,  body) 
Costal — kos'  tal  (L,  costa,  rib) 
Cranial,  craniate,  cranium — kray'  nee  al,  -nee  ate, 

-nee  um  (G,  kranion,  skull) 
Cribriform — krib'  ree  form  (L,  cribrum,  sieve) 
Cricoid — kry'  coid  (G,  kriksos,  ring) 
Crista— kris'  tah  (L,  crest) 
Crocodilia — krok'  oh  dill'  ee  ah  (L,  crocodile) 
Crossopterygii — cross  sop'  ter  yg'  ee  eye  (G,  kros- 

soi,  fringe;  pteron,  wing) 
Crura,  crural,  crus — kru'  rah,  kru'  ral,  kruss  (L, 

crus,  leg) 

Ctenoid — ten'  oid  (G,  ktein,  comb) 
Cuneiform — kiu'  nee  i  form  (L,  cuneus,  wedge) 
Cutaneous — kiu  tay'  nee  us  (L,  cutis,  skin) 
Cuvier — kiu  vyay  ( name  of  French  anatomist) 
Cycloid — sigh'  kloid  (G,  kyklos,  circle) 
Cyclostomata,  cyclostome — sigh'  klo  stow'  ma  tah, 

-stome  (G,  kyklos,  circle;  stoma,  mouth) 
Cystic— sis'  tik  (G,  kystis,  bladder) 


364 


LABORATORY  MANUAL  FOR  VERTEBRATE  ANATOMY 


Defer  ens — deff'  er  enz  (L,  defero,  carry  away) 
Deltoid— dell'  toid  (G,  delta,  fourth  letter  of  the 

Greek  alphabet,  triangular  in  form) 
Dentary — den'  ta  ree  (L,  dens,  tooth) 
Dentine — den'  tin  (or  teen)  (L,  dens,  tooth) 
Dermal,  dermis — derr'  mal,  derr'  miss  (G,  derma, 

skin) 
Dermatome — derr'  ma    tome   (G,  derma,   skin; 

tomos,  cutting) 
Diaphragm — dye'   a   framm    (G,    dia,    between; 

phragnymi,  to  inclose) 
Diastema — dye'  a  stee'  mah  (G,  interval) 


Facet — fas'  et  (L,  fades,  face) 
Facial — fay'  shal  (L,  fades,  face) 
Falciform — fal'  see  form  (L,  falx,  sickle) 
Fallopian— fa   loh'    pee    an    (after    Fallopio,  an 

anatomist) 

Fascia — fash'  ee  ah  (L,  a  band  or  bandage) 
Fasciculus,  fasciculi — fa  sik'  yu  lus,  -yu  lye  (L,  a 

little  bundle) 

Fauces — fa'  sees  (L,  throat) 
Femoral,  femoris — fern'  oh  ral,  -oh  riss  (L,  femur. 

thigh) 
Femur — fee'  mur  (L,  thigh) 


Digastric— dye  gas'  trik  (G,  di,  two;  gaster,  belly)       Fenestra— fee  nes'  trah  (L,  window) 


Digitigrade — dij'  i  ti  grade  (L,  digitus,  finger,  toe; 

gradus,  step,  walk) 
Diphycercal— diff'  ee  sir'  kel  (G,  diphyes,  twofold; 

kerkos,  tail) 
Diplospondyly — dip'  low  spon'  dy  lee  (G,  diploos, 

double;  spondylos,  vertebra) 
Duodenum — diu'  oh  dee'  num  (L,  duodeni,  twelve) 
Dura — diu'  rah  (L,  hard) 


Fibula,  fibulare — fib'  yu  lah,  -yu  lay'  ree  (L,  fibula, 

clasp,  buckle) 
Filoplume — fill'  (or  file)  oh  plume  (L,filum,  thread; 

pluma,  feather) 

Fimbria — fim'  bree  ah  (L,  border) 
Flocculus — flock'  yu  lus  (L,  a  little  piece  of  wool) 
Follicle— foil'  i  kel  (L,  folliculus,  a  little  bag) 
Fontanelle — fon'  ta  nell'  (F,  a  little  fountain) 


Ect,  ecto— ekt,  ek'  toh  (G  prefix,  the  outer,  outside)       Foramen,  foramin 
Edentata— ee'  den  tay'  tab  (L,  e,  without;  dens, 
tooth) 


fo  ray'  men,  -ram'  ee  nah  (L, 
Fornix — for'  niks  (L,  an  arch  or  vault) 


Bff  erent^-eff'  er  ent  (L,  e/ero,  to  bear  away  from)       Fossa— foss'  ah  (L,  a  pit  or  cavity) 


Eff erentia — eff'  er  en'  shi  a  (L,  e/ero,  to  bear  away 
from) 


Frenulum — fren'  yu  lum  (L,  a  little  bridle  or  bit) 
Frontal — frun'  tal  (L,  frons,  brow) 


Elasmobranch,     elasmobranchit — ee     las'     mow      Fundus — fun' dus  (L,  the  bottom) 

brank,  -mow  bran'  kee  eye  (G,  elasmos,  plate;      Fungiform— fun'  ji  form  (L,  fungus,  a  mushroom) 
branchia,  gills) 

Encephalon — en  sef  a  Ion  (G,  enkephalos,  brain) 

End,  endo — end,  en'  dough  (G  prefix,  within,  in- 
side) 

Endostyle — en'    dough   style    (G,   endo,   within; 
stylos,  column) 

Entoderm — en'  toe  derm  (G,  endo,  within;  derma, 
skin) 

Ep,  epi— epp,  epp'  ee  (G,  prefix,  upon) 

Epaxial — epp  axe'  i  al  (G,  epi,  upon;  L,  axis) 

Epidermis — epp'   ee   derr'   miss   (G,   epi,   upon; 
derma,  skin) 

Epididymis — epp'  ee  did'  y  miss  (G,  epi,  upon; 
didymos,  testis) 

Epimere — epp'  ee  mere  (G,  epi,  upon;  meros,  part) 

Epiphysis,  epiphyseal — e  piff'  ee  sis,  epp'  ee  fis'  e  al 
(G,  an  outgrowth) 

Epiploic,  epiploicum — epp'  ee  plo'  ik,  -plo'  ee  kum 
(G,  epiploon,  omentum) 


Funiculus — fiu  nik'  yu  lus  (L,  a  small  rope  or  cord) 
Furcula— fur'  kiu  lah  (L,  a  little  fork) 

Ganglion — gan'  glee  on  (G,  a  tumor) 
Ganoid — gan'  oid  (G,  ganos,  bright) 
Gasserian — ga  see'  ri  an  (from  a  physician  Gasser) 
Gastric,  gastro — gas'    trik,  gas'  troh  (L,  gaster, 

stomach) 
Gastrocentrous — gas'  troh   sen'  trous  (L,  gaster, 

stomach;  centrum,  center) 
Gastrocnemius — gas'  trok  nee'  me  us  (G,  gaster, 

stomach;  kneme,  shank) 
Gastrocoel — gas'  troh   seal  (G,  gaster,  stomach; 

koilos,  hollow) 

Gastrula — gas'  trew  lah  (L,  a  little  stomach) 
G eniculate — ji  nik'  yu  late  (L,  genu,  knee) 
Geniohyoid — ji  nye'  oh  high'  oid  (G,  geneion,  chin; 

upsilon,  a  Y-shaped  letter  of  the  Greek  alphabet) 


Epistropheus— epp'  e  stro'  fee  us  (G,  epi,  upon;       Genital— jen' i  tal  (L,  gigno,  to  reproduce) 


strepho,  turn) 
Esophagus,  esophageal — ee  soph'  a  gus,  ee'  so  faj' 

ee  al  (G,  oisophagos,  gullet) 
Ethmoid — eth'  moid  (G,  ethmoe,  sieve) 
Eustachian — you  stay'  kee  an  (after  Eustachius, 

an  anatomist) 
Ex — (L  prefix,  out,  outside) 
Excretory — eks'  kree  toh  ry   (L,  ex,  out;   cerno, 

separate) 


Genu — jee'  new  (L,  knee) 

Germinativum — jerr'  mi  nay  tiv  um  (L,  germino, 

to  sprout) 

Glans — glanz  (L,  an  acorn) 
Glenoid — glee'  noid  (G,  glene,  a  socket) 
Glomerulus — glow  mer'  yu  lus  (L,  glomus,  a  ball 

of  yarn) 
Glossopharyngeal — gloss'  oh  fa  rin'  jee  al  (or  fare' 

in  jee'  al)  (G,  glossa,  tongue;  pharynx,  pharynx) 


APPENDIX  A 


Glottis — glott'  iss  (G,  glotta,  tongue) 
Gluteus — glu  tee'  us  (G,  gloutos,  rump) 
Gnathostomata,  gnathostotne — nath'  oh  stow'  ma 

tha,  nath'  oh  stome  (G,  gnathos,  jaw;   stoma, 

mouth) 

Gonad — gonn'  ad  (G,  gonos,  seed) 
Graafian — grahf  ee  an  (after  de  Graaf,  a  Dutch 

physician) 

Gracilis — grass'  i  lis  (L,  slender) 
Gubernaculum — giu'   her   nak'   yu    lum    (L,    a 

rudder) 
Gyrus,  gyri — jye'  rus,  -rye  (G,  gyros,  round) 

Habenula — ha  ben'  yu  la  (L,  a  little  band) 

Haemal — hem'  al  (G,  haima,  blood) 

Hamulus — ham'  yu  lus  (L,  a  little  hook) 

Hemichordata — hem'  i  core  day'  tah  (L,  hemi, 
half;  chorda,  cord) 

Hepatic,  hepato — he  pat'  ik,  hep'  a  toe  (G,  hepar, 
liver) 

Hermaphroditic — her  maff'  row  dit  ik  (G,  Hermes, 
Mercury;  Aphrodite,  Venus') 

Heterocercal — het'  er  oh  ser'  kal  (G,  heteros,  dif- 
ferent; kerkos,  tail) 

Heterocoelous — het'  er  oh  see'  lous  (G,  heteros, 
different;  koilos,  hollow) 

Heterodont — het'  er  oh  dahnt  (G,  heteros,  differ- 
ent; odon,  tooth) 

Heteronomous — het'  er  on'  oh  mous  (G,  heteros, 
different;  nomos,  law) 

Hilum — high'  lum  (L,  eye  of  the  bean) 

Hippocampus — hip'  poh  kam'  pus  (G,  hippos, 
horse;  kampos,  sea-monster) 

Holoblastic— holl'  oh  bias'  tik  (G,  holos,  whole; 
blastos,  germ) 

Holostei — ho  loss' tee  eye  (G,  holos,  whole;  osteon, 
bone) 

Homocercal — home'  oh  ser'  kal  (G,  homos,  the 
same;  kerkos,  tail) 

Homonomous — hoh  mon'  oh  mous  (G,  homos,  the 
same;  nomos,  law) 


Ichthyopsida— ik'  thy  op'  si  dah  (G,  ichthys,  fish; 

opsis,  appearance) 
Ileum — ill'  e  um  (G,  eilo,  twist) 
Iliac— ill'  i  ak  (L,  ilium,  flank) 
Ilium — ill'  ee  um  (L,  flank) 
Incisor,  incisiva,  incisive — inn  sigh'  sor,  -sigh'  si 

vah,  -sigh'  sive  (L,  incido,  to  cut  into) 
Incus — inn'  kuss  (L,  anvil) 
Infra — inn'  frah  (L  prefix,  below) 
Inf undibulum — inn'  fun  dib'  yu  lum  (L,  funnel) 
Inguinal — inn'  gwi  nal  (L,  inguen,  groin) 
Innominate — inn  nomm'  i  nate  (L,  innominatus, 

without  a  name) 

Integument — inn  teg'  u  ment  (L,  intego,  to  cover) 
Inter — inn'  ter  (L  prefix,  between) 
Intercalary — inn  terr'  ka  lay  ree  (L,  intercalo,  to 

put  between) 
Intestine,  intestinal — inn  tess'  tin,  inn  tess'  ti  nal, 

(L,  intestinus,  inside) 
Intra — inn'  trah  (L  prefix,  within) 
Invagination — inn  vaj'  i  nay'  shun   (L,  in,  in; 

vagina,  sheath) 
Iris — eye'  riss  (G,  rainbow) 
Ischial,  ischium — iss'  kee  al,  iss'  kee  um  (G,  ischion, 

hip) 
Isolecithal — eye'  so  less'   i  thai  (G,  isos,  equal; 

lekithos,  yolk) 
Iter — eye'  ter  (L,  passage) 

Jejunum — jee  jew'  num  (L,  hungry) 

Jugal — jew'  gal  (L,  jugum,  yoke) 

Jugular — jew'  giu  lar  (L,  jugulum,  the  collar  bone) 

Labia,  labial — lay'  bee  ah,  -bee  al  (L,  labium,  lip) 
Lacertilia — lass'  err  till'  ee  ah  (L,  lacerta,  lizard) 
Lacrimal  (also  spelled  lacrymal) — lack'  ree  mal  (L, 

lacrima,  tear) 

Lagena — la  jee'  nah  (G,  lagynos,  flask) 
Lamella,  lamellae — la  mell'  ah,-mell'  ee  (L,lamina, 

a  thin  sheet) 


Humerus— hiu'  mer  us  (L,  the  bone  of  the  upper      Lamina— lamm'  ee  nah  (L,  a  thin  plate) 
arm) 


Laryngeal,  larynx — la  rin'  jee  al  (or  lar'  in  jee  al), 

lar'  inks  (G,  larynx,  gullet) 
Latissimus — la  tiss'  i  muss  (L,  the  broadest) 
Lepidosteus — lepp'  i  doss'  tee  us  (G,  lepis,  scale; 

osteon,  bone) 

Levator — le  vay'  tor  (L,  a  lifter) 
Lienal,  lieno — lie  ay'  nal,  lie  ay'  no  (L,  lien,  spleen) 
Linea — linn'  ee  ah  (L,  line) 
Lingual— linn'  gwal  (L,  lingua,  tongue) 
Hypogossal-high'  poh  gloss'  al  (G,  hypo,  below;      Lissamphibia_iiss'amnb'eeiah(G,/m,5,  smooth; 

amphibios,  double  life) 
Lobule — lobb'  yule  (G,  lobos,  lobe) 
Longissimus — Ion  jiss'  ee  mus  (L,  the  longest) 
Lorenzini — loh  ren  zee'  nee  (Italian  anatomist) 


Hy,  hyo — high,  high'  oh  (prefix  referring  to  the 

hyoid) 
Hyoid,  hyal — high'  oid,  high'  all  (G,  upsilon,  a  Y- 

shaped  letter  of  the  Greek  alphabet) 
Hyp,  hypo — hipp,  high'  poh  (G  or  L  prefix,  below, 

less  than) 
Hypaxial — hip  axe'  ee  al  (G,  hypo,  below;   axis, 

axis) 


glossa,  tongue) 
Hypomere — high'   poh   mere    (G,    hypo,    below; 

meros,  part) 
Hypophysis — he  poff'  ee  sis  (G,  hypo,  below;  phyo, 

to  cause  to  grow) 
Hypural— hip  your'  al   (G,  hypo,   below;   oura,       Lumbar—  lumm'  bar  (L,  lumbus,  loin) 


Lutea,  lutein — liu'  tee  ah,  -tee  in  (L,  luteus,  yellow) 


366 


LABORATORY  MANUAL  FOR  VERTEBRATE  ANATOMY 


Lymphatic — limm  fat'  ik  (L,  lympha,  clear  water) 
Lymphocytes — limph'  oh  sites  (L,  lympha,  clear 
water;  G,  kytos,  hollow  place) 

Major — may'  jor  (L,  the  greater) 
Malar — may'  lar  (L,  mala,  cheek) 
Malleolus,  malleoli — ma  lee'  oh  lus,  -oh  lye  (L, 

little  hammer) 

Malleus — mahl'  ee  us  (L,  hammer) 
Malpighian,  malpighii — mahl  pig'  ee  an,  -ee  eye 

(after  Malpighi,  an  Italian  biologist) 
Mamillary  (or  mammillary) — mam'  ill  lay  ree  (L, 

mamma,  breast) 

Mammal — mam'  mal  (L,  mamma,  breast) 
Mammalia — ma  may'  lee  ah  (L,  mamma,  breast) 
Mammary— mam'  a  ree  (L,  mamma,  breast) 
Mandible,  mandibular — man'  di  bl,  man  dib'  yu 

lar  (L,  mandibula,  jaw) 
Manubrium — ma  niu'  bree  um  (L,  handle) 
Manus — may'  nuss  (L,  hand) 
Marsupial,  marsupialia — marr  siu'  pee  al,  mar  siu' 

pee  ay'  lee  ah  (L,  marsupium,  pouch) 
Masseter — ma  see'  ter  (G,  maseter,  a  chew) 
Mastoid — mass'  toid  (G,  mastos,  breast) 
Mater — may'  ter  (L,  mother) 
Maxilla,  maxillary — maks  ill'  ah,  maks'  i  lay  ree 

(L,  maxilla,  jawbone) 
Maximus — maks'  i  mus  (L,  the  largest) 
Meatus — mee  ate'  us  (L,  passage) 
Mediastinal,  mediastinum — mee'  dee  ass  tie'  nal, 

-num  (L,  mediastinus,  being  in  the  middle) 
Medius — mee'  di  us  (L,  middle) 
Medulla,  medullary — me  dull'  lah,  medd'  u  lay  ree 

(L,  medulla,  marrow,  pith) 
Meninx,  meninges — mee'  ninks,  mee  nin'  jees  (G, 

membrane) 

Mental — men'  tal  (L,  mentum,  chin) 
Meroblastic — merr'  oh  bias'  tik  (G,    meros,  part; 

blastos,  germ) 

Mes,  meso — mess,  mess'  oh  (G  prefix,  th<»  middle) 
Mesenchyme — mess'  en  kime  (G,   mesas,  middle; 

enchyma,  in  a  fluid) 
Mesenteric,  mesentery — mess'  en  tare'  Ik,  -y  (G, 

mesos,  middle;  enter  on,  gut) 
Mesoderm — mess'  oh    derm  (G,  mesos,  middle; 

derma,  skin) 
Mesogaster — mess'  oh  gas'  ter  (G,  mesos,  middle; 

gaster,  stomach) 
Mesomere — mess'  oh    mere  (G,  mesos,  middle; 

meros,  part) 
Mesopterygium — mess'  op  terr  yg'  ee  um  (G, 

mesos,  middle;  ptefon,  wing) 
Mesorchium — mess  or'  kee  um  (G,  mesos,  middle; 

orchis,  testis) 
Mesotubarium — mess'  oh  tiu  bay'  ri  um  (G,  mesos, 

middle;  L,  tubus,  tube) 
Mesovarium — mess  oh   vay'  ree  um  (G,  mesos, 

middle;  L,  ovum,  egg) 


Met,  meta — met,  met'  ah  (L  or  G  prefix,  between. 

after,  reversely) 
Metacarpal — met'  a  kar'  pal  (G,  meta,  after;  kar- 

POS,  wrist) 
Metacromion — met'  a    krow'  mee  on  (G,  meta, 

after;  akromion,  point  of  the  shoulder) 
Metamere,  metamerism — met'  a  mere,  met'  a  mer 

izm  (G,  meta,  after;  meros,  part) 
Metapleural — met'  a  plu'  ral    (G,  meta,  after; 

pleura,  side) 
Metapterygium — met  ap'  ter  yg'  ee  um  (G,  meta, 

after;  pteron,   wing) 
Metatarsal — met'  a  tar'  sal   (G,  meta,  after;  tar- 

sos,  a  flat  surface) 
Minimus — min'  i  mus  (L,  the  least) 
Minor — my'  nor  (L,  the  lesser) 
Mitral — my'  tral  (F,  mitre,  a  peaked  cap) 
Molar — mow'  lar  (L,  mola,  millstone) 
Monotremata,   monotreme — mon'  oh   tremm'  a 

tah,  -treem   (G,   monos,   one;  trema,   hole    or 

opening) 
Mullerian — me  lerr'  i  an  (after  Miiller,  a  German 

physiologist) 

Multifidus — mull  tiff'  i  dus  (L,  many  cleft) 
Myelon — my'  e  Ion  (G,  marrow) 
Mylohyoid — my'  low    high'  oid  (G,  myle,  mill; 

upsilon,  the  letter  y) 
Myocomma,   myocommata — my'    o   komm'   ah, 

-komm'  a   tah  (G,  mys,  muscle;'  komma,  that 

which  is  cut  off) 
Myotome — my'  oh  tome  (G,  mys,  muscle;  tome, 

cutting) 

Naris,  Nares — nay'  riss,  nay'  rees  (L,  nostril) 

Necturus — nek  too'  rus  (G,  nektos,  swimming; 
oura,  tail) 

Nephros — neff'  ross  (G,  kidney) 

Nephrostome — neff'  row  stome  (G,  nephros,  kid- 
ney; stoma,  mouth) 

Nephrotome — neff'  row  tome  (G,  nephros,  kidney; 
tome,  cutting) 

Neural — niu'  ral  (G,  neuron,  nerve) 

Neurocoel — niu'  row  seal  (G,  neuron,  nerve;  koi- 
los,  hollow) 

Nictitating — nik'  ti  tay'  ting  (L,  nicto,  wink) 

Nodosal — no  dose'  al  (L,  nodosus,  knotty) 

Notocentrous — no'  toe  sen'  trous  (G,  nolos,  back; 
kentron,  center) 

Notochord — no'  toe  kord  (G,  notos,  back;  chorde, 
string) 

Nuchal — new'  kal  (L,  nucha,  nape  of  the  neck) 

Oblongata — ob  Ion  gay'  tah  (L,  oblongus,  oblong) 
Obturator — ob  tiu  ray'  ter  (L,  obturo,  to  close, 

shut) 
Occipital — ok  sip'  i  tal  (L,  occiput,  back  of   the 

head) 
Oculomotor — ok'  yu  loh  mow'  tor  (L,  oculus,  eye 

motor,  mover) 


APPENDIX  A 


367 


Odontoid — o  don'  toid  (G,  odous,  tooth) 
Olecranon — oh'  lee  kray'    non   (G,  olene,   ulna; 

kranion,  skull) 

Olfactory — ol  fak'  toh  ree  (L,  olfacere,  to  smell) 
Omentum — oh  men'  turn  (L,  fat  skin) 
Omo,  om — oh'  mow,  ohm  (G  prefix,  omos,  shoul- 
der) 
Opercular,   operculum — o  per'   kiu   lar,  -him  (L, 

operculum,  lid) 

Ophidia — o  fid'  ee  ah  (G,  ophis,  serpent) 
Ophthalmic — off  thai'  mik  (G,  ophthalmos,  eye) 
Opisto,  opist — o  pis'  tho,  o  pist'  (G  prefix,  opis- 

then,  behind) 

Oral — oh'  ral  (L,  os,  mouth) 
Orbicularis — or  bik'  yu  lay'  riss  (L,  orbiculus,  a 

little  circle) 
Orbital,  orbito — or'  bi  tal,  or'  bi   toe  (L,  orbita, 

orbit) 

Ostium — oss'  tee  um  (L,  mouth  or  entrance) 
Otic — oh'  tik  (G,  otikos,  pertaining  to  the  ear) 
Otolith — oh'  toe  lith  (G,  ous,  ear;  lithos,  stone) 
Ovarian,  ovary — o  va'  ree  an,  oh'  va  ree  (L,  ovum, 

egg) 
Oviduct — oh'  vi  duct  (L,  ovum,  egg;  ductus,  duct) 

Palatal,  palate,  palatine — pal'  a  tal,  pal'  ate,  pal'  a 

tiyn  (or  -tin)  (L,  palatum,  palate) 
Pallium — pahl'  ee  um  (L,  cloak) 
Palpebra — pahl'  pe  brah  (L,  eyelid) 
Pancreas,  pancreatic — pan'  kree  ass,  pan'  kree  at' 

ik  (G,  pas,  all;  kreas,  flesh) 
Papilla,  papillae — pa  pill'  ah,  -ee  (L,  nipple) 
Para — par'  ah  (G  prefix,  beside,  near) 
Paraphysis — pa  raff'  ee  sis  (G,  para,  beside;  phyo, 

produce) 

Parietal — pa  rye'  ee  tal  (L,  paries,  wall) 
Parotid — pa  rot'  id  (G,  para,  beside;  ous,  ear) 
Patella — pa  tell'  ah  (L,  a  small  pan  or  dish) 
Pecten — pek'  ten  (L,  comb) 
Pectineal — pek'ti  nee'  al  (L,  pecten,  comb) 
Pectoral,  pectoralis — pek'  toe  ral,  -ray'  liss  (L,  pec- 

toralis,  referring  to  the  chest) 
Pellucidum — pe  liu'  see  dum  (L,  translucent) 
Pelvic,  pelvis — pell'  vik,  -viss  (L,  pelvis,  basin,  the 

pelvis) 

Penis — pee'  nis  (L,  male  external  sex  organ) 
Peri — perr'  ee  (L  or  G  prefix,  around) 
Pericardial,  pericardium — per'  ee  kar'  deeal,-um 

(G,  peri,  around;  kardia,  heart) 
Perineum — perr'  i  nee'    um  (L,  perineon,  origin 

uncertain) 

Peripheral — pe  riff'  er  al  (G,  peri,  around;  phero, 

bear) 
Peritoneal,  peritoneum — perr'  i  toe  nee'al,  -um  (G, 

peri,  around;  teino,  stretch) 
Peroneal,  peroneus — perr  oh  nee'al, -us  (G,  perone, 

pin  of  a  brooch,  referring  to  the  fibula) 


Petrosal,  petrous — pe  trow'  sal,   pee'   trous   (L 

petrosus,  rocky) 
Phalanges,  phalanx — fay  Ian'  gees,  fay'  lanks  (G, 

phalanx,  battle-line) 
Pharyngeal,  pharynx — fa  rin'  jee  al  (or  far'  in  jee' 

all)  far'  ynks  (G,  pharynx,  throat) 
Phrenic — fren'ik  (G,  phren,  diaphragm) 
Phylum — fye'  lum  (G,  phylon,  race,  tribe) 
Pia — pie'  ah  (L,  pious,  kind) 
Pineal — pin'  ee  al  (L,  pinea,  pine  cone) 
Pinna — pin'  ah  (L,  feather) 
Pisces — piss'  ees  (L,  fish) 
Pituitary — pi  tiu'  i  tay  ree  (L,  pituita,  mucus) 
Placenta,  placentalia — pla  sen'  tah,  plass'  en  tay' 

lee  ah  (L,  placenta,  cake) 
Placoid — plak'oid  (G,  plax,  plate) 
Plantaris — plan  tay'  riss  (L,planta,  sole  of  the  foot) 
Plantigrade — plan'    ti    grade    (L,    planta,    sole; 

gradus,  walk) 

Plastron — plass'  tron  (F,  a  breastplate) 
Platysma — pla  tiz'  mah  (G,  flat  plate) 
Pleura,  pleural — plew'  rah, -ral  (G,  pleura,  rib, side) 
Plexus — picks'  us  (L,  interweaving) 
Plumulae — plew'  miu  lee  (L,  little  feather) 
Pneumatic — niu    mat'   ik   (G,   pneuma,   breath, 

spirit) 
Polyodon — po  lee'  oh  don  (G,  polys,  many;  odous, 

teeth) 

Polypterus — po  lip'  ter  us  (G,  polys,  many;  pteron, 

wing) 

Pons — ponz  (L,  bridge) 
Portal — pour'  tal  (L,  porta,  gate) 
Post — (L  prefix,  behind,  after) 
Pre — pree  (L  prefix,  before) 
Prepuce — pree'  pyuse  (F,  probably  from  L,  pre, 

before;  and  G,  posthion,  penis) 
Primate — pry'  mate  (L,  primus,  first) 
Pro — proh  (L  or  G  prefix,  before) 
Precocious — prob'seelous  (G,  pro,  before;  koilos, 

hollow) 
Proctodaeum — prok'  toe    dee'  um   (G,   proktos, 

anus;  daio,  divide) 
Prostate — press'  tayte  (G.  prostates,  in  the  front 

rank) 
Proventriculus — proh'  ven  trick'  yu  lus  (L,  pro, 

before;  venter,  belly) 
Pseudo — sue'  do  (G  prefix,  pseudes,  false) 
Pterygium — te  ryg'  ee  um  (G,  pterygion,  a  little 

wing  or  fin  or  projection,  from  pteron,  wing) 
Pterygo,  pterygoid — terr'  i  go,  -goid  (G,  pteron, 

wing) 
Pterylae — terr'  y  lee  (G,  pteron,  feather,   wing; 

hyle,  wood) 
Pubic,  pubis — piu'  bik,  -bis  (L.  pubes,  hair,  by 

inference,  maturity) 

Pulmonary — pull'  mow  nay'  ree  (L,  pulmon,  lung) 
PuJviuar — pul  vine'  ar  (L,  pulvinus,  cushion) 


368 


LABORATORY  MANUAL  FOR  VERTEBRATE  ANATOMY 


Pygal— pye'gal  (G,  pyge,  rump) 

Pygostyle — pye'  go  style  (G,  pyge,  rump;  stylos, 

column) 
Pylorus — pye  loh'  rus  (G,  pyloros,  gatekeeper) 

Quadrate — kwad'  rate  (L,  quadratus,  square) 
Quadriceps — kwad'   ree  seps  (L,  quattuor,  four; 
caput,  head) 

Rachis — ray'  kis  (G,  spine) 

Radial,  radiale,  radius — ray'  dee  al,  ray'  dee  ay' 

lee,  -dee  us  (L,  radius,  ray) 
Ramus,  rami — ray'  mus,  ray'  mee  (L,  a  branch) 
Raphe — ray'  fee  (G,  seam) 
Rectrices — rek  try'  sees  (L,  rectus,  straight) 
Rectum,  rectus — rek'  tum,-tus  (L,rectus, straight) 
Remiges — rem'  i  jeez  (L,  remus,  oar) 
Renal — ree'  nal  (L,  renes,  kidneys) 
Reptilia,  reptile — rep  till'  ee  ah,  rep'  till  (L,  rep- 

tihts,  reptile,  from  repo,  creep) 
Respiratory— re  spire'  a  toh'  ree  (or  res'  pee  rah 

toh'  ree)  (L,  respiro,  breathe  back) 
Rete— ree'  tee  (L,  net) 
Retina — ret'  i  nah  (L,  rete,  net) 
Retro — ret'  roh  (L  prefix,  back,  backward) 
Rhinal — rye'  nal  (G,  rhis,  nose) 
Rhomb,  rhombo — romb,  romm'  boh  (L  or  G  prefix, 

referring  to  a  geometric  figure,  a  kind  of  parallel- 
ogram) 
Rhomb oideus — rom  boy'  dee  us  (L,  rhombus,  or  G, 

rhombos,  rhomboid  in  form) 
Rhyncocephalia — rin'  koh  see  fay'  lee  ah  (G,  rhyn- 

chos,  snout;  kephale,  head) 
Rodent,  rodentia — row'  dent,  row  den'  she  ah  (L, 

rodcns,  gnawing) 
Rostral,  rostrum — ross'  tral,  -trum  (L,  rostrum, 

beak) 

Sacculus — sak'  yu  lus  (L,  little  sac) 
Sacral,  sacrum — say'  cral,  -krum  (L,  sacer,  sacred) 
Sagittal — saj'  i  tal  (L,  sagitta,  arrow) 
Salivary— sail'  i  vay'  ree  (L,  saliva,  spit) 
Sartorius — sar  toe'  ree  us  (L,  sartor,  tailor) 
Sauropsida — sah  ropp'  si  dah  (G,  sauros,  lizard; 

opsis,  appearance) 
Scalene,  scalenes — skay'  lean,  skay'  leans  (G, 

skalenos,  uneven) 

Scapula — skap'  yu  lah  (L,  shoulder  blade) 
Sciatic — sigh  at'  ik  (L,  sciaticus,  originally  ischia- 

dicus,  from  G,  ischion,  hip) 
Sclera — sklay'  rah  (G,  skleros,  hard) 
Sclerotic— skle  rot'  ik  (G,  skleros,  hard) 
Sclerotome — skle'    roh    tome  (G,  skleros,  hard; 

tome,  cutting) 
Scrotal,  scrotum — skroh'  tal,  -turn  (L,  uncertain 

origin) 

Scute — skiut  (L,  scutum,  shield) 
Sell  a — sell'  ah  (L,  a  seat  or  saddle) 


Semilunar — semm'  i  liu'  nar  (L,  semi,  half;  luna, 

moon)    - 

Seminal — semm'  i  nal  (L,  semen,  seed) 
Seminiferous — semm'  i    niff'  er    ous  (L,  semen, 

seed;  fero,  to  bear) 
Septum — sepp'  turn  (L,  fence,  wall) 
Serosa — see  row'  sah  (L,  serum,  serum) 
Serratus —  se  rate'  us  (L,  serra,  saw) 
Sesamoid — sess'  a  moid  (G,  sesamon,  a    plant, 

referring  to  the  shape  of  the  seeds) 
Sinus — sigh'  nus  (L,  sinus,  curve) 
Skeletogenous — skell'  e  toj'  ee  nous  (G,  skeleton, 

from  skello,  make  dry) 
Solar — soh'  lar  (L,  sol,  the  sun) 
Soleus — soh'  lee  us  (L,  solea,  sole  of  the  foot) 
Somatic — soh  mat'  ik  (G,  soma,  body) 
Somatopleure — soh'  ma  toe  plure  (G,  soma,  body; 

pleura,  side) 

Somite — soh'  might  (G,  soma,  body) 
Spermatic — sperr  mat'  ik  (L,  sperma,  sperm) 
Sphenodon — sfee'  noh    don    (G,   sphen,   wedge; 

odo n,  tooth) 

Sphenoid — sfee'  noid  (G,  sphen,  wedge) 
Spiracle — spear'  ah  kel  (or  spire'  ah  kel)  (L,  spi- 

raculum,  air  hole) 
Splanchnic — splank'  nik  (G,  splanchnon,  one  of 

the  viscera) 
Splanchnopleure — splank'  noh  plure  (G,  splanch- 

non,  aviscus;  pleura,  side) 
Splenial,  splenic — splee'  nee  al,  splenn'  ik  (G, 

splen,  spleen) 

Splenius — splee'  nee  us  (G,  splenion,  bandage) 
Squamata — skwa  may'  tah  (L,  squama,  scale) 
Squamosal — skwa  moh'  sal  (L,  squama,  scale) 
Stapes — stay'  peez  (L,  stirrup) 
Stegocephala — steg'  oh  seff'  ah  lah  (G,  stego,  cover; 

kephale,  head) 
Sternebra,    sternebrae — stir'  ne  brah,  -bree  (G, 

sternon,  breastbone;  L,  vertebra,  joint) 
Sterno — stir'  noh  (G   combining    word,  sternon, 

breastbone) 

Sternum — stir'  numm  (G,  sternon,  breastbone) 
Stomodaeum — stow'   moh    dee   um    (G,   stoma, 

mouth;  daio,  divide) 

Stratum — stray'  turn  (L,  a  spread  or  cover) 
Striatum — stry  ay'  turn  (L,  stria,  furrow) 
Stylo — sty'  loh  (G  combining  word,  stylos,  column) 
Sub — subb  (L  prefix,  under) 
Subclavian — subb  clay'  vee   an  (L,  subt  under; 

clavis,  key,  referring  to  the  clavicle) 
Sulcus,  sulci — sull'  kuss,  sull'  sigh  (L,  a  furrow  or 

groove) 
Supine — sue'  pine    (L,   supino,    to   put  on   the 

back) 

Supra — siu'  prah  (L  prefix,  above) 
Suture — siu'  chur  (L,  suo,  to  sew) 
Symphysis — sirnrn'  fee  sis  (G,  union) 


APPENDIX  A 


369 


Syn — sin  (L  or  G  prefix,  together,  with) 
Synotic — sy  not'  ik  (G,  syn,  together;  ous,  ear) 
Syrinx — sir'  inks  (G,  pipe) 

Tarsal,  tarsalia,  tarsus — tar'  sal,  tar  say'  lee  ah, 

tar'  sus  (G,  tarsos,  a  flat  surface) 
Tectum — tek'  turn  (L,  tego,  to  cover) 
Tegmentum — teg  men'  turn  (L,  tego,  to  cover) 
Tel,  tele— tell,  tell'  ee  (G,  tele,  far) 
Teleost,  teleostei — tell'  ee  ost,  tell'  ee  os'  tee  eye 

(G,  teleos,  whole,  perfect;  osteon,  bone) 
Teleostome,  teleostomi — tell'  ee  oh  stome,  tell'  ee 

oss'  toe  my  (G,  teleos,  perfect;  stoma,  mouth) 
Telolecithal— tell'  oh  less'  i  thai    (G,  telos,  end; 

lekithos,  yolk) 

Temporal — tern'  poh  ral  (L,  tempus,  temple) 
Tendon — ten'  don  (L,  tendo,  to  stretch) 
Tentorium — ten  toh'  ree  um  (L,  tendo,  to  stretch) 
Teres — tee'  reez  (L,  round) 
Testes,  testis — tess'  teez,  tess'  tiss  (L,  testis) 
Thalamus — thai'  a  mus  (L,  chamber) 
Thecodont — thee'    koh  dont  (G,   theke,    sheath; 

odon,  tooth) 
Thoracic,  thoraco — thoh  ras'  ik,  thoh'  ra  koh  (L, 

thorax,  thorax) 

Thorax — thoh'  raks  (L,  thorax) 
Thymus — thy'  mus  (G,  thymon,  thyme) 
Thyroid  (or  thyreoid) — thy'  roid,  thy'  reeoid  (G, 

thyreos,  shield) 
Tibia,  tibiale,  tibialis—  tibb'  ee  ah,  tibb'  ee  ay'  lee, 

-ay'  lis  (L,  tibia,  the  shin  bone) 
Tonsil — ton'  sill  (L,  tonsilla,  tonsil) 
Trabecula,  trabeculae — tra  bek'  yu  lah,  -lee  (L, 

little  beam) 

Trachea — tray'  kee  ah  (G,  trachys,  rough) 
Trapezius — tra  pee'  zee  us  (G,  trapeza,  table) 
Trapezoid — tra'  pe  zoid  (G,  trapeza,  table) 
Trematic — tree  mat'  ik  (G,  trema,  hole,  opening) 
Triceps — try'  seps  (L,  tres,  three;  caput,  head) 
Tricuspid — try  kuss'  pid  (L,  tres,    three;   cuspis, 

point) 
Trige  minus — try  jem'  i^nus  (L,  three  at  a  birth, 

triple) 

Trochanter — troh  kan'  ter  (G,  trochos,  wheel) 
Trochlea,  trochlear — trock'  lee  ah,  -lee  ar  (L,  troch- 

lea,  pulley) 

Tuber — tiu'  ber  (L,  a  swelling) 
Tubercular,  tuberculum — tiu  burr'  kiu  lar,  -lum 

(L,  tuberculum,  a  little  swelling) 
Tunicata,  tunicate — tiu'  ni  kay'  tah,  tiu'  ni  kate 

(L,  tunica,  tunic) 
Turbinal — turr'  bi  nal    (L,  turbo,  anything  that 

whirls) 

Turcica — turr'  see  kah  (L,  turkish) 
Tympanic,  tympanum — tim  pan'  ik,  tim'  pa  num 

(L,  tympanum,  drum) 

Ulna,  ulnar,  ulnare — ulT  nah,  ulT  nar,  ull  nay'  re 
(L,  ulna,  elbow) 


Umbilical,  umbilicus — um  bill'  i  kel,  um  bi  lye'  kus 

(L,  umbilicus,  navel) 
Uncinate — unn'  see  nate  (L,  uncus,  hook) 
Unguligrade — unn  giu'  li  grade  (L,  ungula,  hoof; 

gradus,  walk) 

Ureter — yu  ree'  ter  (G,  ouron,  urine) 
Urethra — yu  ree'  thrah  (G,  ouron,  urine) 
Urinary — yu  ri  nay'  ree  (L,  urina,  urine) 
Urodela,  urodele — yu'  row  dee'  lah,  yu'  row  deal 

(G,  our  a,  tail;  delos,  evident) 
Urogenital — yu'  row  jen'  i  tal  (G,  ouron,  urine; 

L,  genitalis,  genital) 
Uropygium,  uropygial — yu'  row  pij'  i  um,  -i  al  (G, 

our  a,  tail;  pyge,  rump) 
Urostyle — yu'  row  style  (G,  oura,    tail;    stylos, 

column) 
Uterus,  uterine — yu'  ter  us,  yu'  ter  inn  (or  -ein) 

(L,  uterus,  womb) 
Utriculus— yu  trik'  yu  lus  (L,  a  little  bag) 

Vagina — va  jye'  nah  (L,  a  sheath) 

Vagus — vay'  guss  (L,  wandering) 

Vallate— vail'  ate  (L,  vatto,  to  surround  with  a  wall) 

Vas,  vasa — vass,  vay'  sah  (L,  a  duct) 

Vastus — vass'  tus  (L,  vast) 

Velum — vee'  lum  (L,  veil) 

Venosus — vee  noh'  sus  (L,  vena,  vein) 

Venous — vee'  nous  (L,  vena,  vein) 

Ventricle — ven'  tree  kel  (L,  venter,  belly) 

Vermis — verr'  mis  (L,  worm) 

Vertebra,  vertebrae — verr'  te  brah,  -bree  (L,  a 
joint) 

Vertebrata,  vertebrate — verr'  te  bray'  tah,  -brate 
(L,  vertebra,  joint) 

Vesical — vess'  i  kal  (L,  vesica,  bladder) 

Vibrissa,  vibrissae — vie  briss'  ah,  -ee  (L,  hairs  in 
the  nostrils) 

Villi,  villus — vill'  eye,  -us  (L,  shaggy  hair) 

Viscera,  visceral — viss'  err  ah,  -al  (L,  viscus,  inter- 
nal organ) 

Viscus — viss'  kus  (L,  internal  organ) 

Vitelline — vi  (or  vie)  tell'  inn  (L,  vitellus,  yolk) 

Vitreous — vit'  ree  ous  (L,  glassy) 

Vomei — voh'  mer  (L,  ploughshare) 

Vulva — vull'  vah  (L,  covering) 

Wolffian— wolf  ee  an  (after  K.  F.  Wolff,  German 
anatomist) 

Xiph,  xiphi — ziff,  ziff'  ee  (G,  xiphos,  sword,  used  in 
combining  words) 

Zyg,  zygo — zigg,  zye'  go  (or  zigg'  oh)  (G  combining 

word,  zygon,  yoke) 
Zygapophysis — zye'  (or  zigg)  ga  poff'  ee  sis  (G, 

zygon,  yoke;  apophysis,  process) 
Zygomatic — zye'  go   (or  zigg'  oh)   mat'  ik   (G, 

zygoma,  yoke  or  bar) 


Appendix  B 

PREPARATION   OF   MATERIALS 

1.  Killing  the  specimens. —  Necturus  is  best  killed  by  placing  in  hot  water;    turtles  by 
injection  of  ether  or  chloroform  into  the  cloaca  or  better  the  trachea;  birds  and  mammals  by 
inclosing  them  in  a  tightly  closed  vessel  with  a  wad  of  cotton  soaked  in  ether  or  chloroform.     In 
handling  turtles  pull  the  head  forward  by  inserting  a  stout  hook  behind  the  jaw,  and  pry  open 
the  mouth. 

2.  Preparation  of  skeletons. — Skeletons  are  best  prepared  from  fresh  materials  or,  in  the 
case  of  marine  forms,  those  that  have  been  preserved  in  brine.     It  is  difficult  to  prepare  skele- 
tons from  specimens  that  have  been  preserved  in  formalin.    To  prepare  a  skeleton  remove  the 
skin,  all  of  the  viscera,  and  as  much  of  the  muscles  as  possible  and  soak  the  specimen  in  water. 
The  remaining  flesh  will  decay  and  may  be  removed  with  a  stiff  brush  or  forceps.     This  process 
of  maceration  in  cold  water  takes  some  time.    The  process  may  be  much  shortened,  and  tough 
specimens  are  more  easily  prepared  by  immersing  the  specimen  for  a  few  hours  in  hot  or  boiling 
water  to  which  gold  dust  or  the  following  soap  solution  has  been  added.     Kingsley  gives  the 
following  formula  for  the  soap  solution: 

75  gms.  of  hard  soap 
12  gms.  of  potassium  nitrate  (saltpeter) 
150  c.c.  of  strong  ammonia 
2,000  c.c.  of  distilled  or  soft  water. 

Mix  thoroughly.  In  using,  take  one  part  of  the  soap  solution  to  three  or  four  parts  of  water. 
The  length  of  time  required  before  the  flesh  will  separate  easily  from  the  bones  varies  with 
different  animals  and  is  shorter  for  small  or  young  specimens  than  for  large  and  old  ones.  Davi- 
son  gives  the  time  for  the  cat  as  two  to  four  hours  in  boiling  water  or  better  three  to  six  hours  in 
water  kept  at  75  to  90°  C.  He  also  states  that  if  the  bones  are  heated  for  only  one  to  two  hours 
at  a  temperature  not  above  85°  C.,  the  ligaments  will  be  preserved,  and  when  the  skeleton  is 
dried  the  ligaments  will  harden  and  hold  the  bones  together.  Skeletons  containing  a  consider- 
able amount  of  cartilage  should  not,  of  course,  be  boiled  but  treated  only  with  moderately  hot 
water.  For  fishes  a  few  minutes'  treatment  with  hot  water  is  generally  sufficient.  Cartilagi- 
nous skeletons  should  be  preserved  in  weak  formalin  or  in  70  per  cent  alcohol. 

3.  Injection  of  the  circulatory  system. — To  render  the  blood  vessels  conspicuous  and  more 
easily  followed  it  is  advisable  and  in  fact  practically  necessary  that  the  arteries  at  least  be 
injected  with  a  colored  solution.     Injection  syringes  for  this  purpose  may  be  obtained  from 
dealers  in  laboratory  supplies,  or  an  ordinary  rubber  atomizer  bulb  may  be  used.     A  glass 
cannula  is  inserted  in  the  vessel  to  be  injected.     A  cannula  is  simply  a  piece  of  glass  tubing 
drawn  out  in  the  flame  at  one  end  to  a  size  suitable  for  the  blood  vessel  into  which  it  is  to  be 
inserted.    It  is  also  desirable  that  the  end  to  be  inserted  in  the  vessel  be  slightly  enlarged  as  it 
will  then  hold  more  securely  in  the  blood  vessel.     Loosen  the  vessel  to  be  injected  from  the  sur- 
rounding tissues,  pass  a  cord  under  it,  and  tie  the  cord  in  a  loose  single  knot  above  the  vessel. 
With  a  fine  scissors  make  a  V-shaped  cut  into  the  blood  vessel,  having  the  cut  extend  not  more 
than  halfway  through  the  vessel,  and  immediately  insert  the  cannula  into  the  vessel.     Tighten 
the  knot  around  the  cannula  and  blood  vessel  by  pulling  on  the  two  ends  of  the  cord,  but  make 
only  a  single  knot.     The  cannula  is  connected  with  the  injection  syringe  or  rubber  bulb  by  a 


APPENDIX  B  371 

piece  of  rubber  tubing.  The  whole  system  should  be  filled  with  the  injection  fluid  in  advance, 
in  order  to  avoid  forcing  air  into  the  blood  vessels.  After  everything  is  ready  inject  the  solution 
into  the  blood  vessel  by  a  steady  but  not  too  forceful  pressure  on  the  syringe  or  bulb.  The  suc- 
cess of  the  injection  should  be  determined  by  examining  the  small  vessels  of  the  skin  or  in  the 
intestinal  walls.  When  these  are  deeply  colored  the  injection  is  complete  and  the  cannula  is 
then  withdrawn,  the  cord  being  immediately  tightened  around  the  blood  vessel  and  tied  with  a 
double  knot  to  prevent  leakage.  In  case  only  the  arterial  system  is  injected,  as  is  usually  the 
case,  the  blood  should  not  be  let  out  of  the  animal;  the  injection  will  force  the  blood  around 
into  the  veins  and  distend  them.  If  the  systemic  veins  are  to  be  injected,  the  blood  should  be 
drained  from  the  animal  by  opening  the  vein,  at  the  place  where  it  is  to  be  injected. 
The  following  formula  for  the  injection  fluid  is  given  by  Davison: 

100  c.c.  of  water 
20  c.c.  of  glycerin 
20  c.c.  of  concentrated  formalin 
85  gms.  of  corn  starch 
sufficient  coloring  matter  to  give  a  deep  color. 

Stir  thoroughly  and  strain  through  fine  cheesecloth.  All  lumps  must  be  removed.  As 
the  starch  settles  to  the  bottom  on  standing,  the  mixture  must  be  stirred  before  using.  Color- 
ing matter  used  in  the  injection  mass  is:  for  red  color,  vermilion  (mercuric  sulphide);  for 
yellow  color,  chrome  yellow  (lead  chromate);  for  blue  color,  Prussian  blue.  The  injection 
fluid  will  keep  indefinitely. 

Injection  is  usually  made  into  the  following  vessels:  in  elasmobranchs,  the  arteries  are 
injected  by  way  of  the  caudal  artery,  exposed  by  cutting  across  the  tail;  the  hepatic  portal 
system  is  injected  by  way  of  the  hepatic  portal  vein  near  the  liver  backward  or  in  both  direc- 
tions or  into  the  posterior  mesenteric  vein  forward.  Injection  of  the  systemic  veins  is  more 
difficult  and  is  usually  not  done;  a  method  is  given  by  Rand  in  the  American  Naturalist,  Vol. 
XXXIX  (1905).  In  Necturus  the  arteries  are  most  easily  injected  by  way  of  the  bulbus 
arteriosus.  The  hepatic  portal  system  should  be  injected  in  both  directions  into  the  hepatic 
portal  vein  just  before  it  enters  the  liver.  The  postcaval  can  be  injected,  if  desired,  through 
the  large  hepatic  vein  seen  on  the  ventral  surface  of  the  liver  just  behind  the  transverse  septum. 
The  arterial  system  of  the  turtle  is  injected  either  backward  by  way  of  one  of  the  carotids  or 
into  the  left  aorta  away  from  the  heart.  It  is  practically  impossible  to  work  out  the  renal 
portal  and  hepatic  portal  systems  on  preserved  turtles  unless  they  are  injected.  Injection  is 
very  easily  made  of  both  of  these  systems  by  way  of  the  abdominal  vein.  The  plastron  is,  of 
course,  to  be  removed  by  sawing  across  the  bridges.  Insert  the  cannula  into  one  of  the  abdomi- 
nal veins  about  halfway  between  the  heart  and  pelvis  and  inject  forward.  Injection  of  the 
arterial  system  of  the  pigeon  can  be  done  by  way  of  the  pectoral  artery  toward  the  heart. 
According  to  Parker  the  systemic  veins  and  the  hepatic  portal  system  may  be  injected  by  way  of 
the  coccygeo-mesenteric  vein,  in  both  directions.  The  arterial  system  of  mammals  is  injected 
either  by  way  of  the  carotid  or  the  femoral  artery,  the  cannula  being  directed  toward  the  heart. 
The  veins  are  usually  not  injected,  but  it  is  stated  that  they  may  be  injected  by  way  of  the 
external  jugular  toward  the  heart.  The  hepatic  portal  system  may  be  injected  by  way  of  the 
hepatic  portal  vein  near  its  entrance  into  the  liver.  In  injecting  arteries  and  veins  in  the  same 
specimen,  differently  colored  injection  fluids  should,  of  course,  be  used. 

4.  Preservation  of  specimens. — Specimens  are  usually  preserved  in  5  per  cent  formalin 
(5  c.c.  of  commercial  formalin  plus  95  c.c.  of  water).  A  slit  should  be  made  in  the  body  cavity 
or  the  formalin  should  be  injected  into  the  body  cavity  through  a  small  opening  to  insure  pres- 
ervation of  the  viscera.  A  portion  of  the  roof  of  the  skull  should  be  removed  in  the  smaller 


372       LABORATORY  MANUAL  FOR  VERTEBRATE  ANATOMY 

specimens  in  order  that  the  brain  may  harden.  In  the  case  of  mammals  additional  measures 
are  necessary  in  order  to  insure  preservation.  Mammals  should  be  embalmed.  This  is  done 
by  injecting  an  embalming  fluid  into  the  blood  vessels  before  the  injection  fluid  is  sent  in.  The 
embalming  fluid  may  consist  of  5  per  cent  formalin  or  better  5  per  cent  formalin  plus  one-sixth 
its  volume  of  glycerin.  A  still  better  but  more  expensive  embalming  fluid  is  one  used  for  human 
bodies,  with  the  following  formula: 

Parts  by  Volume 

Formalin 1.5 

Carbolic  acid  (melted  crystals) 2.5 

Glycerin 10.0 

Water 86.0 

The  embalming  fluid  is  injected  preferably  into  the  femoral  artery  through  a  cannula  in  the  same 
way  as  already  described  for  the  injection  fluid.  Bensley  recommends  that  the  injection  of 
the  embalming  fluid  should  be  done,  not  with  a  syringe,  but  by  attaching  the  cannula  to  a 
receptacle  containing  the  embalming  fluid  elevated  about  three  feet  and  allowing  the  fluid  to 
run  into  the  vessel  under  this  pressure  for  about  two  hours.  The  animal  should  be  arranged 
in  a  position  suitable  for  dissection,  with  the  limbs  spread  well  apart  and  the  head  tilted  back- 
ward. After  the  embalming  the  injection  fluid  is  run  into  the  same  cannula  by  attaching  the 
syringe  to  it;  Bensley  recommends  that  twenty-four  hours  elapse  between  the  embalming  and 
the  injection  with  the  colored  fluid. 

If  the  animal  has  been  thoroughly  embalmed,  it  will  keep  without  being  immersed  in  a 
preserving  fluid.  It  should  be  prevented  from  becoming  dry  by  being  placed  in  air-tight 
receptacles  or  wrapped  in  cloths  saturated  with  the  formalin-glycerin  solution.  It  is  best  to 
sponge  the  hair  with  a  mixture  of  alcohol  and  water  containing  2  per  cent  formalin.  The 
animals  may,  however,  if  preferred,  be  immersed  in  i  per  cent  formalin  solution.  During  the 
dissection  the  animals  should  be  kept  wrapped  in  cloths  moistened  with  formalin  glycerin. 

Further  details  on  these  matters  will  be  found  in:  Bensley,  Practical  Anatomy  of  the  Rabbit 
(University  of  Toronto  Press);  Reighard  and  Jennings,  Anatomy  of  the  Cat  (Henry  Holt  and 
Company);  and  Davison,  Mammalian  Anatomy  (P.  Blakiston's  Son  and  Company). 

5.  Dealers. — Preserved  and  injected  material  necessary  in  the  course  can  be  obtained  from: 

The  General  Biological  Supply  House,  1177  East  ssth  Street,  Chicago. 

Supply  Department,  Marine  Biological  Laboratory,  Woods  Hole,  Massachusetts. 

The  Angler's  Company,  1534  West  Lake  Street,  Chicago. 

Live  Necturits,  turtles,  pigeons,  etc.,  are  also  obtainable  from  the  first  and  third  dealers 
named  above.  The  General  Biological  Supply  House  can  also  furnish  prepared  skeletons  of 
any  forms.  For  the  names  of  other  dealers  consult  Science,  Transactions  of  the  American 
Microscopical  Society,  and  similar  journals. 


INDEX 


Abdominal  pores  166 

Abdominal  vein:  development  of  203, 

220-30,  241;    dogfish   209;    mam- 
mals  271;    Necturus   223,   224-25; 

pigeon  248;  skate  an;  turtle  230- 

32,  233,  241 
Abducens  nerve:    definition   of  360; 

elasmobranch  311;    mammal  355; 

Necturus  320;    pigeon  331;    turtle 

326 

Abduction,  definition  of  131 
Accessory  mesonephric  ducts  281,  282, 

283 

Accessory  urinary  bladder  285,  286 
Acetabulum  8 1,  82,  83 
Acipenser:  classification  of  6;  external 

anatomy  ai;    scales  49;  skull  101; 

vertebral  column  63—64 
A  crania,  definition  of  8 
Acrpmion  process  93 
Action  of  muscle,  definition  of  136 
Acustic  area  354,  358 
Acustico-facial  nerve:    Necturus  321; 

turtle  3  26 
Acustico-lateral  area:   elasmobranch 


308;   Necturus  320 
Adduction,  definition  of 


Adduction,  definition  ot  131 

Afferent  branchial  arteries:  dogfish 
215;  Necturus  227;  skate  215 

Agnathostomata,  definition  of  8 

Air-sacs,  pigeon  178-81 

Allantois  163,  276,  292-94;  blood 
vessels  of  203 

Alligator:  atlas  70-71;  axis  71;  jaw 
113-14,  115;  pectoral  girdle  91; 
ribs  71-72;  skull  106,  112-16; 
sternum  91;  vertebral  column 
70-72 

Alveolus,  jaw  116,  124 

Alveolus,  lung  176,  188 

Amblystoma,  classification  of  7 

Amia:  classification  of  6;  external 
anatomy  22;  scales  49;  skull  102; 
vertebral  column  64 

Amnion  292,  293,  294 

Amniota:  definition  of  8;  embryonic 
membranes  of  292-94 

Amphibia:  blastula  32;  cleavage  32; 
development  of  32,  34,  37~30; 
exoskeleton  50;  fore  limb  90;  gas- 
trula  34;  hind  limb  81;  pectoral 
girdle  89-90;  pelvic  girdle  80-81; 
ribs  69;  sternum  86,  90;  vertebral 
column  68-69.  See  further  under 
Necturus 

Amphicoelous  vertebra  65,  67,  69,  76 

Amphioxus  9-11:  blaslula  32;  classi- 
fication of  6;  cleavage  32;  develop- 
ment of  32,  34,  36-37;  external 
anatomy  o-io;  gastrula  34;  in- 
ternal anatomy  10-11 

Amphiplatyan  vertebra  76 

Ampulla,  ear:  elasmobranchs  306; 
Necturus  319 

Ampullae  of  Lorenzini  302 

Analogy  3-4 

Anamniota,  definition  of  8 

Ankle.    See  Tarsus 

Ankylosis,  definition  of  94 

Ansa,  definition  of  335 

Ansa  subclavia  334 

Anterior  cardinal  vein:  development 
of  203;  dogfish  209;  mammal  269; 
Necturus  225,  227;  pigeon  247; 
skate  21 1 ;  turtle  240 

Anterior  chamber,  eye  305,  349 

Anterior  vena  cava.    See  Precaval 

Antibrachium  88.  See  further  under 
Fore  limb 


Anura:  classification  of  7;  coelom 
159,  196;  pectoral  girdle  90; 
sternum  90;  vertebral  column  69 

Aorta:  development  of  202-3;  mam- 
mal 248,  257,  260-64;  pigeon  244, 
245;  turtle  235.  See  further  under 
Dorsal  aorta  and  Ventral  aorta 

Aortic  arches:  development  of  202-3; 
elasmobranchs  214-18;  mammal 
267-69;  Necturus  226-27,  229; 
pigeon  247;  turtle  235-37,  240 

Aponeurosis,  definition  of  136 

Apophyses,  definition  of  62 

Aqueduct,  brain:  elasmobranchs  318; 
mammal  357;  turtle  327 

Aqueous  humor  305,  349 

Arachnoid  341,  353 

Arbor  vitae:  mammal  357;  pigeon 
333 

Arcade:  infratemporal  113;  supra- 
temporal  112 

Arch  of  aorta:  mammal  257;  pigeon 
345 

Arcualia  59-61,  69;  alligator  71; 
Stegocephala  64;  sturgeon  63-64 

Area  acustica  354 

Armadillo:  classification  of  7;  exo- 
skeleton 55 

Arterial  ligament:  mammal  260,  269; 
turtle  236,  240 

Arterial  system:  development  of 
302-3;  elasmobranchs  214-21; 
mammal  257-64;  Necturus  226-28; 
pigeon  244-46;  turtle  235-38 

Artery,  definition  of  200 

Arytenoid  cartilages:  mammal  187; 
Necturus  171;  pigeon  178;  turtle 
175, 176 

Ascending  vena  cava.    See  Postcaval 

Astragalus  84 

Atlas  6p,  70,  71,  73-74 

Auditory  nerve,  360:  elasmobranchs 
314;  mammal  355;  Necturus  321; 
pigeon  332;  turtle  326 

Auditory  tube  162.  301:  mammal  186; 
pigeon  177;  turtle  175.  324 

Auricle:  elasmobranch  207,  221; 
mammal  248,  265-66;  Necturus, 
233,  229;  pigeon  241,  246;  turtle 
230,  238 

Auricular  appendage  248 

Axes  of  body  i 

Axilla  135 

Axillary  artery:  cat  259;  pigeon  244; 
rabbit  258;  turtle  235-36 

Axillary  fossa  135 

Axillary  vein:  cat  254-55;  rabbit 
252;  turtle  234 

Axis,  vertebral  column  70,  71,  74 

Azygos  vein:  cat  253;  development 
of  269;  rabbit  251 

Balanoglossus:  classification  of  6; 
external  anatomy  14 

Basal  plate,  of  skull  97:  elasmo- 
branchs 98;  Necturus  ill 

Basalia:  pectoral  fins  89;  pelvic  fins 
80 

Basals.    See  Basalia 

Basidorsal,  definition  of  59,  60 

Basiventral,  definition  of  59,  60 

Bicuspid  valve  266 

Bilateral  symmetry  a 

Bile  duct  163:  elasmobranch  164; 
mammal  191,  192;  Necturus  170; 
pigeon  182;  turtle  174 

Birds:  exoskeleton  25,  52-54;  ex- 
ternal anatomy  25-27;  fore  limb 
02;  hind  limb  83;  pectoral  girdle 


QI;    pelvic  girdle  82-83;     sternum 

91-92;     vertebral    column    7*~73- 

See  further  under  Pigeon 
Blastocoel  32 
Blastoderm  34,  36 
Blastopore  34,  36 
Blastula  32:   amniotes  34;   Amphibia 

32;  Amphioxus  32 

Blood:  definition  of  200;  origin  of  201 
Blood-vascular  system  200 
Body  cavity.    Set  Coelom 
Bone  58 

Bony  labyrinth  352 
Bowfin.    See  Amia 
Bowman's  capsule  275 
Brachial  artery:    cat  asp;    Necturus 

228;  pigeon  244;  rabbit  258 
Brachial   plexus:    definition  of   299; 

dogfish    301-2;     mammal   335-37; 

Necturus  319;    pigeon  328;    skate 

302;  turtle  322 
Brachial  vein:  cat  255;  Necturus  aas; 

pigeon  243;  rabbit  252;  skate  an; 

turtle  234 
Brachiocephalic  artery:    cat  258-60; 

pigeon     244-45;      rabbit     257-58. 

turtle  235-36 

Brachiocephalic  vein:  cat  «54-55 
Brachium  87.    See  under  Fore  limb 
Brain:  definition  of  296;  development 

of  296-97;    elasmobranchs  307-9, 

functions  of  358-59;  mammal  352- 

S9',  Necturus  319-21;  parts  of  296- 

07,  359-6o;  pigeon  330-31, 332-331 

turtle  324-25,  327 
Branchia.    See  Gill 
Branchiostegal  membrane  30,  21,  22 
Branchiostegal  rays  20 
Breastbone.    See  Sternum 
Bronchus:  mammal  189,  266;  pigeon 

183;  turtle  176 
Buccal    nerve:     elasmobranchs    312, 

313;  Necturus  321 
Bulbourethral  gland  291 
Bulbus  arteriosus:    definition  of  226; 

Necturus  222,  226 
Bursa  of  Fabricius  287 

Caecum:  definition  of  161;  mammal 
J03;  pigeon  182;  turtle  174 

Calcaneus  84 

Canals  of  lateral  line  system:  elasmo 
branchs  302-3;  Necturus  319 

Canals  of  Lorenzini  302 

Canine  teeth  125 

Capillary,  definition  of  200 

Capitulum:  of  humerus  93;  of  rib  6y, 
7i,  75  , 

Capsules  of  sense  organs  97,  104-5 

Carapace  25,  51 

Cardia  190 

Cardiac  nerve  334,  337 

Cardiac  plexus  337 

Carinagi,  92 

Carotid  artery:  elasmobranchs  217- 
18;  mammal  259-60;  Necturus  227; 
pigeon  244-45;  turtle  235-36 

Carpales  88 

Carpus  88:  bird  92;  mammal  94; 
turtle  91 

Cartilage  bone  57-58 

Cartilage  bones  of  skull  103-5 

Cat:  arterial  system  257.  258-60, 
262-64;  brain  352-59;  circulatory 
system  248-66;  classification  of  7; 
coelom  187-90,  196-98;  cranial 
nerves  334,  337,  33^,  342-48; 
digestive  tract  183-86,  180-94;  ex- 
ternal anatomy  27-29;  fore  limb 


373 


374 


LABORATORY  MANUAL  FOR  VERTEBRATE  ANATOMY 


93-94;  heart  188,  248,  265-66;  hind 
limb  84-85;  hyoid  apparatus  126, 
186;  jaw  118-19, 124;  muscles  136- 
57:  nervous  system  333-59;  pec- 
toral girdle  92-93;  pelvic  girdle  83- 
84;  respiratory  system  186-89;  ribs 
75;  sense  organs  345-52;  skull  116- 
26;  spinal  nerves  334-42;  sternum 
93;  sympathetic  system  334~45, 
337-39;  teeth  124-25;  urogenital 
system  288-92;  venous  system  250- 
Si,  253-57,  265;  vertebral  col- 
umn 73-76 

Caval  fold  188 

Cavernous  bodies.  See  Corpora 
cavernosa 

Central  nervous  system:  definition  of 
296;  development  of  296-97; 
elasmobranchs  307-9;  functional 
parts  of  297-98;  mammal  341, 
352-59;  Neclurus  319-21;  pigeon 
330-31,  332-33;  turtle  324-25,  327 

Centrales  79,  88.     See  under  Limbs 

Centrum :  formation  of  61 ;  types  of  76 

Cephalization  3 

Cere  25 

Cerebellar  fossa  121,  351 

Cerebellum:  elasmobranchs  308; 
mammal  353,  357;  Neclurus  320; 
pigeon  331,  333;  turtle  324 

Cerebral  fossa  121,  351 

Cerebral  hemisphere:  elasmobranchs 
307,  318;  mammal  353,  355-56, 
357-58;  Neclurus  320;  pigeon  331. 
332,  333;  turtle  324,  327 

Cerebral  peduncle:  elasmobranch  318; 
mammal  3  54;  Neclurus  321 

Cerebrospinal  fluid  341,  353 

Cervical  plexus,  mammal  334 

Cervicobrachial  plexus:  dogfish 
301-2;  skate  302 

Chevron  bones:  alligator  70;  mam- 
mal 76 

Choana:  definition  of  118;  mammal 
1 86 

Chondrocranium:  bones  derived  from 
103-5;  development  of  96-97; 
dogfish  97-100;  Neclurus  109,  in 

Chordae  tendinae:  mammal  266; 
pigeon  247 

Chordala:  characters  of  5;  classifica- 
tion of  6-7;  embryonic  develop- 
ment of  31-44 

Chordocentrous  vertebra  65 

Chorioid  coat,  eye:  development  300; 
elasmobranch  305;  mammal  349; 
pigeon  329-30 

Chorioid  plexus,  brain:  elasmobranchs 
308;  mammal  353,  356;  Neclurus 
320;  turtle  324 

Chprion  292-94 

Ciliary  body:    mammal  349;    pigeon 


,3.3° 
ilia 


Ciliary  ganglion:    cat   348;    elasmo- 
branch 311 
Ciliary  nerves:  cat  348;  elasmobranch 

~3" 
Ciona  12-13 

Circulation  through  heart:  elasmo- 
branch 221-22;  mammal  266-67; 
Neclurus  229;  pigeon  247;  turtle 
239-40 

Circulatory  system  200-272:  develop- 
ment 201-7;  elasmobranchs  207- 
22;  mammal  248-72;  Neclurus 
222-30;  parts  of  200-201;  pigeon 
241-48;  summary  270-72;  turtle 

Cladoselache  79 

Classification  of  chordates  6-8 

Clava  354,  358 

Clavicle  87,  88;    bird  91;    frog  90; 

mammal  92 
Cleavage  32-34 
Cleithrum  87,  88 

Clitoris:  mammal  289,  290;  turtle  285 
Cloaca  161,  279-80:  dogfish  165.  281; 

mammals    279—80;     Neclurus    169, 

284;    pigeon  182,  287;    skate  165, 

282:  turtle  175,  285-86 
Coccyx:  frog  69;  man  76 


Cochlea  352 

Cochlear  duct  352 

Coeliac  artery:  cat  262;  dogfish  219, 
220;  pigeon  245;  rabbit  260-61; 
skate  220;  turtle  236 

Coeliac  ganglion  338 

Coeliac  plexus:  mammal  338;  pigeon 
328 

Coelom  158-59,  195-98:  definition  of 
158;  development  of  37-38,  39-40, 
44;  elasmobranchs  163-64,  166-67; 
mammal  187-90,  197;  Necturus 
168-69,  170-71;  pigeon  180-83, 
197;  summary  195-99;  turtle  172- 

Coelomoduct*273 

Collector  nerve  301 

Colliculus  353 

Colon:    definition  of   161;    mammal 

193-94;  turtle  174 
Columella:   pigeon  330;   turtle  324 
Columnae  carnae  247 
Common  cardinal  vein:   development 

of  203;   dogfish  207,  209;   mammal 

269;     Necturus    222,    225;     pigeon 

2^7;   skate  210:   turtle  240 
Common    carotid    artery:     mammal 

257,  259-60;   pigeon  244-45 
Common   iliac    artery:     rabbit    263; 

turtle  237-38 

Common  iliac  vein:  cat  265 
Comparisons,     circulatory      system: 

mammal  266-70;   Necturus  229-30; 

pigeon  247-48;   turtle  240-41 
Concha  105,  300;    mammal  121-22, 

350;   pigeon  329;  turtle  323 
Condyle:    of  limbs  83,  84;    of  skull 

99, 109, 114, 117 
Condyloid  process,  jaw  1 24 
Conjunctiva:       elasmobranch      304; 

mammal  345,  349;   pigeon  329 
Contour  feather  25,  52-54 
Conus  arteriosus:   elasmobranch  207, 

221;  Necturus  222,  229;  turtle  230, 

Convergence  4 

Coprodaeum  287 

Coracoid  86.     See  Pectoral  girdle 

Coracoid  process  86,  92 

Corium  45,  46 

Cornea:  development  of  300;  elasmo- 
branch 305;  mammal  349;  pigeon 
329 

Coronary  artery,  of  heart:  elasmo- 
branch 215,  216—17;  mammal  257; 
turtle  235 

Coronary  ligament  160,  166,  195-96; 
elasmobranch  166;  mammal  192; 
Necturus  170;  pigeon  181;  turtle 

Coronary  sinus  253,  265 

Coronary  vein:   cat  253;   rabbit  251 

Coronoid   process:     of   jaw    124;     of 

ulna  93 
Corpora    bigemina    308.    See    Optic 

lobes 
Corpora  cavernosa  295;    turtle  285; 

mammal  291-92 
Corpora  lutea  289 
Corpora  quadrugemina  353,  360 
Corpus  callosum  353,  355-56 
Corpus     striatum:      function,     333; 

mammal  358;    pigeon  333;    turtle 

327 

Corpuscles  200 
Cortex,  brain  357 
Cotylosauria:   classification  of  7;   jaw 

108;    pectoral  girdle  87,  88;    skull 

106, 112 

Cowper's  gland  291 
Cranial    nerves    296,    299,    360-61: 

elasmobranch  309— 15;  mammal  334, 

337-    338,    342-45,     346-47,    348; 

Necturus  320-21;    pigeon   331-32; 

turtle  325-27 
Cranium.     See  Skull 
Cribriform  plate  121,  350 
Cricoid     cartilage:      mammal     187; 

pigeon  178;  turtle  17$ 
Crista  306 
Crop  161,  178 


Crossoplerygii:  classification  of  6; 
fins  79,  89;  ribs  67;  skull  toa 

Crura:  of  diaphragm  338;  of  penis 
292 

Cryptobranchus:  classification  of  7; 
fore  limb  90;  hind  limb  8 1 ;  pectoral 
girdle  89-90;  pelvic  girdle  80-81; 
vertebral  column  68-69 

Ctenoid  scale  49,  50 

Cycloid  scale  49,  50 

Cyclostomata:  anatomy  14-16;  classi- 
fication of  6 

Cystic  duct  191-92 

Delamination  36 

Dental  formula  125 

Dentine:  scales  47;  teeth  49 

Depressor,  definition  of  133,  141 

Dermal  bone  58.  See  Membrane 
bone 

Dermal  exoskeleton,  definition  of  47 

Dermal  papilla:  for  feather  52-53; 
for  hair  55;  for  placoid  scale  47 

Dermatome  43,  45 

Dermis  45,  46:  in  formation  of 
feathers  52-54;  in  formation  of  fish 
scales  47,  49,  50;  in  formation  of 
hair  55;  in  formation  of  reptile 
plates  50 

Descending  vena  cava.    See  Precaval 

Descent  of  testes,  290,  291 

Development  of  chordates  31-44 

Diaphragm  159,  188,  189,  197 

Diastema  125 

Diencephalon  297,  359-60;  elasmo- 
branch 307-8,  317,  318;  mammal 
353,  354,  356,  357,  35^;  Necturus, 
320,  321;  pigeon  331,  333;  turtle 
324,  327 

Digestive  system  159-99:  elasmo- 
branch 164-66,  167-68;  genera! 
159-63;  mammal  183-86,  189-94; 
Necturus  160-70;  parts  of  161-6,3; 
pigeon  176-77,  180,  181-82;  sum- 
mary 198-99:  turtle  173-75 

Digitigrade  walk  28 

Diphycercal  tail  23 

Diplospondyly  61,  64 

Divergence  4 

Dog:  skull  106 

Dogfish:  arterial  system  214-21; 
brain  307-9,  317-18;  chondro- 
cranium  97-99;  classification  of  6; 
coelom  163-64,  166-67;  cranial 
nerves  300-15;  digestive  system 
164-66,  167-68;  exoskeleton  48; 
external  anatomy  16-17;  fins  17, 
80,89;  gills  1 68;  heart  207,  221-22; 
mesenteries  165-66;  muscles  128- 
30;  myotomes  64,  129;  nervous 
system  301-18;  pectoral  fin  89; 
pectoral  girdle  88-89;  pelvic  fin 
80;  pelvic  girdle  79-80;  respiratory 
system  167-68;  ribs  66;  spinal 
nerves  301-2;  urogenital  system 
280-83;  venous  system  207-10; 
212-14;  vertebral  column  64-66; 
visceral  skeleton  99-100 

Dorsal  aorta  202:  elasmobranchs 
218-21;  mammal  260-64;  Necturus 
227-28;  pigeon  244-46;  turtle 
237-38 

Dorsal  column,  spinal  cord  297 

Dorsal  ramus,  spinal  nerve  299 

Down  feather  52-53 

Duct  of  Cuvier  203.  See  Common 
cardinal  vein 

Duodenum:  elasmobranch  164;  mam- 
mal 192-93;  Necturus  169;  pigeon 
181-82;  turtle  174 

Dura  mater:  mammal  341,  352, 
pigeon  330,  turtle  324 

Ear  30,  300-301,  361 :  development  of 
300-301;  elasmobranch  305-6; 
functions  of  306;  lizard  23;  mam- 
mal 27,  350-52;  Necturus  22,  319; 
parts  of  30,  300-301,  361;  pigeon 
25-26,  330;  teleost  20;  turtle  24, 
323-24.  336 


INDEX 


375 


Rar  bones  127,  301:  mammal  120, 
35L  35*;  Necturus  no;  pigeon 
330;  turtle  324 

Ear  drum  162.  See  Tympanic  mem- 
brane 

Ectoderm:  definition  of  34;  deriva- 
tives of  41 

Efferent  branchial  arteries:  elasmo- 
branchs  216-18;  Necturus  227 

Egg:  cleavage  of  32;  kinds  of  31 

Elasmobranchii:  chondrocranium  97- 
100;  circulatory  system  207-22; 
classification  of  6;  coelom  163-64, 
166-67;  digestive  system  164-66, 
167-68;  exoskeleton  47-48;  exter- 
nal anatomy  16-19;  mesenteries 
165-66;  nervous  system  301-18; 
pectoral  fin  89;  pectoral  girdle 
88-8y;  pelvic  fin  80;  pelvic  girdle 
70-80;  respiratory  system  167-68; 
urogenital  system  280-83;  verte- 
bral column  64-66;  visceral  skele- 
ton 90-100 

Elbow  birds  92;  mammals  93 

Elevator,  definition  of  133,  140 

Embryo:   cat  294;   dogfish  294 

Embryology  31-43:  amniotes  34,  36, 
37-39;  Amphibia  32,  34,  35,  37-30'. 
Amphioxus  32,  34,  36 

Embryonic  membranes  292-94 

Enamel:  scale  47,  teeth  49 

Endolymphatic  ducts  17,  98,  306 

Endolymphatic  fossa  98 

Endoskeleton  57-127:  chondrocrani- 
um 96-99;  definition  of  45,  57; 
gill  arches  99-100;  limbs  78-94; 
parts  of  57;  pectoral  girdle  85-94; 
pelvic  girdle  78-85;  ribs  57-77; 
skull  96-127;  sternum  85-94;  ver- 
tebral column  57-66;  visceral 
skeleton  99-100 

Endostyle  n.  13,  161 

Entoderm:  definition  of  34;  deriva- 
tives of  41-42 

Entoglossal  cartilage  177 

Epicondyles  84,  93 

Epicoracoid  87 

Epidermis  45:  in  formation  of 
feathers  52-53,  54;  of  hair  55;  of 
placoid  scales  47;  of  reptilian  scales 
5° 

Epididymis  276;  elasmobranch  283; 
mammal  292;  turtle  286 

Epiglottis  186 

Epimere  39,  41.  43 

Epiphysis  297,  360:  elasmobranch 
318;  mammal  356;  Necturus  320; 
pigeon  331;  turtle  324 

Episternum  87.     See  Pectoral  girdle 

Epistropheus.    See  Axis 

Epithalamus  359,  360:  elasmobranch 
318;  mammal  356;  turtle  327 

Epithelial  bodies  162 

Esophagus  161:  elasmobranch  167; 
mammal  187,  189;  Necturus  171; 
pigeon  177;  turtle  175 

Ethmoid  plate  97,  104,  109,  in 

Ethmoid  region  of  skull  104:  alli- 
gator 115;  mammal  120-22,  350; 
Necturus  in 

Eustachian  tube.    See  Auditory  tube 

Exoskeleton  45-56;  Amphibia  50; 
birds  52-54;  definition  of  45; 
development  of  47;  fishes  47-50; 
mammals  54-55;  reptiles  50-52; 
summary  55-56;  turtle  51-52 

Extension,  definition  of  131 

External  anatomy  9-30:  Amia  22; 
Amphioxus  o-io;  Balanoglossus  14; 
cyclostomes  14-15;  dogfish  16-18; 
elasmobranchs  16-19;  ganoids  21; 
gar  pike  21 ;  lamprey  14-15;  lizard 
23-24;  mammal  27-29;  Necturus 
22-23;  pigeon  25-27;  skate  18-19; 
spoonbill  21 ;  sturgeon  21;  sum- 
mary 29-30;  teleost  19-21;  tuni- 
cate 11-12;  turtle  24-25 

External  auditory  meatusi62  ,  301: 
alligator  112;  mammal  27, 117,351; 
pigeon  25,  330 


External  carotid  artery:  elasmo- 
branch 217;  mammal  259-60;  Nec- 
turus 227;  pigeon  245 

External  ear  30,  301,  361:  lizard 
23;  mammal  27,  351;  pigeon 
25-26.  330 

External  genitalia  29,  289,  290 

External  iliac  artery:  mammal  263- 
64;  turtle  238 

External  iliac  vein:   mammal  264-65 

External  jugular  vein:  cat  254; 
Necturus  225;  rabbit  252;  turtle 
234 

Eye:  development  of  300,  361; 
elasmobranch  304-5;  mammal 
345-491  pigeon  329-30;  turtle  323 

Eyeball:  elasmobranch  304-5;  mam- 
mal 349;  pigeon  329-30;  turtle 
323 

Eyelids:  dogfish  17;  lizard  23;  mam- 
mals 27;  muscles  of,  in  mammals 
346,  347;  pigeon  25;  turtle  24 

Eye  muscles  299,  311:  cat  347-48; 
elasmobranch  304-5;  pigeon  329; 
rabbit  346-47;  turtle  323 

Facet  71,  74,  75 

Facial  nerve  313,  360:  elasmobranch 
313;  mammal  343-44,  355;  Nec- 
turus 321;  pigeon  332;  turtle  326, 
327 

Falciform  ligament  196:  elasmo- 
branch 166;  mammal  192;  Nec- 
turus 170;  pigeon  180;  turtle  173 

Fallopian  tube.    See  Uterine  tube 

Fascia  130,  134,  136 

Fasciculus  135-36 

Feathers  25-26,  52-54 

Femoral  artery:  mammal  264;  Nec- 
lurus  228;  pigeon  246 

Femoral  vein:  mammal  265;  pigeon 
243;  turtle  231 

Femur  79.    See  Hind  limb 

Fenestra,  of  skull  97 

Fibula  79.    See  Hind  limb 

Fibulare  79.    See  Hind  limb 

Filiform  papilla  185 

Filoplume  25,  52,  54 

Fimbria  357,  358 

Fin-fold  theory  78-79 

Fin  rays:  elasmobranch  18,  80; 
teleost  20 

Fins:  Amphioxus  9;  cyclostomes  14; 
dogfish  17-18,  80,  89;  ganoid  89; 
Necturus  23;  skate  18-19;  teleost  20 

Fishes:  classification  of  6;  exoskele- 
ton 47-50;  external  anatomy  16-21; 
fins  17-18,  20;  ribs  66,  67-68; 
scales  47-50;  vertebral  column 
64-68.  See  further  under  Elasmo- 
branchii, Ganoid,  and  Teleost 

Flexion,  definition  of  131 

Flexures  of  brain,  pigeon  330-31 

Floccular  fossa  351 

Flocculus  351 

Foliate  papilla  185 

Follicle:  feather  25,  52;  hair  54 

Fontanelle  97,  98 

Foramen  epiploicum  192 

Foramen  magnum  98, 109.  See  under 
Skull 

Foramina  of  skull:  cat  122-23; 
rabbit  123-24 

Fore  limb  87-88:  bird  92;  mammal 
93-94;  turtle  91;  uTodeles  90 

Forebrain.    See  Prosencephalon 

Form,  principle*  of  1-3 

Fornix  356,  358 

Fossa  rhomboidea  318.  See  under 
Medulla  oblongata 

Frenulum  185 

Frog:  pectoral  girdle  90;  sternum  90; 
vertebral  column  69 

Fungiform  papilla  185 

Funiculi,  of  spinal  cord  341-42 

Furcula  01 

Gall  bladder  163:  elasmobranch  164; 
mammal  191-92;  Necturus  170; 
turtle  174 


Ganglion,  definition  of  296 

Ganoid  fishes:  classification  of  6; 
exoskeleton  49;  external  anatomy 
21-22;  limbs  from  fins  79;  pectoral 
fin  89;  skull  101-3;  vertebral 
column  63-64 

Ganoid  scales  21,  49 

Ganoin  21,  49 

Gar  pike.    See  Lepidosteus 

Gasserian  ganglion.  See  Semilunar 
ganglion 

Gastric  plexus  338 

Gastrocentrous  vertebra  71 

Gastrocoel  34:  amniotes  35;  Am- 
phibia 3<>;  Amphioxus  34 

Gastrula  34:  amniotes  36;  Amphibia 
34;  Amphioxus  34 

Geniculate  bodies  356,  358 

Geniculate  ganglion  313 

Germinal  disk  31 

Germ  layers  34,  36;  development  of 
36-44 

Gill  161:  elasmobranch  168;  Nec- 
turus 22;  teleost  20 

Gill  arch  99-100,  105,  107,  126-27: 
alligator  113,  115-16;  cartilage 
bones  from  105;  dogfish  99-100; 
mammal  126;  membrane  bones  to 
107;  Necturus  111-12;  nerves  315; 
pigeon  177-78;  turtle  115-16,  175 

Gill  slits  5,  161:  Amphioxus  10-11; 
Balanoglossus  14;  dogfish  17,  168; 
lamprey  15;  Necturus  22,  171; 
skate  19,  168;  teleost  20 

Gizzard  180,  181,  182 

Glans,  of  penis:  mammal  291; 
turtle  286 

Glenoid  fossa  90.    See  Pectoral  girdle 

Glomerulus  273,  274,  275 

Glossopharyngeal  nerve  360:  elasmo- 
branch 314;  mammal  343,  355; 
Necturus  321;  pigeon  332;  turtle 
326, 327 

Glottis:  mammal  186;  Necturus  171: 
pigeon  177;  turtle  175 

Gnathustomata,  definition  of  8 

Gonad  276-78 

Graafian  follicles  289 

Gray  matter  296,  297,  342 

Gubernaculum  292 

Gyrus  353 

Habenula  318.  356,  358 

Haemal  arch  60,  61.    See  Vertebral 

column 
Haemal  canal  60,  65.    See  Vertebral 

column 
Haemal  spine  60,  61.    See  Vertebral 

column 
Hair  27,  54-55 
Harderian  gland:   mammal  345,  346, 

348;  pigeon  329;  turtle  323 
Haustra  193 
Head  3,  29,  299-300;    segmentation 

of  299-300 

Heart:  development  of  201-2;  elas- 
mobranch 166-67,  207,  221-22; 

mammal  187,  188,  248-49,  265-67; 

Necturus     170-71,     222,     228-25; 

pigeon  1 80,  241-42,  246-47;   turtle 

172,  230,  238-39 
Hepatic    portal    system     204,     205, 

270:    development  of  204-5;    dog- 
fish    212-13;      mammal     240-51; 

Necturus   223;    pigeon  242;    skate 

213;  turtle  232-33 
Hepatic  portal  vein  204:   cat  250-51; 

dogfish     212-13;      Necturus     223; 

pigeon  242,  rabbit  249-50;    skate 

213;  turtle  232-33 
Hepatic    veins     204:     dogfish    208; 

mammal  255-56;  Necturus  222,  226; 

pigeon  243;  skate  210;  turtle  234 
Heterocercal  tail  17 
Heterocoelous  vertebra  72 
Heterodont  124 
Heteronomy  2 
Hilus  288 
Hind  limb  79:    birds  83;    mammals 

84-85;    turtle  81-82;   urodele  81 


376 


LABORATORY  MANUAL  FOR  VERTEBRATE  ANATOMY 


Hindbrain.    See  Rhombencephalon 

Hippocampus,  357,  3S» 

Holoblastic  cleavage  32 

Homocercal  tail  20,  68 

Homodont  116 

Homology  3 

Homonomy  2 

Humerus  88.    See  Fore  limb 

Hyoid     apparatus:      alligator     115; 

mammal     126,     186-87;      pigeon 

177-78;  turtle  115-16,  175 
Hyoid  arch  100,  105:    bones  derived 

from     105;      elasmobranch     100; 

Neclurus  in-ia 
Hyoidean  artery  217-18 
Hyomandibular  100,  105;    as  stapes 

1 20 

Hyomandibular  nerve  307,  313 
Hypobranchial  nerve  316 
Hypogastric  vein.    See  Internal  iliac 

vein 
Hypoglossal  nerve  361:  mammal  334, 

342-43,  355;    pigeon  332;    turtle 

326,  327 

Hypomere  39,  41;  derivatives  of  44 
Hypophysis  161,  359;    elasmobranch 

317;    mammal  351,  354;    Necturus 

321;  turtle  327 
Hypothalamus    359:      elasmobranch 

318;   mammal  356;   turtle  327 
Hypural  bone  68 

Ichthyopsida,  definition  of  8 

Ileocolic  valve  194 

Ileum  161,  193 

Iliac  artery:  Necturus  228;  turtle 
237-3.8 

Iliac  vein:  dogfish  209;  pigeon  243; 
skate  21 1 ;  turtle  231-32 

Ilium  79.    See  Pelvic  girdle 

Incisor  125 

Inferior  cervical  ganglion:  mammal 
334;  turtle  322 

Inferior  lobes,  of  brain  317 

Inferior  mesenteric  artery:  dogfish 
219;  mammal  263;  pigeon  245, 
246;  skate  220;  turtle  237 

Inferior  mesenteric  ganglion  338 

Inferior  mesenteric  vein:  cat  250; 
pigeon  242,  243;  rabbit  249 

Infra-orbital  gland  184,  345,  346,  348 

Infra-orbital  nerve  305,  312 

Infraspinous  fossa  93 

Infra  temporal  fossa  112 

Infundibulum,  of  brain  359:  elasmo- 
branch 3 17;  mammal  3 54;  Necturus 
321;  pigeon  332;  turtle  327 

Inguinal  canal  290 

Inguinal  ligament  135 

Inguinal  region  135 

Innominate  artery.  See  Brachio- 
cephalic  artery 

Innominate  bone:  birds  82;  mammals 
83 

Insertion,  of  muscle  131,  136 

Interauricular  septum:  mammal  265; 
Necturus  229;  pigeon  246;  turtle 
238 

Intercalary  arch  61,  65 

Interclavicle  87,  88.  See  Pectoral 
girdle 

Interdorsal  59,  60 

Interhaemal  arch  61 

Intermedium  79,  88.  See  under  Limbs 

Internal  carotid  artery:  mammal 
259,354;  Necturus  227;  pigeon  245 

Internal  ear  300-301,361:  develop- 
ment of  300-301;  elasmobranch 
305-6;  function  306;  mammal 
352;  Neclurus  319;  pigeon  330; 
.  turtle  324,  326 

Internal  iliac  artery:  mammal  263-64; 
pigeon  246;  turtle  238 

Internal  iliac  vein:  mammal  264-65; 
pigeon  243;  turtle  232 

Internal  jugular  vein  271;  cat  255; 
Necturus  225,  227;  rabbit  252,  253; 
turtle  234 

Interorbital  septum:  pigeon  329; 
turtle  323 


Interventral  59,  60 

Interventricular  septum:  pigeon  246; 
turtle  239;  mammal  266 

Intervertebral  foramen  76 

Intestine  161, 163:  elasmobranch  164- 
65;  mammal  192-94;  Necturus 
169-70;  pigeon  181-82;  turtle 
174-75 

Intratarsal  joint:  birds  83;  reptiles  82 

Invagination,  definition  of  34,  36 

Involution,  definition  of  36 

Iris  300:  development  of  300;  elasmo- 
branch 305;  mammal  349;  pigeon 

Ischium  79.    See  Pelvic  girdle 
Isolecithal,  definition  of  31 
Isthmus,  of  fauces  186 

Jaw  105-7:  alligator  113,  115;  dog- 
fish 99-100;  mammal  118-19,  124; 
membrane  bones  of  107,  108; 
Necturus  no,  in 

Jugular  ganglion  33  2 
ugular  vein:    Necturus  225;    pigeon 
242-43 

Kidneys  273-76,  294;  development  of 
273;  dogfish  208,  280-81;  mammal 
ipo,  288;  Neclurus  169,  223,  284; 
pigeon  243,  287;  skate  211,  282; 
turtle  232,  285 

Kneecap.    See  Patella 

Lacrimal  gland:  cat  348;  pigeon  329; 

rabbit  346;  turtle  323 
Lamprey:  classification  of  6;  external 

anatomy  14-15;    internal  anatomy 

15-16 

Larynx  199:  cartilages  of  105;  mam- 
mal 186-87;  Neclurus  171;  pigeon 

178;  turtle  175-76 
Lateral  column,  spinal  cord  297 
Lateral  line  canals:    dogfish   302-3; 

skate  303;  Neclurus  319 
Lateral  line  nerve:  elasmobranch  314; 

Neclurus  321 
Lateral  line  system:    dogfish  302-3; 

Neclurus  319;    skate  303 
Lens:    development  of  300;    elasmo- 
branch 305;   mammal  349;    pigeon 

330;  turtle  323 
Lepidosleus:      classification     of     6; 

external  anatomy    21;    scales   49; 

skull  102-3 
Ligament  of  Botallus.    See  Arterial 

ligament 
Ligaments:    of  digestive  tract.    See 

Mesenteries;     of    testis    289;     of 

uterus  289 
Limbs  27-28,  79,  86,  87-88:   bird  26, 

83,   92;    endoskeleton   of    79,   86, 

87-88;   lizard  24;    mammal  27-28, 

84-85,  93-94;   Neclurus  23,  81,  90; 

origin  of  78-79;  position  of  23,  24, 

26,  27-  28;    torsion  of  23,  24,  26, 

27  28;      turtle     25,     81-82,     91; 

urodeles  8 1 ,  oo 
Linea  alba  1 29 
Liver  163,  195-96,  204:  development 

of  163,  195-96,  elasmobranch  164; 

mammal    ipo,    191-92;     Necturus 

169,  170;  pigeon  181;  turtle  173 
Lizard:    classification  of  7;    external 

anatomy  23-24;  jaw  108;  pectoral 

girdle  88;  scales  23,  50-51 
Lorenzini:  ampullae  302;  canals  302 
Lumbosacral  plexus:    dogfish  301-2; 

mammal    339-40;     Neclurus    319; 

pigeon  328;  skate  302;  turtle  322 
Lung  162-63,  196-97,  199:   mammal 

188,  189;    Neclurus  169,  170,  171; 

pigeon  183;   turtle  176 
Lymph  201 

Lymphatic  system  200-201 
Lymph  glands  200;  of  intestine  194 
Lymph  hearts  200 
Lymph  nodules  194 
Lymphocytes  200 


Malleolus  83,  84 

Malpighian  body  275 

Mammalia:  circulatory  system  248- 
70;  coelom  159,  187-90,  190-98; 
digestive  system  183-86,  189- 
94;  exoskeleton  54-55;  external 
anatomy  27-29;  fore  limb  93-94; 
hind  limb  84-85;  muscular  system 
133-57;  nervous  system  333-59; 
pectoral  girdle  88,  92-93;  pelvic 
girdle  83-84;  respiratory  system 
186-88;  ribs  75;  skull  116-26; 
sternum  93;  urogenital  system  288- 
92;  vertebral  column  73-76.  See 
also  under  Cat  and  Rabbit 

Mammary  glands  135 

Mammillary  body:  elasmobranch 
318;  mammal  354,  358 

Mandible.    See  Jaw 

Mandibular  arch  99-100,  ios.  107 

Mandibular  foramen  124 

Mandibular  fossa  118 

Mandibular  nerve:  elasmobranch 
312;  mammal  344-45;  Necturus 
320-21;  pigeon  331-32;  turtle 
325, 326 

Marsupialia:  classification  of  7; 
oviducts  278-79 

Maxillary  nerve:  elasmobranch  312; 
mammal  345,  346,  348;  Neclurus 
320-21;  pigeon  331-32;  turtle 
325,  326 

Meatus,  of  nose  350 

Meckel's  cartilage  99,  105,  107;  alli- 
gator 115;  elasmobranch  99-100; 
Neclurus  in 

Mediastinal  septum  188,  197 

Mediastinum  188,  189 

Medulla  oblongata  297,  360:  elasmo- 
branch 308-9,  317-18;  mamm;il 
353-54;  Neclurus  320;  pigeon  331; 
turtle  324-25 

Medullary  velum  353,  357 

Membrane  bones:  definition  of  57-58; 
of  ganoids  101-3;  of  pectoral  girdle 
87-88;  of  skull  100-103,  105-7. 
See  also  under  Skull  and  Pectoral 
girdle 

Membranes  of  brain.     See  Meninges 

Membranous  labyrinth.  See  Internal 
ear 

Meninges:  elasmobranch  307;  mam- 
mal 341,  352-53;  Necturus  320; 
pigeon  330;  turtle  324 

Mental  foramen  124 

Meroblastic  development  31:  cleav- 
age 34;  egg  31;  gastrula  36;  later 
development  39—41 

Mesencephalon  297,  360;  elasmo- 
branch 308;  mammal 353;  Neclurus 
320;  pigeon  331;  turtle  324 

Mesenchyme:  definition  of  42;  prod- 
ucts of  44 

Mesenteries  158  166;  definition  of 
158,166;  development  of  39, 40, 43; 
elasmobranchs  165-66;  mammal 
190-94;  Neclurus  169-70;  pigeon 
180-82;  turtle  173-76 

Mesoblastic  somite  39,  41 

Mesocardia  158,  202 

Mesoderm:  development  of  36,  37- 
38,  39, 41-44 

Mesomere:  derivatives  of  43-44; 
development  of  39,  41,  43~44;  in 
formation  of  kidneys  273-76 

Mesonephric  duct  275,  276,  295- 
dogfish  281,  283;  mammal  292; 
Necturus  284;  pigeon  287;  skate 
282,  283;  turtle  286 

Mesonephros  274-75,  295'  dogfish 
280;  mammal  292;  Neclurus  284; 
skate  282;  turtle  286 

Mesopterygium  89 

Metacarpals  88.     See  Fore  limb 

Metacromion  process  93 

Metamere  3 

Metamerism  2 

Metanephric  duct.    See  Ureter 

Metanephros  276,  294:  mammal  288; 
pigeon  287;  turtle  285.  286 


INDEX 


377 


Metapterygium  80.  89 
Metatarsals  79.     See  Hind  limb 
Metencephalon  297.     See  Cerebellum 
Midbrain.     See  Mesencephalon 
Middle   cervical  ganglion:     mammal 

334;  turtle  322 

Middle  ear  30, 301 ,  361 :  development 

of  301;  lizard  23;  mammal  351-52; 

pigeon  25,  330;    turtle  24,  323-24. 

See  also  Tympanic  cavity 

Mitral  valve:    mammal  266;    pigeon 

246 

Molar  gland  184 
Molar  teeth  125 
Molgula  12,  13 
Monodelphia  7,  279 
Monotremata:      classification     of     7; 

oviducts  278;    pectoral  girdle  88 
Motor,  definition  of  297-98 
Mouth  cavity.    See  Oral  cavity 
MUllerian  duct.    See  Oviduct 
Muscles:     development   of   43,    128; 
epaxial  128;    hypaxial  128;    inser- 
tion 131;    involuntary  128;    origin 
131;    parietal  128;    parts  of  135; 
somatic    128;     visceral    128,    129; 
voluntary  128 

Muscles,  of  eye.  See  Eye  muscles 
Muscles,  of  mammals:  acromio- 
trapezius  147;  adductor  femoris 
154;  adductor  longus  153,  154; 
adductor  magnus  153;  anconeus 
150;  basioclavicularis  140;  biceps 
brachii  150;  biceps  femoris  151, 
153;  brachialis  150;  caudofemoralis 
153;  cephalobrachial  142;  clavo- 
brachialis  142;  clavodeltoid  140, 
148;  clavotrapezius  I42,i47;cleido- 
mastoid  140,  141;  cutaneous 
maximus  134;  deltoid  140, 145, 148; 
depressor  conchae  posterior  140; 
digastric  141,  142;  extensor  anti- 
brachii  150;  extensor  digitorum 
longus  155,  156;  extensor  hallucis 
longus  155;  external  oblique  135- 
36,  137;  flexor  digitorum  longus 
i55»  157;  gastrocnemius  155,  156; 
geniohyoid  142;  gluteus  maximus 

151,  1.53!  gluteus  medius  152,  153; 
gracilis  152,  154;    iliocostalis  146, 
149;    iliopsoas  339;    infraspinatus 
145,    148;     intercostals    146,    149; 
internal    oblique    137;      latissimus 
dorsi  136,  144,  147;  levator  scapu- 
lae 149;   levator  scapulae  ventralis 

144,  147;     longissimus    146,    149; 
masseter  140,  142;  multifidus  137; 
mylohyoid    141,    142;     panniculus 
carnosus    134;     pectpantibrachialis 
139;    pectoralis  major   139;    pec- 
toralis  minor  139;  pectoralis  primus 
139;   peroneus  155,  156;    plantaris 
155,   156;    platysma   I34~35,   UG; 
psoas     minor     339;       quadriceps 
femoris  152,  154;  rectus  abdominis 
137;      rectus    femoris    152,     154; 
rhomboideus  144, 147;  rhomboideus 
capitis  145,  147;  sacrospinalis  137; 
sartorius   152,   154;    scalenes   145, 
149;    semimembranosus  153,   154; 
semispinalis  dorsi  146,  149;    semi- 
tendinosus     153,     155;      serratus 
dorsalis  146,  149;  serratus  ventralis 

145,  148;  soleus  155,  156;  splenius 
144,     147;      spinotrapezius     147; 
sternohyoid   140,   141;    sternomas- 
toid  140,  141;    sternothyroid  141, 
142;  subscapularis  145, 148;  supra- 
spinatus  144,   148;    temporal  142; 
tensor     fasciae     latae     151,     153; 
tenuissimus  155;    teres  major  145, 
148;    teres  minor  145,  148;    thyro- 
hyoid    141,    143;     tibialis  anterior 
155,    156;     tibialis    posterior    157; 
transverse  137;   trapezius  144,  147; 
triceps  brachii  150;     vastus  inter- 
medius  152,  154;    vastus  lateralis 

152,  154;     vastus    medialis    152, 
154;  xiphihumeralis  139 


Muscular  system  128:  dogfish  128-30; 
mammal  133-57;  Necturus  130-33; 
pigeon  179-80 

Myeloncephalon  297.  See  Medulla 
oblongata 

Myocomma,  definition  of  43,  129 

Myoseptum  43,  129 

Myotome  43,  128:  Amphioxus  9; 
dogfish  129;  head  299-300;  Nec- 
turus 130 

Nares:  alligator  112,  113;  elasmo- 
branch 303;  lizard  23;  mammal  26, 
117-18;  Necturus  22,  171;  pigeon 
25,  177;  turtle  24,  175 

Nasal  cavities  300,  361:  elasmo- 
branch  303;  mammal  349-50; 
Necturus  319;  pigeon  329;  turtle 
323 

Nasopalatine  duct  185 

Nasopharynx  186 

Necturus:  arterial  system  226-28; 
chondrocranium  1 1 1 ;  circulatory 
system  222-30;  classification  of 
7;  coelom  169;  digestive  system 
168-71;  external  anatomy  22-23; 
fore  limb  90;  gill  arches  in,  112; 
heart  222,  228-29;  hind  limb  81; 
jaws  no,  in;  mesenteries  169-70; 
muscular  system  130-33;  nervous 
system  318-21;  pectoral  girdle 
89-00;  pel  vie  girdle  80-8 1 ;  respira- 
tory system  171;  ribs  69;  skull 
107-12;  urogenital  system  283-84; 
venous  system  222-26;  vertebral 
column  68-69 

Nephrostome  273,  275 

Nephrotome  39,  41 

Nervous  system  296-361:  develop- 
ment of  296-97,  359-61;  elasmo- 
branchs  301-18;  functional  com- 
ponents 297-98;  mammal  333-59; 
Necturus  318-21;  parts  of  296; 
pigeon  328-33;  summary  3  59-61; 
turtle  321-28 

Neural  arch  60,  61.  See  Vertebral 
column 

Neural  canal  60,  65.  See  Vertebral 
column 

Neural  crest  296 

Neural  folds  36,  38 

Neural  spine  60,  61.  See  Vertebral 
column 

Neural  tube,  development  of  36,  38, 
296-97 

Neuromere  299-300 

Nodosal  ganglion  343 

Nose.    See  Nasal  cavities 

Notocentrous  vertebra  69 

Notochord  5:  development  of  37,  39; 
in  skull  96-98;  in  vertebral  column 
59-61,  63,  65 

Oblique  septum  159,  181,  196,  199 

Obturator  foramen:  bird  82;  mammal 
84;  turtle  81 

Occipital  condyle  98,  109,  117,  113 

Occipital  nerves  315 

Occipital  region  of  skull  104:  alli- 
gator 114;  mammal  119;  Necturus 
109 

Oculomotor  nerve  360:  cat,  348,  354; 
elasmobranch  308,  310-11;  Nec- 
turus 320;  pigeon  331;  rabbit  346- 
47,354;  turtle  325 

Odontoid  process:  alligator  71; 
mammal  74;  turtle  72 

Olecranon:  bird  92;  mammal  93 

Olfactory  bulb:  elasmobranch  307; 
mammal  353,  354,  358 

Olfactory  capsule  97,  104-5:  alligator 
115;  mammal  121—22;  Necturus, 
in;  in  development  of  skull  97, 
104-5.  See  also  Nasal  cavi- 
ties 

Olfactory  fossa  of  skull  351 

Olfactory  lobe:  elasmobranch  307; 
Necturus  320;  pigeon  331-32; 
turtle  224 


Olfactory  nerve  360:    elasmobranch 

307,  309-10;    mammal  353;    Nec- 

turus 320;   pigeon  329,  331;    turtle 

325 
Olfactory  sacs:    development  of  300, 

361;    elasmobranch  303.     See  also 

Nasal  cavities 
Olfactory  tract:    elasmobranch  307; 

mammal  354 

Omentum,  greater:   190,  191,  198 
Omentum,  lesser:   166,  191 
Operculum  20 
Ophthalmic  nerve  360:    cat  348-49; 

elasmobranch    307,    311-12,    313; 

Necturus     320-21;      pigeon     329 

331-32;    rabbit   347;     turtle    323, 

325-56 

Opisthocoelous  76 
Optic  capsule  97,  105 
Optic   chiasma:     elasmobranch    317; 

mammal  3  54;  Necturus  321;  pigeon 

332;  turtle  327 
Optic  lobes:   elasmobranch  308,  318; 

mammal  353;  Neclurus^ao;  pigeon 

331;  turtle  3  24 
Optic  nerve  360:    cat  348;    elasmo- 

branch   305,   310;     Necturus    320; 

pigeon  331;  rabbit  347;  turtle  325 
Optic  pedicel  98,  305 
Optic  tract:  elasmobranch  31  7;  mam- 

mal 356,  358;   pigeon  332 
Oral   cavity:    elasmobranch    167-68; 

mammal    183-85;     Necturus    171; 

pigeon  176-77;   turtle  175 
Oral  glands  161:  mammal  183-84 
Orbit,   definition   of   98.    See   under 

Skull 

Orbital  fossa  1x7 
Origin,  of  muscle  131,  136 
Ossification  58,  103 
Ostium,  of  oviduct  278:  dogfish  281; 

mammal  289;  Necturus  284;  pigeon 

287;   skate  281;   turtle  284 
Ostrich,  sternum  92 
Otic  capsule  97,  104:    alligator  114; 

mammal  120;    Necturus  100-10 
Otolith  306,  319 
Ovary   278:    dogfish   280;    mammal 

289;    Necturus   283;    pigeon    286; 

skate  281;  turtle  284 
Oviducal  gland  282 
Oviduct    278-79:      dogfish    280-81; 

mammal    278-79,    289;    Necturus 

283-84;     pigeon    286,    287;     skate 

281-82;  turtle  284 

Palate   118,   119:    mammal    184-85; 

S'geon  176-77 
toquadrate  cartilage.    See  Ptery- 


goquadrate  cartilage 
alli 


Pallium:  pigeon  333;  turtle  327 
Pancreas    163,    199:     elasmobranch 

164;    mammal  192;    Necturus  169, 

170;  pigeon  182;   turtle  174 
Papillae,  of  tongue  185 
Papillary  muscles  266 
Parachordal  97:    bones  derived  from 

104 
Paraphysis  297,  318:    elasmobranch 

318;   Necturus  320;   turtle  324 
Parathyroids  162 

Parietal  peritoneum  158,  166,  190.  IQ- 
Parotid  gland  141,  184 
Patella:  bird  83;  mammal  84 
Pecten,  of  eye  330 
Pectoral  artery  244 
Pectoral      fin:       elasmobranch      89, 

Polyplerus  89 
Pectoral  girdle  78-79,  86-87:    birds 

91;    elasmobranch  88-89;    frog  90; 

mammals  92-93;   membrane  bones 

of  87;  origin  of  79;  parts  of  86-87; 

reptiles  90-91;   urodeles  80-90 
Pectoral  vein  242 
Peduncles    of    cerebellum:     elasmo- 

branch   308;     mammal    354     355, 


Pelvic  fin:   elasmobranchs  18,  19,  80 

Pelvic   girdle    78-79:     birds   82,   83; 

elasmobranchs     79-80;      mammals 


378 


LABORATORY  MANUAL  FOR  VERTEBRATE  ANATOMY 


83-84;  origin  of  79;  parts  of  79; 
turtle  81;  urodeles  80-81 

Penis  295:  mammal  29,  291-92; 
turtle  285,  286 

Pericardial  cavity  159,  160,  195-97, 
199:  elasmobranch  166-67;  mam- 
mal 187,  188;  Necturus  170-71; 
pigeon  180;  turtle  172 

Pericardial  sac  160,  196-97;  mammal 
188;  pigeon  180;  turtle  172-73 

Pericardio-peritoneal  canal  167:  dog- 
fish 222;  skate  210,  222 

Pericardium:  elasmobranch  166-67; 
mammal  188;  Necturus  170;  pigeon 
180;  turtle  172 

Periderrn  52,  53,  54 

Perimysium  136 

Perineum  29 

Peripheral  nervous  system  200,  296, 
298-99, 359, 360-61 :  elasmobranchs 
301-2,  309-16;  mammal  334-45, 
346-49;  Necturus  318-19,  320-21; 
pigeon  328,  331-32;  turtle  321-23. 
325-27 

Peritoneal  cavity  159-60,  196-97, 
199:  mammal  189-90;  pigeon  180- 
82 

Petrosal  ganglion:  elasmobranch  314; 
pigeon  332 

Peyer's  patches  194 

Phalanges  79,  88.     See  under  Limb 

Pharynx  161-62,  198-99:  elasmo- 
branch 168;  mammal  185-86;  Nec- 
turus 171;  pigeon  177;  turtle  175 

Phrenic  nerve  334-35 

Pia  mater:  mammal  341,  352-53; 
Necturus  320;  pigeon  330;  turtle 
124 

Pigeon-  air  sacs  178-81;  arterial 
system  244-46;  circulatory  system 
241-48;  classification  of  7,  25; 
coelom  1 80-8 1,  182-83;  digestive 
system  176-77,  180,  181-82;  exo- 
skeleton  52-54;  external  anatomy 
25-26;  feathers  25,  52-54;  fore 
limb  92;  heart  241-42,  246-47; 
hind  limb  83;  muscles  179-80; 
nervous  system  328-33;  pectoral 
girdle  91;  pelvic  girdle  82-83; 
respiratory  system  177-81;  ribs  73; 
sternum  91;  syrinx  183;  urogenital 
system  286-87;  venous  system 
242-44;  vertebral  column  72-73 

Pineal  body.     See  Epiphysis 

Pinna  27,  30,  301,  351,  361 

Pisces  6.     See  Fishes 

Pituitary  body.    See  Hypophysis 

Placenta  294 

Placenta!  mammal  7 

Placoid  scale  47-48 

Planes  of  body  i 

Plantigrade  walk  28 

Plasma  200 

Plastron  25,  52 

Pleura  188;  mammal  1 88, 189,  pigeon 
183 

Pleural  cavity  159,  196-97,  199: 
mammal  187-89;  pigeon  181,  182- 
83 

Pleuroperitoneal  cavity  159,  195-99: 
elasmobranch  163-64;  Necturus 
168-69;  turtle  172-73 

Plexus  of  spinal  nerves  299,  359: 
dogfish  301-2;  mammal  335-37, 
330-40;  Necturus  319;  pigeon  328; 
skate  302;  turtle  322 

Pneumatic  foramen  92,  179 

Polyodon:  classification  of  6;  external 
anatomy  21 

Polypterus:  classification  of  6;  pec- 
toral fin  and  girdle  89;  ribs  67; 
skull  101 

Pons  354-55 

Portal  system  200,  204,  204-6.  See 
further  under  Hepatic  portal  system 
and  Renal  portal  system 

Postaxial  border  of  limb  24 

Postcaval  vein:  development  of 
229-30,  241,  247,  268-70,  271; 
255-56  264-65,  268-70; 


Necturus  225-26;   pigeon  242,  243- 

44, 247;  turtle  233,  234-35,  237,  241 
Posterior  cardinal  vein  203,  270,  271: 

dogfish    208;     mammal  269;    Nec- 
turus 226,  229;    pigeon  247;    skate 

210-11;  turtle  240 
Posterior  chamber  of  eye  305,  349 
Posterior  vena  cava.     See  Postcaval 
Preaxial  border  of  limb  23 
Precavalvein27i;  cat  253-55;  pigeon 

242-43,  247;  rabbit  251-53;   turtle 

233-34,  240 
Prechordal  cartilage  97 ;  bones  derived 

from  104 

Premolar  teeth  125 
Prepuce  29, 291-92 
Precocious  vertebra  69,  70,  76 
Procoracoid  86.     See  under  Pectoral 

girdle 

Procricoid  cartilage  178 
Proctodaeum  160;  pigeon  287 
Prone,  definition  of  28 
Pronephric  duct  273,  275,  294,  295 
Pronephros  273-74,  275,  294 
Propterygium  80,  89 
Prosencephalon  297 
Prostate  gland  291 
Proventriculus  181,  182 
Pseudocentrous  vertebrae  69 
Pterygoid  fossa  113,  118 
Pterygoquadrate    cartilage    99,    105, 

107,  no,  114 

Pubis  79.  See  Pelvic  girdle 
Pudendal  plexus,  pigeon  328 
Pulmonary  artery  267,  272:  mammal 

248,    257;    Necturus   227;     pigeon 

244,  245;  turtle  235,  236 
Pulmonary  plexus  337 
Pulmonary  veins  200,  272:    mammal 

256-57;   Necturus  226;   pigeon  242, 

244;  turtle  235 
Pulp:  feather  52;  scale  47 
Pulvinar  356,  358 
Pupil:    elasmobranch  305;    mammal 

349 

Pygostyle  73 
Pylorus:  elasmobranch  164;  mammal 

190,  194;  Necturus  169 
Pyramid,  of  brain  355,  358 
Pyriform  lobe  354,  358 

Rabbit:  arterial  system  257-58,  260- 
64;  brain  352-59;  circulatory  sys- 
tem 249-65;  classification  of  7; 
coelom  187-89,  196-98;  cranial 
nerves  334,  337,  33.8,  342-45,  346- 
47,  354-55;  digestive  system  183- 
86,  189-94;  external  anatomy  27- 
29;  fore  limb  93-94;  heart  188, 
248,  265-66;  hind  limb  84-85; 
hyoid  apparatus  126,  186;  jaw 
118-19,  124;  muscular  system 
134-56;  nervous  system  333-59; 
pectoral  girdle  92-93;  pelvic 
girdle  83-84;  respiratory  system 
186-89;  ribs  75;  sense  organs 
345-52;  skull  116-26;  spinal 
nerves  334-42;  sternum  93;  sym- 
pathetic system  334~35,  337~39, 
343-44;  teeth  125;  urogenital 
system  288-92;  venous  system 
249-50,  250-53,  255-57,  264-65; 
vertebral  column  73-76 

Radiale  88.     See  Fore  limb 

Radialia:  pectoral  fin  89;  pelvic  fin 
80 

Radials.     See  Radialia 

Radius  88.     See  Fore  limb 

Ramuscommunicans  299:mammal337 
turtle  323 

Raphe  133,  136 

Rectal  gland  165 

Rectrices  26 

Remiges  26 

Renal  corpuscle  275 

Renal  portal  system  204-6,  214,  224, 
229,  240-41,  247,  269-70,  271: 
dogfish  214;  mammal  269-70; 
Necturus  223-25,  229;  pigeon  247; 
skate  214;  turtle  230-32,  240-41 


Renal  portal  vein  206:  elasmobranch 
214;  Neclurus  223-24,  229;  pigeon 
243-44,  247;  turtle  231-32,  240-41 

Reptilia:  classification  of  7;  exo- 
skeleton  50-52;  fore  limb  91; 
hind  limb  81-82;  pectoral  girdle 
90-91;  pelvic  girdle  .81;  skull 
1 1 2-i  6;  sternum  91;  vertebral 
column  70-72.  See  further  under 
Alligator,  Lizard,  and  Turtle 

Respiratory  system  161-63,  198-99: 
elasmobranchs  167-68;  mammal 
186-88;  Necturus  171;  pigeons  177- 
81;  summary  198-99;  turtle  175-76 

Restiform  body:  elasmobranch  308; 
mammal  354,  358 

Rete  testis  276 

Retina  301,  361:  elasmobranch  305; 
mammal  349;  pigeon  330 

Retroperitoneal,  definition  of  176 

Rhinencephalon:  elasmobranchs  307 

Rhombencephalon  297 

Ribs  62,  77:  alligator  71-72;  Am- 
phibia 69;  birds  73;  dogfish  66; 
kinds  of  62,  77;  mammals  75; 
origin  of  62;  teleost  67-68;  turtle  72 

Roots  of  spinal  nerves  298,  359:  elas- 
mobranchs 315,  316;  mammal  341 

Rostrum  17,  97 

Sacculus,  of  ear:  elasmobranch  306; 
mammal  352;  N eclurus  319;  pigeon 
330 

Sacculus  rotundus  193 

Sacral  plexus,  pigeon  328 

Sacrum  69.  See  under  Vertebral 
column 

Salivary  glands  161:  mammal  183-84 

Sauropsida,  definition  of  8 

Scales:  ctenoid  49-50;  cycloid  49; 
fishes  21,  22,  47-50;  ganoid  21,  49; 
mammals  55;  reptiles  23,  34,  25, 
50-52 

Scapula  86.    See  Pectoral  girdle 

Sciatic  nerve:  mammal  340;  Nec- 
turus 319;  pigeon  328;  turtle  322 

Sciatic  vein:  pigeon  243;  turtle  231 

Sclera  300:  elasmobranch  305;  mam- 
mal 349;  pigeon  329 

Sclerotic  coat.     See  Sclera 

Sclerotome  43;  in  development  of 
vertebrae  58-61 

Scrotum  29,  290 

Scute:   armadillo  55;    reptiles  50-52 

Segment  2 

Segmentation  2:  of  head  299-300, 
315;  of  circulatory  system  204,  206 

Segmentation  cavity  32 

Sella  turcica  121,  351 

Semicircular  canals:  mammal  352; 
pigeon  330 

Semicircular  ducts:  elasmobranch 
306;  mammal  352;  Necturus  319; 


pigeon  330;  turtle  326 
emilunai 


Semilunar   ganglion:     Necturus    320; 

pigeon  332;  turtle  325,  326 
Semilunar  valves:   elasmobranch  221; 

mammal  266;  Necturus  229;  pigeon 

246,  247;  turtle  239 
Seminal  vesicle:    elasmobranch   283; 

rabbit  291 

Seminiferous  tubules  276 
Sense  organs:    development  of  300- 

301,    361;     elasmobranchs    302-6; 

mammal    345-52;     Necturus    319; 

pigeon  329-30;   turtle  323-24 
Sensory,  definition  of  297-98 
Septum,    of    brain:     mammal    356; 

pigeon  333;  turtle  327 
Septum,  of  nose:    mammal  117,  121, 

350;  turtle  323 
Septum  pellucidum  356 
Serosa,  definition  of  158 
Sesamoid  bone  83 
Seymouria:    pectoral  girdle  88;    skull 

106 

Shell  gland  281 
Sinus    venosus:     elasmobrancb    207 

221;    mammal  248;    Necturus  222 

228-29:   turtle  230,  238 


INDEX 


379 


Skate:  arterial  system  215-19;  brain 
307-9,  316-18;  circulatory  system 
aio-ii,  213,  214,  215-19;  classi- 
fication of  6;  coelom  163-67; 
cranial  nerves  300-16;  digestive 
system  164-66;  exoskeleton  47; 
external  anatomy  18-19;  gills 
168;  'heart  207,  221-22;  nervous 
system  301-18;  respiratory  system 
168;  scales  47;  sense  organs  302-6; 
spinal  nerves  301-2;  urogenital 
system  281-83;  venous  system 
2IO-H,  213,  214 

Skeletogenous  regions  57 

Skeletogenous  septa  57,  58 

Skeleton:  definition  of  45,  57; 
exoskeleton  45-56',  gill  arches  99- 
roo,  111-12;  girdles  and  appen- 
dages 78-94;  ribs  57-77;  skull 
06-127;  sternum  85-86,  90-93,  95; 
summary  76-77,  94-95;  126-27. 
vertebral  column  57-77;  visceral 
skeleton  99-100 

Skin  45-55:  microscopic  structure  of 
45-46 

Skull  96-97,  99,  loo-ioi,  103-7: 
alligator  112-16;  bones  of  10377; 
cartilage  bones  of  103-5;  definition 
of  96;  elasmobranchs  97-100; 
ganoids  101—3;  mammal  116-26; 
membrane  bones  of  100-103,  105-7; 
Necturus  107-12;  origin  of  96-97 

Solar  plexus  338 

Somatppleure,  definition  of  41 

Somatic  mesoderm  36,  41 

Somatic  motor  column  309,  325 

Somatic  muscles  128 

Somatic  sensory  column:  elasmo- 
branch  308-9;  mammal  354; 
turtle  324 

Somite  2 

Spermatic  cord  290 

Sperm  sac  283 

Sphenodon:  classification  of  7;  verte- 
bral column  71 

Sphenoid  region  of  skull  104:  alli- 
gator 114-15;  bones  of  104;  mam- 
mal 1 20 

Sphenopalatine  ganglion  345 

Spinal  accessory  nerve  361:  mammal 
342,  355;  pigeon  332;  turtle  326 

Spinal  cord  296-98:  definition  of  296; 
development  of  296-97;  functional 
components  298;  mammal  341 

Spinal  ganglion  298:  elasmobranchs 
315;  mammal  341;  Necturus  319; 
pigeon  328;  turtle  322 

Spinal  nerves  296,  298-99,  359;  com- 
ponents 298;  definition  of  296; 
elasmobranchs 301-2, 315-16;  mam- 
mal 334-42;  Necturus  318-19;  parts 
of  298-99;  pigeon  328;  turtle 
321-23 

Spiracle  17,  168 

Spiral  valve  164-65 

Splanchnic  mesoderm  36,  41 

Splanchnic  nerves  338 

Splanchnocranium  99-100 

Splanchnopleure  41 

Spleen  200:  elasmobranch  164; 
mammal  190;  Necturus  169;  pigeon 
182;  turtle  174 

Spoonbill.     See  Polyodon 

Stegocephala:  classification  of  7; 
exoskeleton  50;  jaw  108;  pectoral 
girdle  87,  88;  skull  101,  106;  verte- 
bral column  64 

Sternebia  93 

Sternum  78,  85-86,  95:  alligator  91; 
bird  91;  definition  of  78;  frog  90; 
mammal  93;  origin  of  85-86; 
urodeles  90 

Stomach  161:  elasmobranch  164; 
mammal  190-91;  Necturus  169; 
pigeon  181,  182;  turtle  174 

Stratum  corneum  45,  46  See  Exo- 
skeleton 

Stratum  germinativum  45,  46.  See 
Exoskeleton 

Sturgeon.     See  A  cipenser 


Subcardinal  vein  204-6,  224,  270-71: 
elasmobranchs  214;  in  formation 
of  postcaval  vein  229,  241,  268, 
270-71 

Subclavian  artery:  cat  258-59; 
elasmobranch  218;  Necturus  227; 
pigeon  244;  rabbit  257-58;  turtle 
235 

Subclavian  vein:  cat  254;  dogfish 
209;  Necturus  225;  pigeon  242-43; 
rabbit  251;  turtle  234 

Subintestinal  vein  203,  205,  206,  270 

Sublingual  gland  184 

Submaxillary  gland  140,  184 

Subscapular  fossa  93 

Sulcus  353 

Summary:  appendages  94-96;  circu- 
latory system  270-72;  coelom  195- 
99;  digestive  system  198-99; 
exoskeleton  55-56;  external  anat- 
omy 29-30;  girdles  94-96;  muscu- 
lar system  157;  nervous  system 
359-61;  respiratory  system  198-99; 
ribs  77;  sense  organs  361;  skull 
126-27;  sternum  95;  urogenital 
system  294-95;  vertebral  column 
76-77 

Superior  cervical  ganglion:  mammal 
343;  turtle  327 

Superior  mesenteric  artery:  cat  262; 
pigeon  245;  rabbit  261;  turtle  237 

Superior  mesenteric  ganglion  338 

Superior  mesenteric  vein:  cat  250; 
pigeon  242;  rabbit  249 

Supine,  definition  of  28 

Supracondyloid  foramen  93 

Suprascapula  86-87.  See  Pectoral 
girdle 

Supraspinous  fossa  93 

Supratemporal  arcade  106,  112 

Supratemporal  fossa  112 

Suspensory  ligament:  of  lens  349; 
of  liver.  See  Falciform  ligament 

Suture,  definition  of  108,  117 

Swim  bladder  163 

Symmetry  1-2 

Sympathetic  system  296,  299,  359: 
mammal  334-35,  337~39,  343-44J 
pigeon  328,  332;  turtle  321,322-23, 
327 

Symphysis:  ischial  81,  84;  jaws  115; 
pubic  8 1,  84 

Synotic  tectum  97,  104,  in 

Synsacrum  73 

Syrinx  183 

Systemic  veins  200,  203-4:  definition 
of  200;  development  203-4;  dog- 
fish 207-10;  mammal  251-56,  264- 
65;  Necturus  225-26;  pigeon  242- 
44;  skate  210-11;  turtle  233-35, 
237 

Tarsales  79-    See  Hind  limb 

Tarsus  79:  bird  83;  mammal  84-85; 
turtle  82 

Teeth 48-49;  alligator  116;  homology 
of  48-49;  mammal  124-25;  Nec- 
turus 109,  171;  structure  of  48-49 

Tegmentum  357 

Telencephalon  297.  359:  elasmo- 
branch 307,  318;  Necturus  320. 
See  Cerebral  hemisphere 

Teleostei:  classification  of  6;  external 
anatomy  19-21;  pectoral  girdle 
87,  89;  ribs  67-68;  scales  49-50; 
vertebral  column  67-68 

Teleostomi:  classification  of  6.  See 
further  under  Ganoids 

Telolecithal,  definition  of  31 

Temporal  fossa  117 

Tendon,  definition  of  136 

Tendon  of  Achilles  155 

Tentorium  121,  351 

Testis  276,  295:  descent  of  290; 
elasmobranch  282;  mammal  290, 
292;  pigeon  287;  turtle  286 

Thalamencephalon.  See  Diencepha- 
lon 

Thalamvis  }.sy-6o;  elasmobranch  318; 
mammal  156,  358;  turtle  327 


Thecodont  116, 124 

Thoracic  duct  260 

Thorax  27,  187-89:  muscles  of  139, 
144-50 

Thymus  162:  mammal  189;  turtle 
235 

Thyroid  cartilage  142,  187 

Thyroid  gland  161,  162:  elasmo- 
branch 215;  mammal  187;  turtle 
235 

Tibia  79,  80.     See  Hind  limb 

Tibiale  79,  80.     See  Hind  limb 

Tongue:  elasmobranch  168;  mammal 
185;  Necturus  171;  pigeon  177; 
turtle  175 

Tonsils  162,  186 

Trabecula  97.     See  Prechordal 

Trabeculae  carnae  266 

Trachea  162-63,  199:  mammal  187; 
Necturus  171;  pigeon  177,  178,  183; 
turtle  175 

Transverse  septum  159,  195-98,  203- 
4:  elasmobranch  166,  167;  mam- 
mal 189,  197;  Necturus  170,  171; 
pigeon  180-81,  197;  turtle  173,  196 

Trapezoid  body  355,  358 

Tricuspid  valve  266 

Trigeminus  nerve  360:  elasmobranch 
311-13;  mammal  344~45,  346,  347, 
348,  355;  Necturus  320-21;  pigeon 
331-32;  turtle  325-26 

Trochanter  83,  84 

Trochlea:  of  eye  346,  347;  of  humerus 
93 

Trochlear  nerve  360:  elasmobranch 
307,  308,  310:  mammal  347,  349, 
354;  Necturus  320;  pigeon  329,331; 
turtle  323,  325 

Tuber  cinereum  354 

Tuberculum  of  rib  69,  71,  75 

Tuberculum  cuneatum  354,  358 

Tunicata:  classification  of  6;  external 
anatomy  11-12;  internal  anatomy 
12-13;  pharynx  13 

Turbinals  300.    See  Conchae 

Turbinated  bones  121-22  Set 
Conchae 

Turtle:  arterial  system  235-38;  brain 
324-25,  327;  carapace  51-52;  cir- 
culatory system  230-41;  classifica- 
tion of  7;  coelom  172-73;  cranial 
nerves  325-27;  digestive  system 
173-75;  exoskeleton  51-52;  exter- 
nal anatomy  24-25;  fore  limb  91; 
heart  230.  238-39;  hind  limb  81-82; 
hyoid  apparatus  115-16;  neryou? 
system  321-28;  pectoral  girdle 
90-91;  pelvic  girdle  81;  plastron 
52;  respiratory  system  175-76; 
sense  organs  323-25;  spinal  nerves 
321-23;  sympathetic  system  322- 
23;  urogenital  system  284-86; 
venous  system  230-35,  237;  verte- 
bral column  72 

Tympanic  bulla  117,  119,  120,  351-52 

Tympanic  cavity  162,  300-301,  361: 
mammal  351-52;  pigeon  330; 
turtle  324 

Tympanic  membrane  162,  301,  361: 
cat  352;  pigeon  330;  rabbit  351; 
turtle  324 

Ulna  88.    See  Fore  limb 

Ulnare  88.     See  Fore  limb 

Umbilical  vein  203,  294 

Uncinate  process  73 

Unguligrade  walk  28 

Ureter  276,  294:  mammal  288;  pigeon 

287;   turtle  285,  286 
Urethra  279,  288,  290,  291 
Urethral  groove  286 
Urinary  bladder  163,  276,  292,  295: 

mammal   190,   194,   288;    Necturus 

169,   170,   284;    skate  282;    turtle 

174,  285,  286 
Urodaeum  287 
Urodela:  classification  of  7;  external 

anatomy  22-23;  fore  limb  90;  hind 

limb    81;     pectoral    girdle    89-90; 

pelvic      girdle     80-8 1;      vertebral 


LABORATORY  MANUAL  FOR  VERTEBRATE  ANATOMY 


column  68-69.  See  further  under 
Necturus 

Urogenital  canal  or  sinus  279,  295: 
elasmobranch  283;  mammal  290, 
291-92 

Urogenital  system  273-95:  develop- 
ment of  273-79;  elasmobranch 
280-83;  mammal  288-92;  Nec- 
turus 283-84;  parts  of  273-79; 
pigeon  286-87;  summary  294-95; 
turtle  284-86 

Uropygial  gland  27 

Uropygium  25 

Urostyle:  fish  68;  frog  69;  man  76 

Uterine  tube  278,  279,  295:  mammal 
289 

Uterus  278-79,  295:  dogfish  281; 
ligaments  of  289;  mammal  289; 
skate  281-82 

Utriculus:  elasmobranch  306;  mam- 
mal 352;  Necturus  319;  pigeon 
330 

Vagina  279,  289 

Vagus  nerve  360-61:  elasmobranch 
314-15;  mammal  334,  337,.  338, 
342-43,  355;  Necturus  321;  pigeon 
332;  turtle  326,  327 

Valla te  papilla  185 

Vas  deferens  276,  295:  mammal  288, 
290-92;  pigeon  287;  turtle  286 

Vasa  efferentia  276:  elasmobranch 
283;  mammal  292;  Necturus  284; 
turtle  286 

Vein,  definition  of  200 

Velum  transversum  318 

Venous  system  203-7,  270-72:  elas- 
mobranch 207-14;  mammal  249-57, 
264-65,  a68-6g;  Necturus  322-26, 


229-30;  pigeon  24*-44,  247-4*1 
turtle  230-35,  237,  240-41 

Ventral  aorta  202,  270,  272:  dogfish 
215;  Necturus  226-27;  skate  215 

Ventral  column,  spinal  cord  297 

Ventral  ramus,  spinal  nerve  299 

Ventricle,  of  brain  297,  360:  elasmo- 
branch 308,  317-18;  mammal  356- 
57;  Necturus  320;  pigeon  333; 
turtle  327 

Ventricle,  of  heart  271,  272:  elas- 
mobranch 207,  221;  mammal  248, 
266;  Necturus  222,  229;  pigeon 
241,  246-47;  turtle  230,  238-39 

Vermiform  appendix  193 

Vertebra  58-62,  76-77:  definition  of 
58;  development  of  58-61;  parts 
of  61-62.  See  further  under  Verte- 
bral column 

Vertebral  artery:  cat  259;  Necturus 
227;  pigeon  244;  rabbit  258; 
skate  218;  turtle  236 

Vertebral  column  57-77:  alligator 
70-72;  Amphibia  68-69;  bird  72- 
73;  bowfin  64;  development  of 
58-61;  dogfish  64-66;  mammal  73- 
76;  reptiles  70-72;  Stegocephala 
64;  sturgeon  63-64;  summary  75- 
77;  teleosts  67-68;  turtle  72 

Vertebral  vein:  cat  254;  pigeon  243' 
rabbit  251;  turtle  232,  234 

Vertebraterial  canal  71 

Vertebrate:  characters  of  5;  classifica- 
tion of  6-7 

Vestibule,  mouth  184 

Visceral  arch  161,  162,  168:  elasmo- 
branch 1 68;  Necturus  171 

Visceral  branch  of  vagus  nerve  314-15 

Visceral  cleft  161.    See  Gill  slits 


Visceral  furrow  161 

Visceral  motor  column  309 

Visceral  peritoneum  158,  166,  107 

Visceral  pouch  161,  162:  elasmo- 
branch 1 68;  glands  from  162; 
Necturus  171 

Visceral  ramus  of  spinal  nerve  299 

Visceral  sensory  column  309 

Visceral  skeleton  99-100,  105,  107: 
elasmobranch  99-100;  Necturus 
IH-I2.  See  further  under  Gill 
arches  and  Hyoid  apparatus 

Vitelline  veins  202,  204,  205,  206,  270, 
271 

Vitreous  body.     See  Vitreous  humor 

Vitreous  humor  305,  349 

Vocal  cords  187 

Vulva  29 

White  matter  296,  297,  342 

Windpipe.    See  Trachea 

Wings:    external  anatomy  26;    skele 

ton  of  92 
Wishbone  91 

Wolffian  body.     See  Mesonephros 
Wolffian  duct.  See  Mesonephric  duct 

and  Vas  deferens 
Womb.    See  Uterus 

Yolk  31,  32:  effect  on  cleavage  31-35; 

on  form  of  embryo  40 
Yolk  plug  34-35 
Yolk  sac  40,  161,  163,  292-94:   blood 

vessels  of  202-3 
Yolk  stalk  40 


Zygapophyses,  definition  of  68. 
under  Vertebral  column 

Zygoma  tic  arch  117,  118 


Set. 


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